Environmental Biotechnology β€” Master Practical Exam Guide (BTBio 425)

A unified, exam-optimized synthesis of all lab manuals, lecture transcripts, sample specimens, and the doctor's verbal "exam alerts." Built for fast recall under time pressure.
🚨 Exam Alert / Trap βœ“ Key Concept βš™ Mechanism πŸ’‘ Doctor's Tip

1. Exam Blueprint & Doctor's Priorities

🚨 GUARANTEED on the exam (doctor stated explicitly):
  • Dilution / CFU calculation problem β€” "There will definitely be a calculation problem regarding dilutions." Watch the units (mL ↔ L conversion is a known trap).
  • Selective & Differential Media identification from a photo β€” name the medium, what it is selective for (and the inhibitory agents), and how it differentiates.
  • Phosphate Solubilization sheet β€” measure colony & zone diameter with a ruler and calculate Solubilization Efficiency (SE%).
  • BOD & COD β€” definitions, procedures, and simple calculations (the formulas are tested).
  • Gram-stain identification from a photo + theory of the staining process.
βœ“ Exam structure (per doctor): 25 marks. Photos used in the exam are the same images used in lab sessions. Specific details (e.g., naming the inhibitory agents in a medium) carry significant weight.

What the doctor repeatedly emphasized

ThemeWhy it matters / What to write
Indicator organism logicWhy test for E. coli instead of every pathogen β†’ cheaper, safer, faster, more abundant. Compare to "pH indicator."
Why use 1, 2, 4 in API 20EBecause 3 would create ambiguous sums (e.g., 1+2 = 3 same as tube 3 alone). Powers of 2 give unique sums.
"Zero" rulePotable water = 0 coliforms / 100 mL.
Congo Red trapRhizobium = colorless / white; red colonies are contaminants. Easy to flip.
Heat-stable enterotoxinsS. aureus can leave toxins in cooked food even after cells are dead.
White-rot fungiThe classical example for laccase / lignin degradation (and PAH biodegradation).
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2. High-Yield Concepts (Must-Memorize Core)

2.1 PGPR β€” Plant Growth Promoting Rhizobacteria

Definition (verbatim style): PGPR are a heterogeneous group of bacteria found in the rhizosphere, at root surfaces, and in association with roots, which can improve the extent or quality of plant growth directly and/or indirectly.
Direct mechanismsIndirect mechanisms
  1. Producing/altering plant growth regulators: indole-acetic acid (IAA), gibberellic acid, cytokinins, ethylene
  2. Symbiotic Nβ‚‚ fixation
  3. Solubilization of mineral phosphates and other nutrients
  1. Antagonism against phytopathogens (siderophores, hydrocyanic acid)
  2. Biocontrol via antibiotic production
  3. Triggering induced local or systemic resistance

Genera reported: Pseudomonas, Azospirillum, Azotobacter, Klebsiella, Enterobacter, Alcaligenes, Arthrobacter, Burkholderia, Bacillus, Serratia.

2.2 Key Definitions Cheat-Sheet

TermDefinition
RhizosphereSoil zone directly surrounding the plant root system.
RhizoplaneThe actual surface of the root.
EndophyteMicroorganism (e.g., Rhizobium) that lives inside plant tissues.
Nitrogen fixationReduction of atmospheric Nβ‚‚ β†’ NH₃, by the enzyme nitrogenase (only in prokaryotes).
DiazotrophProkaryote that uses Nβ‚‚ gas as its sole N source.
SiderophoresIron-chelating compounds; "starve" pathogens of Fe³⁺.
LeghaemoglobinPink/red pigment in active N-fixing nodules; protects nitrogenase from Oβ‚‚.
Mineralization (P)Release of inorganic P from organic P by microbes.
ColiformsGram-negative, non-spore-forming, lactose-fermenting rods of family Enterobacteriaceae.
Indicator organismNon-pathogen present in greater numbers than pathogens, used as a proxy for contamination.
BODOxygen required by microbes to break down organic matter (5-day incubation, 20 Β°C, dark).
CODOxygen required by a chemical oxidant to oxidize organic + inorganic matter.
CFUColony Forming Unit β€” counts only viable cells.
OD (Optical Density)Turbidity-based count; counts dead + live + debris (not viable-only).

2.3 Coliform Hierarchy (KNOW THIS)

GroupHabitatLactose ferments atSignificanceKey enzyme
Total ColiformsSoil, water, plants, intestines35 Β°C, 24–48 hGeneral contaminationΞ²-galactosidase
Fecal ColiformsIntestines of warm-blooded mammals44.5 Β°C, 24–48 hRecent fecal contaminationΞ²-galactosidase
E. coliHuman/animal gut (best indicator)44.5 Β°CDefinitive fecal indicatorΞ²-galactosidase + Ξ²-glucuronidase
🚨 The "Zero Rule": Potable water = 0 coliforms / 100 mL. Detection methods rely on coliforms' ability to ferment lactose β†’ acid + gas.

2.4 Three Methods for Coliform Monitoring

MethodTypePrincipleKey media / signal
Presence-Absence (P-A)QualitativeYes / no in 100 mLColilert (ONPG β†’ yellow), Lauryl Sulfate Tryptose broth
Membrane Filter (MF)QuantitativeFilter water β†’ place membrane on selective agar β†’ count coloniesm-Endo agar β€” red colonies with metallic sheen = coliforms
Multiple Tube Fermentation (MTF / MPN)Quantitative (statistical)3 sets Γ— 5 tubes (10 mL / 1 mL / 0.1 mL); count gas/yellow tubes; read MPN tableLactose broth β†’ Brilliant Green Lactose Bile broth β†’ EMB agar

2.5 Bioremediation & Ligninolytic Enzymes

The Triple Threat (memorize): Laccase (Lac) + Lignin Peroxidase (LiP) + Manganese Peroxidase (MnP). All three are produced by White-Rot Fungi (basidiomycetes), the only organisms able to completely mineralize lignin to COβ‚‚ + Hβ‚‚O.
FeatureLaccaseLiP / MnP
CofactorCopper (blue copper oxidase)Heme (Fe-protein)
ReactionOne-electron oxidation of phenolics β†’ e⁻ to Oβ‚‚ β†’ Hβ‚‚OPeroxidase (uses Hβ‚‚Oβ‚‚)
Lab assay substrateGuaiacol β†’ Tetraguaiacol (brown), Ξ» = 470 nmβ€”
EC numberEC 1.10.3.2 (benzenediol:Oβ‚‚ oxidoreductase)β€”

2.6 PAHs (Polycyclic Aromatic Hydrocarbons)

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3. Mechanisms & Reasoning (The "Why")

3.1 Why MSA selects + differentiates S. aureus
βš™ Mechanism: Two layers act simultaneously.
  1. Selection β€” 7.5% NaCl creates a hyper-osmotic environment. Most bacteria lose water and cannot grow. Staphylococci are halotolerant and survive.
  2. Differentiation β€” The medium contains mannitol + phenol red indicator. S. aureus ferments mannitol β†’ acid β†’ indicator drops to acidic pH β†’ red turns yellow (a yellow halo around colonies). Coagulase-negative S. epidermidis does not ferment mannitol β†’ colonies stay pink/red on a red background.

Memory hook: "aureus" = Latin aurum = gold = yellow.

3.2 Why MacConkey is selective and differential
βš™ Mechanism:
  • Selection: Crystal violet + bile salts inhibit Gram-positive organisms.
  • Differentiation: Lactose + neutral red indicator. Lactose fermenters (e.g., E. coli) produce acid β†’ bile salts precipitate β†’ neutral red is absorbed β†’ brick-red / pink colonies with a precipitated bile zone. Non-fermenters (Salmonella, Shigella) β†’ colorless / transparent (alkaline reaction).
3.3 Why XLD shows red colonies with black centers
βš™ Three-stage mechanism:
  1. Stage 1 (early): Salmonella ferments xylose β†’ acid β†’ medium turns yellow (just like E. coli, but E. coli further ferments lactose & sucrose, locking the medium yellow β†’ masking).
  2. Stage 2 (xylose exhausted): Salmonella decarboxylates lysine β†’ amines β†’ pH rises again to alkaline β†’ indicator (phenol red) reverts to red.
  3. Stage 3: Salmonella reduces thiosulfate β†’ Hβ‚‚S, which reacts with ferric ammonium citrate to form a black precipitate (FeS) β†’ black-centered red colonies.
  4. Shigella cannot ferment xylose at all + does NOT make Hβ‚‚S β†’ plain red colonies.
3.4 Why Baird-Parker shows black colonies + halo for S. aureus
βš™ Mechanism (3 components):
  • Selection: Lithium chloride + 1% potassium tellurite suppress non-staphylococci.
  • Black color: S. aureus reduces tellurite β†’ telluride β†’ grey-to-black colonies.
  • Clear halo: S. aureus produces lecithinase that breaks down lecithin in the egg yolk in the medium β†’ clear "halo" around the black colony.
  • Opaque halo: Coagulase converts fibrinogen (if present) β†’ fibrin β†’ opaque halo (coagulase-positive identification).
3.5 Gram-stain mechanism (and its alternatives)
βš™ Mechanism: Gram⁺ cells have a thick peptidoglycan wall (50–90% of envelope) that traps the crystal violet–iodine complex during decolorization β†’ stay purple. Gram⁻ cells have a thin peptidoglycan layer (~10%) inside an outer membrane that is dissolved by alcohol β†’ CV-I washes out β†’ counterstain with safranin β†’ pink/red.

Steps

  1. Heat-fix smear β†’ apply crystal violet (2 min) β€” primary stain
  2. Add iodine β€” mordant; binds CV, traps it inside cell
  3. Rapid decolorization with ethanol/acetone (critical step β€” over-decolorizing = false Gram-negative)
  4. Counterstain with safranin (2 min)

Alternatives the doctor mentioned

TestPrinciplePositive resultImplies
KOH String Test3% KOH lyses Gram⁻ cells β†’ DNA leaks β†’ solution becomes viscous & "strings"Stringy/viscous within 60 sGram-negative
Aminopeptidase TestL-alanine aminopeptidase (Gram⁻ only) cleaves L-alanine-4-nitroanilide β†’ 4-nitroaniline (yellow)Yellow color (5 min)Gram-negative
Fluorescent WGAWheat germ agglutinin binds N-acetylglucosamine in exposed peptidoglycan of Gram⁺Green fluorescenceGram-positive
🚨 KOH Trap: KOH-positive (stringy) = Gram-negative. KOH-negative (no string) = Gram-positive. The result/identity are inverted β€” easy to flip on the exam.
3.6 Coliscan / ONPG / MUG (the enzyme-substrate logic)
βš™ The clever idea: Coliforms ferment lactose, which means they have specific enzymes. Detect the enzymes directly using engineered substrates that release a colored or fluorescent product when cleaved.
EnzymeSubstrateProduct / SignalIdentifies
Ξ²-galactosidaseONPG (o-nitrophenyl-Ξ²-galactoside)o-nitrophenol = yellow (read at 420 nm)All coliforms (general)
Ξ²-glucuronidaseMUG (4-methylumbelliferyl-Ξ²-D-glucuronide)4-methylumbelliferone = fluorescence under UVE. coli specifically

On a Coliscan Easygel plate: dark blue / purple = E. coli; pink / red = other coliforms; teal / green = other organisms.

ONPG nuance: Some "non-lactose fermenters" lack permease but still have Ξ²-galactosidase β€” they show ONPG-positive but ferment lactose slowly = "late lactose fermenter" (e.g., Citrobacter).

3.7 Phosphate solubilization mechanism
βš™ Mechanism: Plants take up P only as orthophosphate (Hβ‚‚PO₄⁻ at pH < 7; HPO₄²⁻ at pH > 7.2). In soil, P is "locked":
  • In acidic soils: P binds to Fe³⁺ and Al³⁺ surfaces (sorption).
  • In calcareous (alkaline) soils: P precipitates with Ca²⁺ as insoluble calcium phosphates.
Phosphate-Solubilizing Microorganisms (PSMs) β€” fungi (Aspergillus, Penicillium) and bacteria (Pseudomonas, Bacillus, Rhizobium, Azospirillum) β€” release organic acids that chelate the cations or lower pH, freeing soluble P.

On agar with insoluble Ca-phosphate, this appears as a clear halo (solubilization zone) around the colony.

3.8 Why API 20E uses 1, 2, 4 (not 1, 2, 3)
βš™ Mechanism (combinatorial logic): 21 tests (20 strip + 1 oxidase) split into 7 triads. In each triad: tube 1 = 1 pt, tube 2 = 2 pts, tube 3 = 4 pts. Sum the positives in each triad β†’ 7-digit profile β†’ look up in API database.

Why 4 and not 3? Because every subset must give a unique sum:

PositivesSum (1,2,3)Sum (1,2,4) βœ“
Tube 1 only11
Tube 2 only22
Tube 3 only3 ⚠ same as 1+24
1+23 ⚠3
1+345
2+356
1+2+367

With (1, 2, 4) every combination has a unique sum (0–7). This is the same logic as binary encoding.

Pre-step: Oxidase test must be negative (Enterobacteriaceae are oxidase-negative).

Why add reagents AFTER incubation, not before? Because the reagents could inhibit bacterial growth.

3.9 Why use indicator organisms instead of direct pathogen detection
βš™ Doctor's "pH indicator" analogy: You don't measure every acid in a solution β€” you use one indicator. Same for water: don't search for every pathogen.

An ideal indicator must be:

  • Present in greater numbers than the pathogen
  • Survive longer (gives time to detect)
  • Non-pathogenic (safe to handle)
  • Easy to test for in routine labs
  • Reflect contamination (present only when water is contaminated)
  • Useful for all water types
3.10 BOD vs COD β€” what they actually measure
ParameterBODCOD
What's measuredOβ‚‚ used by microbes to break down organicsOβ‚‚ used by chemical oxidant to break down everything (organic + inorganic)
MethodDO at day 0 vs day 5 (20 Β°C, dark)Kβ‚‚Crβ‚‚O₇ + Hβ‚‚SOβ‚„ digestion 2 h at 150 Β°C β†’ titrate excess
Sees acetate?Yes (microbes metabolize it)No (resists oxidation)
Sees cellulose?No (slow for microbes within 5 d)Yes (oxidant breaks it)
Time5 days~2 hours
InterpretationHigher = more pollution (more "food" β†’ more Oβ‚‚ used)Higher = more pollution
🚨 Trap: Initial DO is always high in fresh samples. The drop over 5 days = BOD. Don't get confused by the high starting reading.
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4. Practical Protocols Breakdown

4.1 Pour-Plate Procedure (Isolation by Dilution)
  1. Pipette 1 mL or 0.1 mL of a known dilution into the bottom of a sterile Petri dish (before adding media).
  2. Within 20 min, add 12–15 mL of warm (46–48 Β°C) fluid medium.
  3. Cover the dish.
  4. Swirl gently in a figure-eight pattern (flat on the bench) to disperse the inoculum.
    • Why? Ensures homogeneous distribution; avoids splashing onto the lid.
  5. Allow to cool and solidify.
  6. Invert the plate (medium-side down) and incubate.
    • Why invert? Prevents condensation droplets from pooling on the agar surface β€” pools could let surface colonies merge or migrate.

Key constraint: Agar must be at 46–48 Β°C β€” hot enough to stay liquid, cool enough not to kill the cells.

4.2 Streak-Plate Procedure
  1. Aseptically introduce inoculum to a small area = "the well".
  2. Re-flame loop, cool on remote corner, then streak from the well across one quadrant.
  3. Re-flame after each streak β€” this dilutes the cell density progressively.
  4. 3–5 streaks per plate; the last quadrant should yield isolated colonies.

Both pour-plate and streak-plate aim to produce single, well-isolated colonies assumed to arise from a single cell β€” so each colony = one strain.

4.3 Counting rule β€” the "30–300" Window
  • Lower limit (30): Statistical accuracy minimum.
  • Upper limit (300): Beyond this, accurate counting becomes impossible.
  • Pick the dilution plate that falls in this window for CFU/mL calculation.
4.4 Isolating Pathogenic Staphylococci from Water (full design)
  1. Prepare Mannitol Salt Agar (MSA) containing: mannitol (fermentable sugar), phenol red indicator, 7.5% NaCl (selective).
  2. Pipette 1 mL of a known dilution into a sterile Petri dish (before media addition).
  3. Within 20 min, add 12–15 mL warm (46–48 Β°C) fluid MSA.
  4. Cover & swirl (figure-8) to disperse evenly.
  5. Cool, solidify.
  6. Invert and incubate.
  7. Read: pathogenic S. aureus = small colonies surrounded by yellow zones (mannitol fermentation β†’ acid β†’ phenol red β†’ yellow).
4.5 Isolating Rhizobium from Root Nodules
  1. Locate legume plants in the field; dig a 15-cm radius / 20-cm-deep soil clump around each.
  2. Carefully wash roots under gentle running water β€” keep nodules on lateral roots intact. Do NOT freeze nodules (ice ruptures bacteroids).
  3. An active N-fixing nodule has a pink, red, or brown interior (leghaemoglobin); ineffective rhizobia β†’ white interiors.
  4. Sever each nodule (~0.5 cm of root either side); use forceps on the appendage, not the nodule itself.
  5. Surface sterilization: 95% ethanol or isopropanol 5–10 s β†’ 2.5–3% sodium hypochlorite 2–4 min β†’ rinse with sterile water.
  6. Crush nodule in a sterile test tube with a sterile glass rod.
  7. Streak one loopful onto YMA + Congo Red; another onto YMA + Bromothymol Blue.
  8. Incubate in the dark (CR can be photoactivated).
  9. Read: White / colorless glistening colonies = Rhizobium; red colonies = contaminants.

YMA composition (per L): Kβ‚‚HPOβ‚„ 0.5 g; MgSOβ‚„Β·7Hβ‚‚O 0.2 g; NaCl 0.1 g; yeast extract 1 g; mannitol 10 g; Hβ‚‚O up to 1000 mL; pH 6.8–7.0; agar 18 g/L; Congo Red 0.025 g/L. Sterilize 121 Β°C / 20 min.

🚨 Congo Red Trap: Rhizobium = colorless/white. Red colonies = contaminants. Reverse intuition.
4.6 Phosphate Solubilization Assay
  1. Inoculate isolate centrally onto Pikovskaya / NBRIP medium containing insoluble Ca-phosphate.
  2. Incubate; the colony grows + dissolves nearby insoluble P β†’ clear "solubilization halo".
  3. Measure with a ruler:
    • Diameter of colony (Dcolony)
    • Diameter of solubilization zone (Dzone) β€” entire halo edge to edge.
  4. Calculate Solubilization Efficiency (formula in Β§7).
4.7 Bacillus Isolation Protocol
  1. 1 g soil β†’ 9 mL nutrient broth/saline.
  2. Heat 10 min at 80 Β°C β€” destroys vegetative bacteria/fungi; spores survive (selects for spore-formers like Bacillus).
  3. Make tenfold serial dilutions (10⁻¹ to 10⁻⁷).
  4. Plate 100 Β΅L aliquots in duplicate on nutrient agar (spread plate).
  5. Incubate 28–30 Β°C / 24 h.
4.8 Laccase Enzyme Assay (Guaiacol Method)

Reaction mixture:

  • 3.9 mL acetate buffer (10 mmol/L, pH 5.0)
  • 1 mL guaiacol (2 mmol/L)
  • 0.1 mL diluted enzyme solution
  • Incubate 35 Β°C / 30 min
  • Read absorbance at 470 nm; blank uses buffer instead of guaiacol.

Why guaiacol? Laccase oxidizes it to tetraguaiacol (brown). Color intensity ∝ activity.

(Formula in Β§7.)

4.9 BODβ‚… Procedure
  1. Take two parallel water samples.
  2. Sample A: measure dissolved oxygen immediately (DOβ‚€).
  3. Sample B: incubate in the dark at 20 Β°C for 5 days; then measure DOβ‚….
  4. BOD = DOβ‚€ βˆ’ DOβ‚… (mg/L).

Why dark? Prevents photosynthesis, which would add Oβ‚‚ and falsify the result.

4.10 COD Procedure

Reagents: distilled Hβ‚‚O; 0.250 N Kβ‚‚Crβ‚‚O₇; conc. Hβ‚‚SOβ‚„ + Agβ‚‚SOβ‚„; HgSOβ‚„; 0.25 N ferrous ammonium sulfate; ferroin indicator.

  1. 50 mL sample + 1 g HgSOβ‚„ (binds Cl⁻ to prevent interference).
  2. Add 5 mL conc. Hβ‚‚SOβ‚„; swirl until HgSOβ‚„ dissolves.
  3. In ice bath, slowly add 25 mL of 0.025 N Kβ‚‚Crβ‚‚O₇ with swirling.
  4. Add 70 mL Hβ‚‚SO₄–Agβ‚‚SOβ‚„ solution slowly with swirling.
  5. Reflux at 150 Β°C for 2 hours.
  6. Cool, dilute to ~300 mL with distilled water, cool to room T.
  7. Add 8–10 drops ferroin β†’ titrate excess dichromate with 0.25 N FAS to endpoint (red/brown).
  8. Run a blank in parallel (low-COD water in place of sample).

Mercury is added because chloride is the main interferent (Cl⁻ would consume oxidant via oxidation, falsely raising COD).

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5. Selective & Differential Media β€” Master Identification Table

For every photo on the exam, you must answer: (1) name of medium, (2) what is it selective for + the inhibitory agents, (3) how it differentiates (mechanism + color logic), (4) expected positive vs negative result.

Medium Type Target / Selective for Selective Agent(s) Differential Mechanism Positive (+) appearance Negative (βˆ’) appearance
Mannitol Salt Agar (MSA) Selective + Differential Staphylococci (halotolerant) 7.5% NaCl Mannitol fermentation + phenol red indicator S. aureus: yellow colonies + yellow halo Other staph (S. epidermidis): pink/red colonies on red background; non-staph: no growth
Baird-Parker Agar Selective + Differential Coagulase-positive S. aureus Lithium chloride; 1% potassium tellurite Tellurite reduction β†’ black; lecithinase β†’ halo; coagulase β†’ opaque ring Black colonies with clear halo (Β± opaque halo) No growth or non-black colonies
Blood Agar Differential β€” β€” Hemolysin β†’ RBC lysis pattern Ξ±: greenish/brown (partial; S. pneumoniae); Ξ²: clear zone (full lysis; S. aureus, S. pyogenes) Ξ³: no change (Neisseria)
ORSAB (Oxacillin Resistance Screening Agar Base) Selective + Differential MRSA (methicillin-resistant S. aureus) 5.5% NaCl; lithium chloride; Oxacillin (2 mg/L); polymyxin B Mannitol fermentation + aniline blue indicator Intense blue colonies = MRSA MSSA inhibited by oxacillin; non-staph inhibited by salt
MacConkey Agar Selective + Differential Gram-negative enteric bacteria Bile salts + crystal violet (inhibit Gram⁺) Lactose fermentation + neutral red indicator Lactose fermenters (E. coli): brick-red / pink colonies with bile precipitate zone Non-fermenters (Salmonella, Shigella): colorless / transparent
EMB Agar (Eosin Methylene Blue) Selective + Differential Gram-negative enterics Eosin Y + methylene blue (inhibit Gram⁺) Strong lactose fermenters precipitate dye complex at low pH E. coli: green metallic sheen; other lactose+ : blue-black Salmonella, Shigella: colorless / translucent
XLD Agar Selective + Differential Salmonella, Shigella Sodium deoxycholate Xylose ferment β†’ lysine decarboxylation β†’ Hβ‚‚S production with ferric ammonium citrate Salmonella: red colonies with black centers; Shigella: plain red E. coli: yellow β†’ yellow-red colonies
Salmonella-Shigella Agar (SS) Selective + Differential Salmonella, Shigella Bile salts + brilliant green + citrates Lactose fermentation (neutral red) + Hβ‚‚S (Na thiosulfate + ferric citrate) Salmonella: colorless colonies with black centers; Shigella: colorless, no center Lactose fermenters: red colonies (e.g., E. coli if present)
m-Endo Agar Selective + Differential Total coliforms (membrane-filter method) Sodium lauryl sulfate; sodium deoxycholate (inhibit Gram⁺) Lactose fermentation + basic fuchsin / sodium sulfite + aldehyde formation Coliforms: red colonies with green metallic sheen Non-fermenters: clear, colorless colonies
Coliscan Easygel Selective + Differential Coliforms vs E. coli in water Inhibitors of non-coliforms in gel Two chromogenic substrates: Ξ²-galactosidase + Ξ²-glucuronidase E. coli = dark blue/purple; other coliforms = pink/red Other organisms = teal/green or none
YMA + Congo Red (Yeast-Mannitol Agar) Differential (CR) Rhizobium from nodules β€” Congo red absorption test Rhizobium: white / colorless glistening colonies (does NOT absorb) Contaminants: red colonies (absorb dye)
Lauryl Sulfate Tryptose (LST) broth Selective (presumptive) Coliforms Sodium lauryl sulfate (inhibits Gram⁺) Lactose fermentation β†’ acid + gas (in Durham tube) Acid (color change) + gas bubble in inverted Durham No color change, no gas
Brilliant Green Lactose Bile broth Selective (confirmed) Coliforms Oxbile + brilliant green Gas in Durham tube (lactose fermentation) Gas formation within 48 h at 35 Β°C No gas
Levine's EMB (completed) Selective + Differential Gram-neg enteric bacilli Methylene blue (inhibits Gram⁺) Eosin pH-responsive (acid β†’ black) E. coli: blue-black with green metallic sheen Non-fermenters: colorless, translucent
Mineral Salt Medium (MSM) Selective Pollutant-degrading microbes Pollutant is the only carbon source (e.g., crude oil) Growth = ability to use pollutant as C source Growth on plate No growth β†’ cannot use pollutant
Lactose Broth + phenol red (MTF presumptive) Presumptive Coliforms — Lactose → acid (red→yellow) + gas (Durham) Yellow + gas bubble at 24 h, 35 °C = positive Red, no gas
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6. Visual Specimen Recognition (The Lab Photo Bank)

These are the 19 specimens that appeared in your sample-photos PDF β€” and per the doctor: "the images used in the exam will be the same ones you saw during lab sessions." Memorize the visual cue β†’ name β†’ mechanism for each.

πŸ“· Phosphate Solubilization Plate (3 photos)

What you see: A Petri dish with a small bacterial colony in the center; around it is a clear / lighter halo (the solubilization zone). A ruler is positioned across the plate.

Identify as: Phosphate Solubilization assay (Pikovskaya / NBRIP-type medium with insoluble Ca-phosphate).

What to do: Measure Dcolony and Dzone with the ruler. Report SE% using the formula.

SE% = [(D_zone βˆ’ D_colony) / D_colony] Γ— 100

πŸ“· Mannitol Salt Agar (split plate)

What you see: Petri dish split visually β€” one side YELLOW with growth (Staphylococcus aureus), the other side RED with NO growth (Serratia marcescens).

Identify as: MSA β€” selective medium.

Reasoning: 7.5% NaCl inhibits non-staph (Serratia); S. aureus ferments mannitol β†’ acid β†’ phenol red turns yellow.

πŸ“· Baird-Parker Agar (Specimen A)

What you see: Two plate views β€” small black colonies surrounded by clear (lecithinase) halos.

Identify as: Baird-Parker for S. aureus.

Mechanism: Tellurite reduction β†’ black; lecithinase β†’ clear halo; coagulase β†’ opaque inner ring (if present).

πŸ“· Fluorescence-Labeled WGA Method

What you see: Two microscope fields β€” green-glowing rod cells on the LEFT (Gram-positive); dark-grey, non-fluorescent rods on the RIGHT (Gram-negative).

Identify as: Wheat-germ agglutinin (WGA) fluorescence = alternative to Gram stain.

Mechanism: WGA binds N-acetylglucosamine in the exposed peptidoglycan of Gram-positive cells. In Gram-negative cells, peptidoglycan is hidden under the outer membrane β†’ no labeling.

🚨 Memory aid: Glowing = positive (Gram+). Dark = negative (Gramβˆ’).

πŸ“· Potassium Hydroxide (KOH) String Test

What you see: 4-panel image showing a loop being lifted off a slide. Some panels show a thick, viscous string following the loop; others show no string.

Identify as: KOH 3% String Test β€” Gram alternative.

Read it: Stringy & viscous (within 60 s) = Gram-negative (DNA released by alkali lysis). No string = Gram-positive.

🚨 The classic flip-trap. Positive test (string) β‰  Gram positive! Stringy = Gram-NEGATIVE.

πŸ“· API 20E Strip

What you see: Three strips of microtubes β€” Positive control (multiple distinct colors), Negative control (mostly uniform/yellow), and Test sample (mixed pattern). A 7-digit code interpretation table is provided.

Identify as: API 20E system β€” phenotypic identification of Enterobacteriaceae.

To answer:

  1. Compare each tube to the reference chart β†’ mark + or βˆ’ (and read the oxidase test result for the 21st position).
  2. Group into 7 triads of 3 tubes each.
  3. Within each triad, sum: tube 1 (+) = 1, tube 2 (+) = 2, tube 3 (+) = 4.
  4. Concatenate the 7 group sums β†’ 7-digit profile code.
  5. Look up code in API database for species ID.

Worked example from sheet: Triad results 7-0-3-1-6-4-5 β†’ code 7031645.

βš™ Why 1-2-4? Powers of 2 ensure unique sums for any subset of positives (binary encoding). 1-2-3 would create ambiguity (1+2 = 3 same as tube 3 alone).

πŸ“· XLD Agar

What you see: Red plate covered in many tiny black-centered colonies.

Identify as: XLD agar.

Organism: Salmonella sp.

Mechanism (3-stage): xylose ferment β†’ acid (initial yellow) β†’ lysine decarboxylated to alkaline (back to red) β†’ thiosulfate β†’ Hβ‚‚S + ferric ammonium citrate β†’ black FeS centers.

Compare: Shigella = plain red, no black; E. coli = yellow to yellow-red.

πŸ“· Eosin Methylene Blue (EMB) Agar

What you see: Dark red agar with two streaks. One streak is dark purple/black; the other is green-metallic-sheen.

Identify as: EMB agar.

Read it: Green metallic sheen = E. coli (strong lactose fermenter, very acidic local pH precipitates dye). Purple/blue-black streak = other lactose fermenter (e.g., Enterobacter). Colorless = non-fermenter (Salmonella, Shigella).

πŸ“· Blood Agar

What you see: Reddish blood agar plate with grey/white colonies; around them are clear/colorless zones where the red blood cells have been completely lysed.

Identify as: Blood Agar β€” differential medium.

Pattern: Clear zone = Ξ²-hemolysis (complete lysis) β€” typical of S. aureus and S. pyogenes.

Other patterns: Ξ± = greenish-brown discoloration (partial lysis, e.g. S. pneumoniae); Ξ³ = no change (e.g. Neisseria).

πŸ“· ORSAB Selective Supplement

What you see: Plate stained intense BLUE with small blue colonies (matrix is dark blue).

Identify as: ORSAB (Oxacillin Resistance Screening Agar Base).

Organism: MRSA (methicillin-resistant S. aureus).

Mechanism: Oxacillin (2 mg/L) inhibits MSSA. MRSA grows + ferments mannitol β†’ drops pH β†’ aniline blue indicator β†’ intense blue colonies. NaCl (5.5%) + lithium chloride suppress non-staph; polymyxin B suppresses Proteus.

πŸ“· Salmonella-Shigella Agar (SSA)

What you see: Pale/tan plate with abundant colorless colonies that have black centers.

Identify as: SSA.

Organism: Salmonella.

Mechanism: Bile salts + brilliant green + citrate inhibit Gram⁺ & coliforms. Salmonella doesn't ferment lactose (colorless), but reduces thiosulfate β†’ Hβ‚‚S β†’ black FeS centers via ferric citrate. Shigella = colorless, no black center.

πŸ“· MacConkey Agar (Media for Salmonella)

What you see: Dark red/brown agar with two streaks β€” both appear red/pink here, indicating lactose-positive growth in this image.

Identify as: MacConkey Agar.

Read: Brick-red/pink colonies = lactose fermenters (E. coli, coliforms). Colorless transparent = non-fermenters (Salmonella, Shigella).

Selective agents: Crystal violet + bile salts (inhibit Gram⁺).

πŸ“· Test-tube Rack β€” Multiple Tube Fermentation (MTF)

What you see: Rack of cotton-plugged test tubes containing red, yellow, and orange liquids β€” corresponds to lactose broth + phenol red tubes from MPN setup.

Identify as: MTF / MPN test (presumptive).

Read: Yellow tubes = positive (lactose ferment → acid → red→yellow); also gas in inverted Durham tubes. Red tubes = negative.

Setup: 5-5-5 series (3 sets Γ— 5 tubes inoculated with 10 mL / 1 mL / 0.1 mL of sample). Count yellow tubes per set β†’ look up MPN/100 mL in reference table.

πŸ“· Pour-Plate Series (red plates with colonies)

What you see: Four red Petri dishes with isolated visible colonies dispersed throughout the agar (not just surface) β€” classic look of a pour-plate dilution series.

Identify as: Pour-plate technique on a colored selective medium (looks like MacConkey or m-Endo from coloration).

Use: Count colonies in the 30–300 range β†’ calculate CFU/mL.

πŸ“· Coliscan Easygel Guide

What you see: Reference plate with numbered colonies: β‘  dark blue/purple colonies = E. coli; β‘‘ pink/red colonies = other coliforms; β‘’ teal/green colonies = other organisms.

Identify as: Coliscan Easygel β€” quantitative method that uses two chromogenic substrates simultaneously.

Reasoning: ONPG-type substrate cleaved by Ξ²-galactosidase = pink (all coliforms); MUG-type cleaved by Ξ²-glucuronidase = blue/purple (E. coli only). When BOTH enzymes act, the colors superimpose β†’ dark blue/purple.

πŸ“· m-Endo Media (membrane filter)

What you see: Pink/red agar with abundant tiny dark/red colonies, some showing a metallic sheen.

Identify as: m-Endo agar β€” used for membrane-filter method (quantitative coliform detection).

Read: Red colonies with green metallic sheen = total coliforms (lactose fermentation β†’ aldehyde + basic fuchsin/sodium sulfite reaction). Clear/colorless colonies = non-coliforms.

Selective agents: Sodium lauryl sulfate, sodium deoxycholate (inhibit Gram⁺).

πŸ“· Test-Tube Rack β€” biochemical test series

What you see: Rack of small tubes containing red, pink, orange, and yellow liquids β€” classic biochemical fermentation tubes.

Identify as: Sugar fermentation / pH-indicator tubes (e.g., phenol-red broth) or part of an MPN run.

Read: Yellow = acid (positive fermentation); red = no fermentation; orange = partial.

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7. Calculations Mastery β€” Every Formula, Solved

🚨 Doctor's Warning: "They might give you a value in milliliters but ask for it in liters, or vice versa. Always write the unit for the final result." Convert before plugging in. Always.

7.1 CFU/mL Formula

CFU/mL = (Number of colonies Γ— Dilution Factor) / Volume plated (mL)

Variable definitions:

Solved Example 1 β€” Final-Answers PDF Question 1

For a soil sample, it was initially diluted 1:100. A subsequent 1:10 dilution was prepared. Finally, 0.2 mL of the final dilution was inoculated on a plate; 64 colonies were counted. Calculate CFU/mL.

Step 1 β€” Total Dilution Factor: DF = 100 Γ— 10 = 1000

Step 2 β€” Plug in: CFU/mL = (64 Γ— 1000) / 0.2 = 3.2 Γ— 10⁡ CFU/mL

Solved Example 2 β€” Lab Manual Problem

5 mL of bacterial culture is added to 45 mL of sterile diluent. From this suspension, two serial 1:100 dilutions are made, and 0.1 mL is plated from the last dilution. After incubation, 137 colonies are counted. Calculate CFU/mL.

Step 1 β€” Initial dilution: final volume / sample volume = 50/5 = 10 β‡’ first DF = 10 (or 1:10).

Step 2 β€” Two more 1:100 dilutions: 10 Γ— 100 Γ— 100 = 10⁡

Step 3 β€” Plug in: CFU/mL = (137 Γ— 10⁡) / 0.1 = 1.37 Γ— 10⁸ CFU/mL

🚨 The classic trap: When 5 mL is added to 45 mL of diluent, the total volume is 50 mL (not 45). The dilution = 5/50 = 1:10. Forgetting to add the sample volume gives you 5/45 = 1:9 (wrong) and is the leading "near-miss" multiple-choice option.

Solved Example 3 β€” Lab Manual Quick Example

The plate of the 10⁢ dilution yielded 130 colonies. Find CFU/mL.

Bacteria/mL = 130 Γ— 10⁢ = 1.3 Γ— 10⁸ CFU/mL (volume plated β‰ˆ 1 mL implied).

7.2 Solubilization Efficiency (SE%) β€” Phosphate Solubilization

SE% = [(Diameter of solubilization zone βˆ’ Diameter of colony) / Diameter of colony] Γ— 100

Variable definitions:

Worked Example

A bacterial colony has Dcolony = 0.8 cm and a clear halo with Dzone = 3.2 cm. Calculate SE%.

SE% = [(3.2 βˆ’ 0.8) / 0.8] Γ— 100 = (2.4 / 0.8) Γ— 100 = 300%

πŸ’‘ Tip: Always include cm or mm. Whatever unit you use, both diameters must match. Higher SE% = better phosphate solubilizer.

7.3 Laccase Enzyme Activity

U/mL/min = (Ξ”Abs Γ— 10⁢) / (Ξ΅ Γ— R Γ— T)

Variable definitions:

Worked Example

After a 30-min reaction with 0.1 mL enzyme, Ξ”Abs at 470 nm = 0.404. Calculate laccase activity.

U/mL = (0.404 Γ— 10⁢) / (6,740 Γ— 0.1 Γ— 30)
= 404,000 / 20,220
β‰ˆ 20 U/mL

🚨 Units matter! The doctor said: "Always include the unit U/mL or you'll lose points!"

7.4 BOD Calculation

BOD (mg/L) = DOβ‚€ βˆ’ DOβ‚…

Higher BOD ↔ more pollution (more organic matter for microbes).

Worked Example (instructor's example)

Sample A: BOD = 8 mg/L. Sample B: BOD = 5 mg/L. Which is more polluted?
Sample A β€” more Oβ‚‚ was consumed, meaning more organic matter was decomposed.

7.5 COD Calculation

COD (mg/L) = [8000 Γ— N Γ— (A βˆ’ B)] / S

Worked Example

Blank titration: A = 30 mL FAS. Sample titration: B = 20 mL FAS. N = 0.25. Sample volume S = 50 mL. Calculate COD.

COD = [8000 Γ— 0.25 Γ— (30 βˆ’ 20)] / 50
= [8000 Γ— 0.25 Γ— 10] / 50
= 20,000 / 50
= 400 mg/L Always state the unit!

Trap: writing the formula with 8 instead of 8000 gives 0.4 β€” off by a factor of 1000. The 1000 converts the sample volume from mL to L.

πŸ’‘ Doctor's note: COD > BOD almost always, because COD oxidizes everything (organic + inorganic), while BOD only counts what microbes can degrade in 5 days. The ratio BOD/COD tells you how biodegradable the wastewater is.

7.6 API 20E 7-digit Profile (worked from sample sheet)

From the sample sheet provided in the photo:

TriadIIIIIIIVVVIVII
Tubes1,2,34,5,67,8,910,11,1213,14,1516,17,1819,20,oxidase
Reaction+,+,+βˆ’,βˆ’,βˆ’+,+,βˆ’+,βˆ’,βˆ’βˆ’,+,+βˆ’,βˆ’,++,βˆ’,+
Points1+2+40+0+01+2+01+0+00+2+40+0+41+0+4
Sum7031645

7-digit code: 7031645 β†’ look up in API database for species ID.

7.7 Common Calculation Mistakes

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8. Predicted Exam Questions + Model Answers

8.1 Confirmed Question Templates (from sample sheet)

Q1 β€” Dilution / CFU Problem (template)
Setup: Soil/water diluted in stages β†’ final dilution plated β†’ colonies counted β†’ calculate CFU/mL or CFU/g.
Strategy: (1) Compute total DF by multiplying each step. (2) For first dilution involving "x mL into y mL diluent," remember total = x + y. (3) Plug into CFU = (colonies Γ— DF) / V_plated. (4) Always state units.
Q2 β€” Define PGPR and mention the mechanisms used by PGPR which directly and indirectly promote plant growth.
Definition: PGPR = Plant growth promoting rhizobacteria β€” a heterogeneous group found in the rhizosphere, at root surfaces and in association with roots.

Direct growth promotion β€” PGPR provides plant growth-promoting substances or facilitates uptake of nutrients:
  • (i) Production / change in concentration of plant growth regulators: indole-acetic acid, gibberellic acid, cytokinins, ethylene.
  • (ii) Symbiotic Nβ‚‚ fixation.
  • (iii) Solubilization of mineral phosphates and other nutrients.
Indirect growth promotion β€” PGPR lessens or prevents deleterious effects of phytopathogens:
  • (i) Antagonism by production of siderophores, hydrocyanic acid.
  • (ii) Acting as biocontrol agents through antibiotic production, triggering induced local or systemic resistance.
Q3 β€” Design an experiment to isolate pathogenic staphylococci from a water sample.
  1. Prepare Mannitol Salt Agar (MSA) containing mannitol, phenol red indicator (red→yellow at acid pH), and 7.5% NaCl (high salt inhibits non-staph).
  2. Pipette 1 mL of a known dilution into the bottom of a sterile Petri dish (before adding media).
  3. Within 20 min add 12–15 mL warm (46–48 Β°C) fluid medium.
  4. Cover and swirl gently in a figure-8 to disperse the sample.
  5. Cool, solidify.
  6. Invert the Petri dish (medium-side down) and incubate.
  7. Read after incubation.
Result: Pathogenic S. aureus produces small colonies surrounded by yellow zones.
Reason: S. aureus ferments mannitol β†’ acid β†’ indicator changes red to yellow.
Q4 β€” Identify the provided specimen (MacConkey Agar) and write short notes.
MacConkey's Agar
  • Selective for: Gram-negative enteric bacteria.
  • Differential: differentiates lactose fermenters from lactose non-fermenters.
  • Inhibitor: Crystal violet + bile salts inhibit Gram-positives.
  • Use: Detection / isolation of dysentery, typhoid, and paratyphoid; differentiating Salmonella typhosa from coliforms; supports Salmonella & Shigella.
  • Brick-red colonies with bile precipitate zone = lactose-fermenting coliforms.
  • Colorless / transparent colonies = non-fermenters (dysentery, typhoid, paratyphoid bacilli).
  • Reason for color: Acid from lactose fermentation acts on bile salts; neutral red is absorbed by the precipitated salts. Transparent ones don't ferment lactose β€” alkaline reaction.

8.2 Likely Additional Questions (built from the doctor's emphases)

Q5 β€” Why are E. coli used as the indicator of fecal contamination instead of testing for pathogens directly?
  • Pathogens are usually present in low numbers and are difficult/expensive/unsafe to detect directly.
  • E. coli lives in the intestines of warm-blooded mammals β†’ its presence in water = fecal contamination = high probability of pathogens.
  • Indicator criteria met: present in greater numbers, non-pathogenic, easy to test, reflects environmental change, useful for all water types.
  • Specifically, E. coli uniquely possesses Ξ²-glucuronidase, distinguishing it from other coliforms.
Q6 β€” Identify the medium and specimen: red plate, abundant tiny black-centered colonies on red background. Explain the mechanism.
Medium: XLD agar. Organism: Salmonella.
Mechanism (3-stage): (1) Salmonella ferments xylose initially β†’ yellow. (2) Xylose exhausted β†’ decarboxylates lysine β†’ alkaline reaction β†’ indicator (phenol red) reverts to red. (3) Reduces thiosulfate β†’ Hβ‚‚S; with ferric ammonium citrate β†’ black FeS precipitate at colony center. Shigella doesn't ferment xylose or produce Hβ‚‚S β€” appears as plain red colonies.
Q7 β€” Define BOD. Describe the procedure and interpret a result.
BOD = amount of oxygen required by microorganisms (typically aerobic) to break down organic materials in a water sample.

Procedure (BODβ‚…):
  1. Take two parallel samples.
  2. Measure DO of sample 1 immediately (DOβ‚€).
  3. Incubate sample 2 in the dark at 20 Β°C for 5 days.
  4. Measure DO at day 5 (DOβ‚…).
  5. BOD = DOβ‚€ βˆ’ DOβ‚… (mg/L).
Interpretation: BOD = 8 mg/L means more polluted than BOD = 5 mg/L (more organics β†’ more Oβ‚‚ consumption).
Why dark? Prevents algal photosynthesis from artificially adding Oβ‚‚.
Q8 β€” Define COD. Why do we add HgSOβ‚„ and Agβ‚‚SOβ‚„? Give the formula.
COD = amount of oxygen required to chemically oxidize all organic + inorganic matter in water.
HgSOβ‚„ β€” binds Cl⁻ (the main interferent) to prevent its oxidation falsely raising COD.
Agβ‚‚SOβ‚„ β€” catalyst that promotes oxidation of straight-chain alcohols and certain organics.
Formula: COD (mg/L) = [8000 Γ— N Γ— (A βˆ’ B)] / S, where A = blank titration volume, B = sample titration volume, N = FAS normality, S = sample volume.
Q9 β€” Identify the test (image showing a viscous string from a loop). State what it tells you and the underlying mechanism.
Test: KOH 3% String Test β€” a Gram-stain alternative.
Result: A loopful of cells from a colony mixed with a drop of 3% KOH on a slide β†’ if the mixture becomes viscous & "strings" within 60 s = positive = Gram-NEGATIVE.
Mechanism: The dilute alkali lyses Gram-negative cells (thin peptidoglycan + lipid outer membrane), releasing chromosomal DNA. The released DNA makes the suspension viscous.
Gram-positive cells resist lysis β†’ no string = negative test.
Q10 β€” Explain how the API 20E system works and how the 7-digit code is derived.
API 20E is a phenotypic identification kit for Enterobacteriaceae and other non-fastidious Gram-negative rods. The strip has 20 microtubes containing dehydrated substrates. A bacterial suspension rehydrates the substrates β†’ some tubes give color reactions immediately; others need post-incubation reagents and/or mineral oil for anaerobiosis.

Pre-step: oxidase test must be NEGATIVE (Enterobacteriaceae are oxidase-negative).

Scoring (1, 2, 4 logic): The 21 tests (20 + oxidase) are grouped into 7 triads. Inside each triad the tubes are weighted 1, 2, and 4. Sum the positives in each triad β†’ 7 numbers concatenated β†’ 7-digit profile code β†’ look up in API database.

Why 1, 2, 4 (not 1, 2, 3)? To guarantee unique sums for every subset of positive results. With 1+2+3, the sum 3 could mean "tube 3 only" or "tubes 1 + 2" β€” ambiguous. With 1+2+4 every combination is unique (binary encoding).
Q11 β€” Explain the Coliscan test. What do blue, pink, and other colors mean?
Principle: Coliscan uses two chromogenic substrates simultaneously, each cleaved by a coliform-specific enzyme: ONPG by Ξ²-galactosidase (all coliforms), and a glucuronide substrate by Ξ²-glucuronidase (E. coli only).
Color interpretation:
  • Dark blue / purple = E. coli (both enzymes active).
  • Pink / red = other coliforms (Ξ²-galactosidase only).
  • Teal / green = other organisms.
Total coliform count = blue/purple + pink/red.
Q12 β€” A solubilization plate shows Dcolony = 1 cm and Dzone = 4 cm. What is the SE% and what does it mean biologically?
SE% = [(4 βˆ’ 1) / 1] Γ— 100 = 300%.
Biologically, the bacterium is releasing organic acids that solubilize insoluble Ca-phosphate (or chelate Fe³⁺/Al³⁺ in the medium), freeing soluble orthophosphate. The bigger the halo relative to the colony, the more efficient the strain is at making P bioavailable to plants.
Q13 β€” Identify the medium (intense blue colonies / blue background). What organism does it select for and how?
Medium: ORSAB. Organism: MRSA.
Oxacillin (2 mg/L) inhibits methicillin-sensitive S. aureus; high salt + lithium chloride suppress non-staph; polymyxin B suppresses Proteus. MRSA grows + ferments mannitol β†’ acid β†’ aniline blue indicator β†’ intense blue colonies.
Q14 β€” Why does Baird-Parker agar show a clear halo around S. aureus colonies?
Egg yolk in the medium contains lecithin. S. aureus produces lecithinase (a phospholipase) that hydrolyzes lecithin into diglycerides β†’ the precipitated lecithin is dissolved β†’ clear "halo" around the black colony. The black color comes from tellurite reduction. An additional opaque halo indicates coagulase activity (fibrinogen β†’ fibrin).
Q15 β€” A Gram-stain photo shows pink/red rod-shaped cells. State Gram reaction, shape, and theory of staining.
Gram reaction: Negative (pink/red, counterstained by safranin).
Shape: Bacilli (rods).
Theory: Gram-positive cells have thick peptidoglycan (50–90% of envelope) that traps the crystal violet–iodine complex through alcohol decolorization β†’ stay purple. Gram-negative cells have thin peptidoglycan + outer membrane that is dissolved by alcohol β†’ CV–I washes out β†’ safranin counterstain β†’ pink/red.
Q16 β€” What is the laccase enzyme assay? Write the formula and one application.
Laccase activity is assayed using guaiacol as substrate. Reaction mix: 3.9 mL acetate buffer (pH 5), 1 mL guaiacol (2 mM), 0.1 mL diluted enzyme, incubated at 35 Β°C for 30 min, absorbance read at 470 nm. Blank = guaiacol replaced by buffer.
Formula: U/mL/min = (Ξ”Abs Γ— 10⁢) / (Ξ΅ Γ— R Γ— T), where Ξ΅ = 6,740 M⁻¹ cm⁻¹.
Applications: degradation of PAHs, dyes, phenolic compounds in wine, biopulping/bioleaching of paper, biosensors.
Q17 β€” In an MTF / MPN test, which set of tubes is positive and how do you read it?
Three sets of 5 tubes are inoculated with 10 mL, 1 mL, and 0.1 mL of sample respectively. After 24 h at 35 Β°C in lactose broth + phenol red:
Positive: tube turns from red (pH 6.8) to yellow (pH 5.2) due to acid from lactose fermentation, AND gas trapped in the inverted Durham tube.
Count yellow/gas-positive tubes per set β†’ look up the combination in an MPN table β†’ result reported as MPN/100 mL. The MPN is a statistical estimate, not an exact count.
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9. Exam Traps & Common Mistakes

TrapThe wrong answerThe correct answer
Congo Red on YMA"Red colonies = Rhizobium"Rhizobium = white/colorless. Red = contaminant.
KOH String Test"Positive (string) = Gram positive"String = Gram negative (DNA leak from lysed cell wall).
Initial dilution math5 mL into 45 mL = 1:95 mL into 45 mL = 1:10 (5/50, not 5/45).
Volume in CFU formulaUsing broth volumeUse the volume plated (e.g., 0.1 or 0.2 mL).
SE% denominatorDividing by DzoneDivide by Dcolony.
Selective vs differentialCalling MSA "selective only"MSA is both selective (NaCl) and differential (mannitol + phenol red).
BOD vs CODSaying COD < BODCOD β‰₯ BOD (chemical oxidant attacks more substances than microbes can in 5 days).
EMB green sheenIdentifying it as SalmonellaGreen metallic sheen = E. coli. Salmonella = colorless/translucent.
API oxidaseSkipping the oxidase testIt's the 21st test. Enterobacteriaceae are oxidase negative β€” must check first.
Why 4 in API"Random number"Powers of 2 give unique sums (binary encoding).
30–300 ruleCounting any plateBelow 30 = unreliable; above 300 = inaccurate.
BOD incubation"In light at 25 Β°C"5 days, dark, 20 Β°C. Dark prevents photosynthesis adding Oβ‚‚.
Mannitol fermentation color"Yellow β†’ Red"Phenol red goes red β†’ yellow in acid (drops on pH).
Forgetting the unitGiving "1.37 Γ— 10⁸"1.37 Γ— 10⁸ CFU/mL. Doctor explicitly said: write the unit.
Rhizobium location"Lives on the root surface"It's an endophyte β€” lives inside the nodule.
OD vs CFU"Both count viable cells"OD counts everything (alive, dead, debris). CFU counts only viable cells.
Pathogenic identification on MacConkey"Pink colony = Salmonella"Pink/brick-red = lactose fermenter (E. coli). Salmonella = colorless.
Cooking kills risk"Cooked food is always safe from S. aureus"S. aureus enterotoxins are heat-stable β€” toxin remains even after cells are killed.
HemolysisConfusing Ξ± (clear zone) and Ξ² (greenish)Ξ² = clear (full lysis); Ξ± = green/brown (partial); Ξ³ = no change.

9.2 Confusable Concept Pairs

Selective vs Differential
Selective = some can't grow at all (inhibited). Differential = all grow but look different. Most exam media are both.
Total vs Fecal coliforms
Total: 35 Β°C / environment-wide. Fecal: 44.5 Β°C / gut origin. E. coli is the gold-standard fecal indicator (Ξ²-glucuronidase positive).
Rhizosphere vs Rhizoplane
Rhizosphere = soil volume around the root. Rhizoplane = the root surface itself.
Ξ± vs Ξ² vs Ξ³ hemolysis
Ξ± = partial (greenish-brown methemoglobin). Ξ² = complete (clear zone). Ξ³ = none.
P-A vs MTF vs MF
P-A = qualitative (yes/no). MF = quantitative (count colonies on m-Endo). MTF/MPN = statistical estimate via tube series.
ONPG vs MUG
ONPG β†’ Ξ²-galactosidase β†’ yellow β†’ ALL coliforms. MUG β†’ Ξ²-glucuronidase β†’ fluorescence β†’ E. coli only.
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10. Rapid Revision Sheet β€” One-Page Cheat Sheet

FORMULAS β€” memorize cold:
  • CFU/mL = (colonies Γ— DF) / Vplated  |  DF is reciprocal of dilution
  • SE% = [(Dzone βˆ’ Dcolony) / Dcolony] Γ— 100
  • Laccase U/mL = (Ξ”Abs Γ— 10⁢) / (Ξ΅ Γ— R Γ— T)  |  Ξ΅ = 6,740 M⁻¹ cm⁻¹
  • BOD = DOβ‚€ βˆ’ DOβ‚… (mg/L)  |  5 d, 20 Β°C, dark
  • COD = [8000 Γ— N Γ— (A βˆ’ B)] / S (mg/L)
MEDIA QUICK MAP β€” color β†’ organism
  • MSA β€” S. aureus = yellow halo; others = pink/red on red
  • Baird-Parker β€” S. aureus = black colonies + clear halo
  • Blood agar β€” Ξ² = clear (S. aureus); Ξ± = green; Ξ³ = none
  • ORSAB β€” MRSA = intense blue
  • MacConkey β€” E. coli = brick-red/pink; Salmonella, Shigella = colorless
  • EMB β€” E. coli = green metallic sheen; Salmonella = colorless
  • XLD β€” Salmonella = red colonies + black centers; Shigella = plain red
  • SS agar β€” Salmonella = colorless + black centers; Shigella = colorless
  • m-Endo β€” coliforms = red colonies + green metallic sheen
  • YMA + Congo Red β€” Rhizobium = white/colorless; contaminants = red
  • Coliscan β€” E. coli = blue/purple; coliforms = pink
TOP 12 EXAM-DAY ESSENTIALS:
  1. Always state the unit on every numerical answer.
  2. For dilutions: total volume = sample + diluent (5 + 45 = 50, not 45).
  3. 30–300 colony rule.
  4. SE% denominator is Dcolony.
  5. Potable water = 0 coliforms / 100 mL.
  6. BOD: 5 days, dark, 20 Β°C.
  7. COD > BOD (always).
  8. Rhizobium on Congo Red = WHITE (not red).
  9. KOH-positive string = Gram-NEGATIVE.
  10. API: 1-2-4 β†’ unique 7-digit code; oxidase first, must be negative.
  11. Mannitol fermentation: phenol red goes red β†’ yellow.
  12. Petri dishes incubate inverted (no condensation on agar).
KEY ENZYMES & SUBSTRATES
  • Ξ²-galactosidase + ONPG β†’ yellow β†’ all coliforms
  • Ξ²-glucuronidase + MUG β†’ fluorescence β†’ E. coli
  • Laccase + guaiacol β†’ tetraguaiacol (brown, 470 nm) β†’ white-rot fungi
  • Lecithinase + lecithin β†’ diglycerides β†’ halo on Baird-Parker
  • Coagulase + fibrinogen β†’ fibrin β†’ opaque halo / clot
  • Aminopeptidase + L-alanine-4-nitroanilide β†’ 4-nitroaniline (yellow) β†’ Gram-NEG
  • Nitrogenase: Nβ‚‚ β†’ NH₃ (only in prokaryotes; protected by leghaemoglobin in nodules)
5-second specimen recognition (visual cues):
  • 🟑 Yellow halo on red plate β†’ MSA + S. aureus
  • ⚫ Black colonies + clear halo β†’ Baird-Parker + S. aureus
  • 🌈 Green metallic sheen β†’ EMB or m-Endo + E. coli
  • πŸ”΄βš« Red colonies w/ black centers β†’ XLD + Salmonella
  • 🟦 Intense blue colonies on blue β†’ ORSAB + MRSA
  • βšͺ White colonies on red plate β†’ YMA + Congo Red + Rhizobium
  • 🟣🩷 Mixed dark blue + pink colonies β†’ Coliscan (blue = E. coli, pink = coliforms)
  • πŸ’‘ Green-glowing rods + dark rods side by side β†’ Fluorescent WGA (Gram⁺ vs Gram⁻)
  • πŸ”΄βšͺ Halo around colony with ruler β†’ Phosphate Solubilization β†’ measure Dcolony & Dzone β†’ SE%
  • πŸ§ͺ Tubes + Durham + yellow color β†’ MTF / MPN positive
  • πŸ§ͺ 20-tube colored strip β†’ API 20E β†’ 7-digit code

Good luck. Aim for the bullet points, name the inhibitors, write the units.

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