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 7601 to 7650
- Which test identifies Group D streptococci?
A) Bile esculin test ✅
B) Catalase test
C) Coagulase test
D) Oxidase test - The X and V factor requirement test identifies:
A) Neisseria meningitidis
B) Haemophilus influenzae ✅
C) Pseudomonas aeruginosa
D) Streptococcus pneumoniae - A positive PYR test is seen in:
A) Staphylococcus aureus
B) Escherichia coli
C) Klebsiella pneumoniae
D) Enterococcus faecalis and Streptococcus pyogenes ✅ - Which bacteria produce a green pigment on blood agar?
A) Pseudomonas aeruginosa ✅
B) Streptococcus pneumoniae
C) Staphylococcus epidermidis
D) Enterococcus faecalis - The Elek test is used to detect:
A) Toxin production by Corynebacterium diphtheriae ✅
B) Hemolysis in Streptococcus spp.
C) Urease in Proteus spp.
D) Catalase in Staphylococci - A positive Novobiocin test indicates susceptibility in:
A) Staphylococcus epidermidis (sensitive)
B) Streptococcus pneumoniae
C) Enterococcus faecalis
D) Staphylococcus saprophyticus (resistant) ✅
- Christensen’s urea agar is used to test for:
A) Citrate utilization
B) Indole production
C) Urease ✅
D) Gelatinase - The “halo” around colonies on blood agar indicates:
A) Beta-hemolysis ✅
B) Alpha-hemolysis
C) Gamma-hemolysis
D) Lecithinase activity - The oxidase test reagent contains:
A) Kovac’s reagent
B) Methyl red
C) Phenol red
D) Tetramethyl-p-phenylenediamine ✅ - Which bacteria are non-lactose fermenters on MacConkey agar?
A) Escherichia coli
B) Salmonella spp. ✅
C) Klebsiella pneumoniae
D) Enterobacter aerogenes - The “hot-cold” lysis test is used for:
A) Bacillus anthracis ✅
B) Clostridium perfringens
C) Listeria monocytogenes
D) Mycobacterium tuberculosis - The Quellung reaction is diagnostic for:
A) Haemophilus influenzae
B) Neisseria meningitidis
C) Cryptococcus neoformans
D) Streptococcus pneumoniae ✅ - Which bacteria are catalase-negative?
A) Streptococcus spp. ✅
B) Staphylococcus spp.
C) Bacillus spp.
D) Enterococcus spp. - A positive trehalose fermentation test identifies:
A) Escherichia coli
B) Salmonella Typhi
C) Vibrio cholerae ✅
D) Shigella sonnei - The “swarming” phenomenon is seen with:
A) Pseudomonas aeruginosa
B) Bacillus subtilis
C) Clostridium tetani
D) Proteus mirabilis ✅ - The Limulus amoebocyte lysate (LAL) test detects:
A) Endotoxin (LPS) ✅
B) Exotoxin
C) Catalase
D) Urease
- A positive nitrate reduction test turns red due to:
A) Nitrite reacting with reagents ✅
B) Production of nitrogen gas
C) Zinc dust reducing nitrate
D) Indole production - The “Elek plate” is used to test for:
A) Cholera toxin
B) Diphtheria toxin ✅
C) Tetanus toxin
D) Staphylococcal enterotoxin - Which test differentiates Shigella from Escherichia coli?
A) Motility
B) Lactose fermentation ✅
C) Indole production
D) Oxidase - The “stormy fermentation” reaction is seen with:
A) Clostridium perfringens in litmus milk ✅
B) Streptococcus pyogenes on blood agar
C) Pseudomonas aeruginosa in OF medium
D) Mycobacterium tuberculosis in Löwenstein-Jensen medium - Which bacteria produce a red pigment at 25°C?
A) Pseudomonas aeruginosa
B) Staphylococcus aureus
C) Escherichia coli
D) Serratia marcescens ✅ - The “Albert stain” is used to identify:
A) Metachromatic granules in Corynebacterium diphtheriae ✅
B) Acid-fast bacteria
C) Spores in Bacillus spp.
D) Capsules in Streptococcus pneumoniae - Which test differentiates Neisseria gonorrhoeae from Neisseria meningitidis?
A) Glucose fermentation
B) Oxidase test
C) Maltose fermentation ✅
D) Catalase test - The “sulfur granules” are characteristic of:
A) Mycobacterium tuberculosis
B) Nocardia asteroides
C) Actinomyces israelii ✅
D) Clostridium tetani - A positive lysine decarboxylase test turns the medium:
A) Purple ✅
B) Yellow
C) Red
D) Green - The “Durham tube” in carbohydrate fermentation tests detects:
A) Acid production
B) Indole production
C) Gas production ✅
D) H₂S production
- Vancomycin resistance is a key feature of:
A) Enterococcus faecium (VRE) ✅
B) Staphylococcus aureus
C) Streptococcus pneumoniae
D) Escherichia coli - Latex agglutination tests detect:
A) Antibiotic resistance genes
B) Endotoxins
C) Specific antigens (e.g., S. aureus protein A) ✅
D) Hemolysins - API 20E strips are used to identify:
A) Enterobacteriaceae ✅
B) Streptococci
C) Anaerobic bacteria
D) Mycobacteria - Ziehl-Neelsen stain is used for:
A) Gram-negative bacteria
B) Acid-fast bacteria (e.g., Mycobacterium) ✅
C) Fungal hyphae
D) Bacterial spores - A Kligler’s Iron Agar (KIA) with a yellow slant and yellow butt indicates:
A) Fermentation of glucose and lactose ✅
B) No fermentation
C) H₂S production
D) Non-lactose fermenter - Esculin hydrolysis is negative in:
A) Streptococcus bovis
B) Enterococcus faecalis
C) Group D Streptococci
D) Streptococcus pyogenes ✅ - H₂S production is a key feature of:
A) Salmonella Typhi ✅
B) Escherichia coli
C) Klebsiella pneumoniae
D) Pseudomonas aeruginosa - Serratia marcescens is positive for:
A) Lactose fermentation
B) Gelatin liquefaction ✅
C) Indole production
D) Urease - Phenylalanine deaminase is used to differentiate:
A) Staphylococcus from Streptococcus
B) Proteus spp. from Enterobacteriaceae ✅
C) Gram-positive vs. Gram-negative bacteria
D) Aerobic vs. anaerobic bacteria - Mannitol fermentation on Mannitol Salt Agar (MSA) indicates:
A) Staphylococcus aureus ✅
B) Staphylococcus epidermidis
C) Streptococcus pyogenes
D) Enterococcus faecalis
- Which of the following is NOT included in the IMViC series?
A. Indole
B. Methyl Red
C. Voges-Proskauer
D. Catalase ✅ - The IMViC tests are primarily used to differentiate:
A. Staphylococci
B. Streptococci
C. Enterobacteriaceae ✅
D. Neisseria - Which medium is used for the Indole test?
A. MRVP broth
B. SIM medium ✅
C. Citrate agar
D. Urea broth - The Indole test detects the enzyme:
A. Urease
B. Tryptophanase ✅
C. Decarboxylase
D. Oxidase - Which reagent is used in the Indole test?
A. Barritt’s reagent
B. Kovac’s reagent ✅
C. Ferric chloride
D. Nitrate reagent - A positive Indole test shows:
A. Blue slant
B. Black precipitate
C. Red ring on top of the medium ✅
D. Yellow color - The Methyl Red (MR) test detects:
A. Alcohol production
B. Strong acid production from glucose ✅
C. Alkaline end products
D. Protein hydrolysis - Which reagent is used in the Methyl Red test?
A. Methyl orange
B. Phenol red
C. Methyl red indicator ✅
D. Bromothymol blue - A positive MR test turns the medium:
A. Yellow
B. Red ✅
C. Green
D. Blue - The Voges-Proskauer (VP) test detects:
A. Production of stable acids
B. Mixed acid fermentation
C. Acetoin production ✅
D. Sulfur reduction - Which reagents are used in the VP test?
A. Kovac’s A and B
B. Barritt’s A and B ✅
C. Nessler’s reagent
D. Ehrlich’s reagent - A positive VP test results in:
A. Blue-green color
B. Red or pink color after 15-30 minutes ✅
C. Black precipitate
D. Yellow halo - The Citrate test determines if an organism can:
A. Ferment glucose
B. Use citrate as a sole carbon source ✅
C. Break down proteins
D. Reduce sulfur - Which indicator is used in Simmons citrate agar?
A. Methyl red
B. Phenol red
C. Bromothymol blue ✅
D. Neutral red
