MCQs on Biochemical Tests for Identifying Bacteria in Clinical Laboratories:
Biochemical tests identify microorganisms by analyzing their metabolic reactions to specific compounds, aiding phenotypic classification. These traditional methods, still used for rapid detection, rely on observable changes like turbidity (indicating growth) or pH-induced color shifts. They can differentiate microbial groups or species based on physiological traits, offering cost-effective simplicity. However, drawbacks include lengthy processing times (days), labor-intensive procedures, and potential false positives among similar species. While valuable for basic diagnostics, these limitations highlight the balance between accessibility and efficiency in microbial identification.
Key Themes Covered:
- Catalase & Coagulase Tests: Differentiate Staphylococci and Streptococci.
- IMViC Series: Identify Enterobacteriaceae (e.g., E. coli vs. Klebsiella).
- Urease Test: Detect Proteus and Helicobacter pylori.
- TSI Agar: Assess sugar fermentation and H₂S production.
- Nitrate Reduction: Determine anaerobic respiration pathways.
- Hemolysis Patterns: Alpha, beta, and gamma hemolysis on blood agar.
- Specialized Tests: Optochin, bacitracin, CAMP, and bile solubility for Streptococci identification.
- Enzyme Activity: DNase, lipase, lecithinase, and gelatinase tests.
- Fungal Tests: Germ tube test for Candida albicans.
Questions 7701 to 7750
- Positive bile esculin test turns the medium:
A. Yellow
B. Blue
C. Black ✅
D. Red - The hippurate hydrolysis test helps in identifying:
A. Staphylococcus aureus
B. Streptococcus agalactiae ✅
C. Streptococcus pyogenes
D. Proteus mirabilis - Which indicator is used in bile esculin agar?
A. Bromothymol blue
B. Ferric citrate ✅
C. Methyl red
D. Phenol red - Which bacteria hydrolyze esculin in the presence of bile?
A. Streptococcus pyogenes
B. Enterococcus faecalis ✅
C. Staphylococcus epidermidis
D. E. coli - The ONPG test detects:
A. Lactose fermentation via permease
B. Beta-galactosidase activity ✅
C. Indole production
D. Hydrogen sulfide - ONPG-positive organisms appear:
A. Green
B. Yellow ✅
C. Purple
D. Black - Which test distinguishes Enterobacter from Klebsiella based on motility?
A. Urease
B. TSI
C. SIM ✅
D. Citrate
- The SIM test can detect all of the following EXCEPT:
A. Sulfur reduction
B. Indole production
C. Motility
D. Catalase activity ✅ - Indole production in the SIM test requires which reagent?
A. Methyl red
B. Kovac’s reagent ✅
C. Barritt’s A
D. Ferric chloride - A cherry red ring in the SIM test after adding reagent indicates:
A. Urease production
B. Indole production ✅
C. H₂S production
D. Negative result - Motility in SIM medium is demonstrated by:
A. Gas bubbles
B. Black precipitate
C. Cloudy or diffuse growth away from the stab line ✅
D. No color change - A positive citrate utilization test turns the media:
A. Yellow
B. Green
C. Blue ✅
D. Red - The pH indicator in Simmon’s Citrate Agar is:
A. Phenol red
B. Bromothymol blue ✅
C. Methyl red
D. Crystal violet - Which enzyme is detected in the malonate test?
A. Citrate permease
B. Succinyl-CoA synthetase ✅
C. Indole oxidase
D. Lactase - Malonate positive test changes color to:
A. Blue ✅
B. Pink
C. Yellow
D. Black - Beta-galactosidase activity is measured by which test?
A. Catalase
B. Oxidase
C. ONPG ✅
D. SIM - Which test detects the breakdown of tryptophan to indole?
A. Methyl red
B. Voges-Proskauer
C. SIM ✅
D. Nitrate - A positive Voges-Proskauer (VP) test indicates the production of:
A. Acetic acid
B. Butylene glycol pathway products (acetoin) ✅
C. H₂S
D. Indole - Which reagent is used in the VP test?
A. Methyl red
B. Barritt’s A and B ✅
C. Kovac’s reagent
D. Phenol red - VP test helps differentiate between:
A. E. coli and Enterobacter ✅
B. Proteus and Pseudomonas
C. Staph and Strep
D. Bacillus and Clostridium - The MR-VP tests are part of which IMViC series?
A. Indole, MR, VP, Citrate ✅
B. Indole, Mannitol, Voges-Proskauer, Catalase
C. Methyl red, Urease, Oxidase, Citrate
D. MR, H₂S, Indole, Citrate
- The Snyder agar test is used to assess susceptibility to:
A) Urinary tract infections
B) Tuberculosis
C) Dental caries (Lactobacillus acidophilus) ✅
D) Fungal sepsis - The “reverse CAMP test” is positive for:
A) Streptococcus agalactiae
B) Staphylococcus aureus
C) Listeria monocytogenes
D) Clostridium perfringens (arrowhead hemolysis) ✅ - Thioglycollate medium is used to determine:
A) Carbohydrate fermentation
B) Urease activity
C) Oxygen requirements of bacteria ✅
D) Nitrate reduction - A positive Löwenstein-Jensen culture indicates growth of:
A) Candida albicans
B) Clostridium difficile
C) Streptococcus pyogenes
D) Mycobacterium tuberculosis ✅ - The “Nagler reaction” is inhibited by:
A) Alpha antitoxin ✅
B) Beta-lactamase
C) Catalase
D) Oxidase - The “E-test” is used to determine:
A) Hemolytic patterns
B) Minimum inhibitory concentration (MIC) of antibiotics ✅
C) Motility of bacteria
D) Toxin production - Which bacteria hydrolyze hippurate to glycine and benzoic acid?
A) Streptococcus agalactiae ✅
B) Escherichia coli
C) Pseudomonas aeruginosa
D) Proteus mirabilis - The “asculin hydrolysis test” helps differentiate:
A) Staphylococcus aureus from Staphylococcus epidermidis
B) Gram-positive vs. Gram-negative bacteria
C) Enterococcus spp. (positive) from Streptococcus spp. ✅
D) Aerobic vs. anaerobic bacteria - The “spot indole test” is performed using:
A) Para-dimethylaminocinnamaldehyde reagent ✅
B) Kovac’s reagent
C) Oxidase reagent
D) Methyl red - Which bacteria produce a “mustard-yellow” pigment on nutrient agar?
A) Micrococcus luteus ✅
B) Serratia marcescens
C) Pseudomonas aeruginosa
D) Staphylococcus aureus
- The “bile solubility test” uses sodium deoxycholate to lyse:
A) Staphylococcus aureus
B) Streptococcus pneumoniae ✅
C) Haemophilus influenzae
D) Neisseria meningitidis - The “Schaeffer-Fulton stain” identifies:
A) Acid-fast bacteria
B) Capsules
C) Flagella
D) Bacterial endospores ✅ - Which bacteria are identified using the “growth in thiosulfate citrate bile salts sucrose (TCBS) agar”?
A) Salmonella Typhi
B) Shigella dysenteriae
C) Vibrio cholerae ✅
D) Escherichia coli - A positive “litmus milk reduction test” turns the medium:
A) White (due to litmus reduction) ✅
B) Pink (acidic)
C) Blue (alkaline)
D) Curdled (clot formation) - The “FTA-ABS test” is used to diagnose:
A) Tuberculosis
B) Lyme disease
C) Syphilis (Treponema pallidum) ✅
D) Leptospirosis - The “Weil-Felix test” detects antibodies against:
A) Streptococcus pyogenes
B) Mycobacterium leprae
C) Hepatitis B virus
D) Rickettsiae (using Proteus antigens) ✅ - Which test differentiates Bacillus anthracis from Bacillus cereus?
A) Susceptibility to penicillin ✅
B) Hemolysis on blood agar
C) Motility
D) Catalase production - The “Paraffin bait technique” is used to culture:
A) Mycobacterium tuberculosis
B) Clostridium botulinum
C) Candida albicans
D) Nocardia spp. ✅ - A positive “Dienes test” indicates:
A) Urease activity
B) Compatibility between Proteus mirabilis strains ✅
C) Nitrate reduction
D) Gelatin liquefaction - The “CAMP test” enhances hemolysis due to synergy between:
A) Streptococcus agalactiae and Staphylococcus aureus ✅
B) Clostridium perfringens and Bacillus cereus
C) Escherichia coli and Klebsiella pneumoniae
D) Pseudomonas aeruginosa and Proteus mirabilis
- A sputum sample from a cystic fibrosis patient grows green-pigmented colonies on blood agar. Which test confirms Pseudomonas aeruginosa?
A) Indole test
B) Oxidase test ✅
C) Coagulase test
D) Catalase test - A patient with rice-water stools tests positive for oxidase and grows on TCBS agar. Which organism is likely?
A) Escherichia coli
B) Salmonella Typhi
C) Shigella sonnei
D) Vibrio cholerae ✅ - A gram-positive coccus isolated from a wound is catalase-positive and coagulase-positive. Which treatment is prioritized?
A) Vancomycin (for MRSA) ✅
B) Penicillin
C) Ceftriaxone
D) Metronidazole - A urease-positive, gram-negative spiral bacterium in gastric biopsy indicates:
A) Helicobacter pylori ✅
B) Escherichia coli
C) Campylobacter jejuni
D) Salmonella enterica - A beta-hemolytic, bacitracin-resistant Streptococcus from a throat culture suggests:
A) Group A Streptococcus (S. pyogenes)
B) Streptococcus pneumoniae
C) Group B Streptococcus (S. agalactiae) ✅
D) Enterococcus faecalis - MALDI-TOF mass spectrometry identifies bacteria based on:
A) Protein profiles ✅
B) DNA sequencing
C) Carbohydrate fermentation
D) Antibiotic resistance - The “ESBL confirmatory test” detects resistance in:
A) Extended-spectrum beta-lactamase-producing Enterobacteriaceae ✅
B) Methicillin-resistant Staphylococcus aureus
C) Vancomycin-resistant Enterococci
D) Carbapenem-resistant Pseudomonas - The “Etest” for H. pylori detects:
A) Urease activity
B) Clarithromycin resistance ✅
C) Catalase production
D) Motility - The “GeneXpert MTB/RIF assay” rapidly diagnoses:
A) Tuberculosis and rifampin resistance ✅
B) Malaria
C) HIV viral load
D) Hepatitis C - The “IMViC” profile for Klebsiella pneumoniae is:
A) + + – –
B) + – + –
C) – + – +
D) – – + + ✅
Possible References Used
