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 7551 to 7600
- Which test differentiates Staphylococcus from Streptococcus?
A) Catalase ✅
B) Oxidase
C) Coagulase
D) Indole - Staphylococcus aureus is positive in which test?
A) Catalase
B) Oxidase
C) Coagulase ✅
D) Urease - Which bacteria is oxidase positive?
A) Escherichia coli
B) Klebsiella pneumoniae
C) Enterobacter aerogenes
D) Pseudomonas aeruginosa ✅ - Which organism is indole positive?
A) Klebsiella pneumoniae
B) Escherichia coli ✅
C) Enterobacter cloacae
D) Proteus mirabilis - A positive methyl red test indicates:
A) Mixed acid fermentation ✅
B) Butanediol fermentation
C) Citrate utilization
D) Urease activity - VP test detects:
A) Mixed acids
B) Acetoin ✅
C) Indole
D) Citrate permease - Simmons citrate agar turns blue when the organism:
A) Utilizes citrate as a carbon source ✅
B) Ferments citrate
C) Produces hydrogen sulfide
D) Reduces nitrate - Which is urease positive?
A) Proteus vulgaris ✅
B) Escherichia coli
C) Shigella sonnei
D) Salmonella Typhi - A TSI slant with a red slant and yellow butt indicates:
A) Lactose and/or sucrose fermentation
B) No fermentation
C) Glucose fermentation only ✅
D) H₂S production - After adding nitrate reagents, a red color indicates:
A) Nitrate reduced to nitrite ✅
B) Nitrate not reduced
C) Nitrate reduced to nitrogen gas
D) Zinc added - Which indicator is commonly used in carbohydrate fermentation tests?
A. Methyl Red
B. Phenol Red ✅
C. Crystal Violet
D. Bromothymol Blue - What does a yellow color in a carbohydrate fermentation tube indicate?
A. No fermentation
B. Alkaline pH
C. Acid production ✅
D. Protein degradation - The Durham tube is used in fermentation tests to detect:
A. pH change
B. Gas production ✅
C. Color change
D. Protein breakdown - A positive fermentation result includes:
A. Only color change
B. Color change and gas production ✅
C. Color change and turbidity
D. Only gas production - In a fermentation tube, gas formation is visible as:
A. Bubbles in the medium
B. Precipitate
C. Gas bubbles in the Durham tube ✅
D. Turbid ring - Which sugar is commonly used in basic fermentation tests?
A. Galactose
B. Ribose
C. Glucose ✅
D. Maltose - What is the pH range at which phenol red turns yellow?
A. Below 7.0
B. Below 6.8 ✅
C. Above 7.4
D. Between 7.2–8.2 - A red color after incubation in a carbohydrate tube indicates:
A. Acid production
B. Alkaline reaction ✅
C. Neutral reaction
D. Protein fermentation - Which of the following bacteria is a strong lactose fermenter?
A. Salmonella
B. E. coli ✅
C. Shigella
D. Proteus - Non-fermenters usually produce which result in fermentation tests?
A. Yellow medium with gas
B. Red or unchanged color with no gas ✅
C. Black precipitate
D. Turbid yellow - Sucrose fermentation is used to differentiate:
A. Gram-negative cocci
B. Streptococcus species
C. Enteric bacilli ✅
D. Non-motile organisms - Which sugar is not typically tested in standard fermentation tests?
A. Lactose
B. Sucrose
C. Mannitol
D. Cellulose ✅ - A positive glucose fermentation with acid and gas indicates:
A. Fermentative organism ✅
B. Oxidative organism
C. Inactive metabolism
D. Proteolytic organism - What is the role of peptone in the fermentation broth?
A. Source of carbon
B. Source of nitrogen and growth factors ✅
C. Indicator for acid
D. Fermentable carbohydrate - If an organism utilizes peptone but not the carbohydrate, the result will be:
A. Yellow
B. Pink or red with no gas ✅
C. Cloudy yellow
D. Bubbles in Durham tube - Motility Indole Ornithine (MIO) medium tests for all EXCEPT:
A) Motility
B) Indole production
C) Ornithine decarboxylase
D) Urease activity ✅
- A positive phenylalanine deaminase test is seen in:
A) Proteus spp. ✅
B) Escherichia coli
C) Klebsiella spp.
D) Enterobacter spp. - Bile esculin agar turns black due to:
A) Hydrolysis of esculin ✅
B) Fermentation of bile
C) Production of indole
D) Citrate utilization - Starch hydrolysis is detected using:
A) Methyl red
B) Kovac’s reagent
C) Iodine ✅
D) Oxidase reagent - DNase test agar is used to identify:
A) Streptococcus pyogenes
B) Pseudomonas aeruginosa
C) Enterococcus faecalis
D) Staphylococcus aureus ✅ - The Optochin test is used to differentiate:
A) Staphylococcus aureus from coagulase-negative staphylococci
B) Neisseria species
C) Streptococcus pneumoniae (sensitive) from other streptococci ✅
D) Enterobacteriaceae - Bacitracin susceptibility helps identify:
A) Group B Streptococcus (S. agalactiae)
B) Enterococcus spp.
C) Group A Streptococcus (S. pyogenes) ✅
D) Streptococcus bovis - A positive CAMP test is associated with:
A) Streptococcus agalactiae ✅
B) Streptococcus pneumoniae
C) Staphylococcus aureus
D) Listeria monocytogenes - Salt tolerance (6.5% NaCl) is a feature of:
A) Enterococcus spp. ✅
B) Streptococcus pyogenes
C) Escherichia coli
D) Pseudomonas aeruginosa - The ONPG test detects:
A) Oxidase activity
B) Urease activity
C) Citrate utilization
D) β-galactosidase activity ✅
- Lecithinase production is a characteristic of:
A) Bacillus cereus
B) Clostridium perfringens ✅
C) Staphylococcus aureus
D) Escherichia coli - Lipase activity is tested using which medium?
A) Tributyrin agar ✅
B) Blood agar
C) MacConkey agar
D) TSI agar - Germ tube formation is diagnostic for:
A) Candida albicans ✅
B) Cryptococcus neoformans
C) Aspergillus fumigatus
D) Candida glabrata - Hippurate hydrolysis is positive in:
A) Group A Streptococcus (S. pyogenes)
B) Enterococcus faecalis
C) Group B Streptococcus (S. agalactiae) ✅
D) Streptococcus pneumoniae - The satellite phenomenon is seen with:
A) Haemophilus influenzae ✅
B) Neisseria meningitidis
C) Streptococcus pneumoniae
D) Staphylococcus aureus - A positive bile solubility test confirms:
A) Staphylococcus aureus
B) Enterococcus faecalis
C) Pseudomonas aeruginosa
D) Streptococcus pneumoniae ✅ - The “string test” is used to identify:
A) Escherichia coli
B) Vibrio cholerae ✅
C) Klebsiella pneumoniae
D) Proteus mirabilis - Urease-positive bacteria in gastric biopsies suggest:
A) Escherichia coli
B) Helicobacter pylori ✅
C) Salmonella Typhi
D) Shigella spp. - Which test differentiates Klebsiella from Enterobacter?
A) Indole
B) Oxidase
C) Catalase
D) Motility ✅ - The O-F test distinguishes:
A) Oxidative vs. fermentative metabolism ✅
B) Aerobic vs. anaerobic bacteria
C) Gram-positive vs. Gram-negative bacteria
D) Spore-forming vs. non-spore-forming bacteria
- A positive gelatinase test indicates:
A) Liquefaction of gelatin ✅
B) Coagulation of plasma
C) Fermentation of glucose
D) Production of hydrogen sulfide - The reverse CAMP test is used to identify:
A) Listeria monocytogenes
B) Streptococcus agalactiae
C) Clostridium perfringens ✅
D) Staphylococcus aureus - Which bacteria produce a “fried egg” colony on agar?
A) Mycoplasma spp. ✅
B) Mycobacterium tuberculosis
C) Bacillus anthracis
D) Corynebacterium diphtheriae - The Nagler reaction detects:
A) Hyaluronidase of Streptococcus pyogenes
B) Coagulase of Staphylococcus aureus
C) Enterotoxin of Escherichia coli
D) Alpha toxin (lecithinase) of Clostridium perfringens ✅
Possible References Used
