1.biochemical test- composition Indication 2.difference bw stroke and Kirby baur disc diffusion method 3.difference bw typhi and paratyphi colonies Give me answers for 3 marks microbiology
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Salmonella typhi vs paratyphi colony morphology differences microbiology
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1. Biochemical Tests - Composition & Indications
Biochemical tests detect specific metabolic activities of bacteria, used to differentiate and identify bacterial species.
Common Biochemical Tests
| Test | Composition / Reagent | Indication |
|---|---|---|
| Catalase test | H₂O₂ (3% hydrogen peroxide) | Differentiates Staphylococcus (catalase +ve) from Streptococcus (catalase -ve) |
| Coagulase test | Rabbit/human plasma (citrated) | Identifies S. aureus (coagulase +ve) from coagulase-negative staphylococci (e.g., S. epidermidis) |
| Oxidase test | Tetramethyl-p-phenylenediamine (TMPD) reagent | Detects cytochrome C oxidase; differentiates Pseudomonas (oxidase +ve) from E. coli / Enterobacteriaceae (oxidase -ve) |
| Indole test | Kovac's reagent (p-dimethylaminobenzaldehyde + HCl + amyl alcohol) | Detects tryptophanase enzyme breaking tryptophan to indole; E. coli is indole +ve; Salmonella, Klebsiella are -ve |
| Urease test | Christensen's urea agar (urea + phenol red indicator) | Detects urease production; Proteus, H. pylori are urease +ve |
| Citrate utilization | Simmons citrate agar (sodium citrate as sole carbon source + bromothymol blue indicator) | Klebsiella (citrate +ve) vs E. coli (citrate -ve); color turns blue when positive |
| H₂S production | TSI agar or lead acetate paper | Detects sulfur reduction; Salmonella is H₂S +ve, Shigella is -ve |
| ONPG test | O-Nitrophenyl-β-D-galactopyranoside substrate | Detects β-galactosidase in late/slow lactose fermenters (e.g., Salmonella arizonae) |
| Sugar fermentation (IMViC) | Peptone water + Durham tube + specific sugar + indicator | Identifies Enterobacteriaceae by their fermentation pattern (acid/gas production) |
Key principle: These tests detect enzymes or metabolic products and produce a color change or visible reaction that allows taxon-level identification of bacteria.
- Jawetz, Melnick & Adelberg's Medical Microbiology, 28th Ed.
2. Difference Between Stroke Method and Kirby-Bauer Disc Diffusion
Both are antibiotic susceptibility testing (AST) methods, but they differ significantly:
| Feature | Stroke Method (Lawn/Carpet inoculation) | Kirby-Bauer Disc Diffusion |
|---|---|---|
| Principle | Bacteria inoculated by streaking (flooding/spreading) a lawn on agar | Standardized lawn of bacteria seeded; antibiotic-impregnated paper discs placed on surface |
| Inoculum | Bacterial suspension spread uniformly across entire plate | Standardized inoculum (0.5 McFarland turbidity standard) seeded on Mueller-Hinton agar (MHA) |
| Medium | Any nutrient/non-selective agar can be used | Mueller-Hinton Agar specifically recommended (standardized thickness ~4 mm) |
| Antibiotic application | Antibiotic discs placed on the seeded plate | Commercially prepared paper discs with fixed, specified amounts (not concentrations) of antibiotic |
| Principle of result | Zone of inhibition around disc indicates susceptibility | Antibiotic diffuses radially from disc; zone diameter is inversely correlated with the MIC |
| Interpretation | Qualitative (zone present/absent) | Quantitative-qualitative: zone diameter (mm) interpreted as Susceptible / Intermediate / Resistant using CLSI/EUCAST breakpoint tables |
| Standardization | Less standardized | Highly standardized (CLSI guidelines); breakpoint tables differ per drug-organism combination |
| Bacteria tested | General method | Mainly for rapidly growing aerobic and facultatively anaerobic bacteria (Enterobacteriaceae, Pseudomonas, staphylococci) |
| Cost | Low | Low; simple, flexible, inexpensive |
| Limitation | Less reproducible without standardization | Manual/labor intensive; not suitable for slow-growing or fastidious organisms without special media |
Key point: In the Kirby-Bauer method, the zone diameter is compared to established breakpoints from a linear regression relating zone size to MIC values - this is what makes it a standardized, clinically reliable method.
- Sherris & Ryan's Medical Microbiology, 8th Ed.; Henry's Clinical Diagnosis and Management by Laboratory Methods
3. Difference Between S. typhi and S. paratyphi Colonies
| Feature | Salmonella typhi | Salmonella paratyphi A |
|---|---|---|
| Nutrient agar | Large, 2-3 mm, smooth, moist, slightly convex, off-white/translucent colonies | Similar size but slightly more opaque; may appear whiter |
| MacConkey agar | Colorless/pale (non-lactose fermenter); no H₂S black center | Colorless (non-lactose fermenter); typically no black center |
| XLD / SS agar | Colorless/pale colonies with prominent black centers (H₂S +ve) | Pink to red colonies; no black centers (H₂S -ve or trace only) |
| H₂S production | Positive - black colonies on H₂S-detecting media | Negative (Paratyphi A) - no H₂S production |
| Gas production (TSI) | No gas (non-gas producer) | Gas produced (acid + gas in butt) |
| Ornithine decarboxylase | Positive | Negative |
| Vi antigen | Present (capsular polysaccharide - enhances virulence) | Absent |
| LIA (lysine iron agar) butt | Alkaline (K) | Acid (A) - distinctive feature |
| Widal test antigens | O (somatic) and H (flagellar) - classic Widal reaction | "d" H antigen for Paratyphi A |
| Disease caused | Typhoid fever (more severe, higher mortality) | Paratyphoid fever (milder clinical course) |
Key distinguishing features:
- S. typhi produces H₂S (black colonies on XLD/SS agar); S. paratyphi A does not
- S. typhi produces no gas in TSI; S. paratyphi A produces gas
- S. typhi is ornithine +ve; S. paratyphi A is ornithine -ve
- Only S. typhi carries the Vi (virulence) capsular antigen
- Jawetz, Melnick & Adelberg's Medical Microbiology; APHL Salmonella Identification Guidelines
I asked for 3 marks only
Got it! Here are concise 3-mark answers:
1. Biochemical Tests - Composition & Indication
| Test | Composition | Indication |
|---|---|---|
| Catalase | 3% H₂O₂ | Staph (+ve) vs Strep (-ve) |
| Coagulase | Citrated plasma | S. aureus (+ve) vs S. epidermidis (-ve) |
| Oxidase | TMPD reagent | Pseudomonas (+ve) vs E. coli (-ve) |
| Indole | Kovac's reagent | E. coli (+ve) vs Salmonella (-ve) |
| Urease | Christensen's urea agar | Proteus, H. pylori (+ve) |
| Citrate | Simmons citrate agar | Klebsiella (+ve) vs E. coli (-ve) |
2. Stroke Method vs Kirby-Bauer Disc Diffusion
| Feature | Stroke Method | Kirby-Bauer |
|---|---|---|
| Inoculum | Streaked (less standardized) | 0.5 McFarland standard (standardized) |
| Medium | Any nutrient agar | Mueller-Hinton Agar only |
| Interpretation | Qualitative (zone present/absent) | S / I / R using CLSI breakpoint tables |
| Standardization | Less standardized | Highly standardized |
| Result | Zone diameter not measured precisely | Zone diameter (mm) inversely correlates with MIC |
3. S. typhi vs S. paratyphi Colonies
| Feature | S. typhi | S. paratyphi A |
|---|---|---|
| H₂S production | +ve (black center on XLD/SS agar) | -ve (no black center) |
| Gas in TSI | No gas | Gas produced |
| Vi antigen | Present | Absent |
| Ornithine decarboxylase | +ve | -ve |
| XLD appearance | Pale/colorless with black center | Pink-red, no black center |
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