Bacteriology — MCQs
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On what basis are E. coli subtypes classified?
What is the mechanism of action of diphtheria toxin?
What is the generation time of Mycobacterium tuberculosis?
Stalactite growth in ghee broth is due to which of the following organisms?
Which organism is partially acid-fast?
What is the causative agent of Legionnaire's disease?
Which organism is known for its ability to survive in anaerobic conditions?
Species of Shigella most commonly causing arthritis
Which organism is most commonly associated with a fishy odour on growth?
The Griffith classification is based on which of the following?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 2331: On what basis are E. coli subtypes classified?
- A. Virulence properties
- B. Lactose fermentation
- C. Maltose fermentation
- D. Somatic O antigen (Correct Answer)
Explanation: ***Somatic O antigen*** - *E. coli* subtypes are commonly classified based on the **somatic O antigen**, which is part of the **lipopolysaccharide (LPS)** layer on the bacterial surface. - This antigenic classification system, along with H (flagellar) and K (capsular) antigens, helps in **epidemiological tracking** and identifying pathogenic strains. *Virulence properties* - While *E. coli* subtypes do possess different **virulence properties** (e.g., toxins, adhesins), these are characteristics of specific strains rather than the primary basis for their initial laboratory classification. - Virulence factors determine disease manifestation but are not the standard phenotypic markers used for general subtyping. *Lactose fermentation* - **Lactose fermentation** is a general characteristic of *E. coli* as a species, used to distinguish it from **non-lactose fermenting enterics** like *Salmonella* and *Shigella*. - It does not differentiate between different pathogenic or non-pathogenic *E. coli* subtypes. *Maltose fermentation* - **Maltose fermentation** is a biochemical test that can be used to differentiate certain bacteria but is not a primary or standard method for classifying *E. coli* into subtypes. - *E. coli* generally ferments maltose, but this characteristic does not provide the specificity needed for subtyping.
Question 2332: What is the mechanism of action of diphtheria toxin?
- A. Inhibiting glucose synthesis
- B. Inhibiting protein synthesis (Correct Answer)
- C. Promoting acetylcholine release
- D. Altering cyclic GMP levels
Explanation: ***Inhibiting protein synthesis*** - Diphtheria toxin is an **A-B toxin** that enters host cells and catalyzes the **ADP-ribosylation** of **elongation factor-2 (EF-2)**. - This modification inactivates EF-2, which is crucial for the translocation step of **protein synthesis**, thereby blocking translation and leading to cell death. *Inhibiting glucose synthesis* - This is not the mechanism of action of diphtheria toxin; its primary target is the **eukaryotic protein synthesis machinery**. - While metabolic processes might be indirectly affected by cell death, directly inhibiting glucose synthesis is incorrect. *Promoting acetylcholine release* - This mechanism is associated with other toxins, such as **black widow spider venom**, which promotes the release of neurotransmitters. - Diphtheria toxin specifically targets **protein synthesis in host cells**, not neurotransmitter release. *Altering cyclic GMP levels* - Toxins like **heat-stable enterotoxins of E. coli** or **guanylin** can alter cGMP levels, affecting fluid and electrolyte balance. - Diphtheria toxin's action is distinct, involving the **inactivation of EF-2** to halt protein production.
Question 2333: What is the generation time of Mycobacterium tuberculosis?
- A. 10-15 hours
- B. 1-2 hours
- C. 5-10 hours
- D. 15-20 hours (Correct Answer)
Explanation: ***15-20 hours*** - *Mycobacterium tuberculosis* is known for its **slow growth rate**, with a generation time of approximately **15-20 hours** (some sources cite up to 18-24 hours). - This unusually long generation time contributes to the **prolonged incubation period** required for culture (3-8 weeks) and makes its isolation and identification a time-consuming process. - The slow growth is due to its **complex, lipid-rich cell wall** and unique metabolic characteristics. *1-2 hours* - A generation time of 1-2 hours is typical for **rapidly growing bacteria** such as *Escherichia coli* (which has a generation time of approximately 20 minutes under optimal conditions). - *Mycobacterium tuberculosis* is characterized by its **much slower growth rate**, distinguishing it from most common bacterial pathogens. *10-15 hours* - While this is slower than typical bacteria, it is still at the **lower end** of the range for *M. tuberculosis*. - The more widely accepted generation time is **15-20 hours**, making this option slightly inaccurate. *5-10 hours* - A generation time of 5-10 hours is **too short** for *Mycobacterium tuberculosis*. - Members of the *Mycobacterium tuberculosis complex* are known for their **exceptionally long replication cycles** compared to most other bacteria.
Question 2334: Stalactite growth in ghee broth is due to which of the following organisms?
- A. C. perfringens
- B. Y. pestis
- C. C. diphtheriae (Correct Answer)
- D. H. influenzae
Explanation: ***C. diphtheriae*** - **Corynebacterium diphtheriae** exhibits characteristic **stalactite (or stalagmite) growth** when cultured in **ghee broth** or on **Loeffler's serum slope** (which contains ghee). - This distinctive growth pattern appears as **pendant drop-like projections** hanging from the surface, resembling stalactites in caves. - This is a classic microbiological feature used for presumptive identification of C. diphtheriae. *Y. pestis* - *Yersinia pestis*, the causative agent of plague, does not show stalactite growth. - In liquid media, Y. pestis may form a **pellicle** or show floccular growth. - On solid media, it may show **"inverted fir tree" appearance** in stab cultures, not stalactite growth. *C. perfringens* - *Clostridium perfringens* is an anaerobic bacterium causing **gas gangrene** and food poisoning. - Growth in broth is typically **turbid with gas production**, without stalactite formation. *H. influenzae* - *Haemophilus influenzae* requires **X (hemin) and V (NAD) factors** for growth. - Typically grows as **small colonies on chocolate agar**, not in ghee broth. - Does not exhibit stalactite growth patterns.
Question 2335: Which organism is partially acid-fast?
- A. Actinomyces israelii
- B. Mycobacterium bovis
- C. Nocardia asteroides (Correct Answer)
- D. Mycobacterium tuberculosis
Explanation: ***Nocardia asteroides*** - *Nocardia* species are distinguished by their **Gram-positive, beaded, branching filamentous** morphology and their unique cell wall structure. - They are considered **partially acid-fast** because their cell walls contain **mycolic acid**, which retains carbolfuchsin stain but is decolorized by weak acid alcohol, unlike the stronger acid-fastness of Mycobacteria. *Mycobacterium bovis* - *Mycobacterium bovis* is a **fully acid-fast** organism. - Its cell wall contains a high concentration of **mycolic acid**, making it resistant to decolorization by strong acid-alcohol solutions. *Actinomyces israelii* - *Actinomyces* species are **Gram-positive, branching filamentous bacteria** similar in morphology to Nocardia. - However, they are **NOT acid-fast** at all, as their cell walls lack mycolic acid. - This is a key distinguishing feature from Nocardia. *Mycobacterium tuberculosis* - This is a classic example of a **fully acid-fast** bacterium. - The presence of a thick, waxy layer of **mycolic acid** in its cell wall prevents decolorization even with strong acid-alcohol.
Question 2336: What is the causative agent of Legionnaire's disease?
- A. Klebsiella pneumoniae
- B. Streptococcus pneumoniae
- C. Staphylococcus aureus
- D. Legionella pneumophila (Correct Answer)
Explanation: ***Legionella pneumophila*** - This bacterium is the definitive **causative agent** of Legionnaire's disease, a severe form of pneumonia. - It thrives in **warm water environments** like air conditioning systems and water pipes, and is transmitted via aerosols. *Staphylococcus aureus* - While *S. aureus* can cause various infections, including **pneumonia**, it does not cause Legionnaire's disease. - *S. aureus* is commonly associated with **skin infections**, bloodstream infections, and hospital-acquired pneumonia. *Streptococcus pneumoniae* - This bacterium is the most common cause of **community-acquired pneumonia** but is not responsible for Legionnaire's disease. - *S. pneumoniae* typically causes lobar pneumonia with characteristic clinical and radiographic findings. *Klebsiella pneumoniae* - *K. pneumoniae* is a significant cause of **hospital-acquired pneumonia**, particularly in immunocompromised individuals and alcoholics. - It is known for causing severe pneumonia with **"currant jelly" sputum**, but it is not the agent of Legionnaire's disease.
Question 2337: Which organism is known for its ability to survive in anaerobic conditions?
- A. Micrococcus
- B. Clostridium (Correct Answer)
- C. B. anthracis
- D. Corynebacterium
Explanation: ***Clostridium*** - Species like *Clostridium perfringens* and *Clostridium tetani* are **obligate anaerobes**, meaning they can only grow in the absence of oxygen. - They produce **spores** that allow them to survive in harsh aerobic environments until anaerobic conditions are met. *Micrococcus* - **Micrococcus** species are typically **aerobic** or facultatively anaerobic bacteria. - They require oxygen for optimal growth and are not known for thriving in strictly anaerobic conditions. *B. anthracis* - **Bacillus anthracis** is a **facultative anaerobe**, meaning it can grow either with or without oxygen, but prefers aerobic conditions. - While it can survive without oxygen, it is not primarily known for its anaerobic capabilities in the same way *Clostridium* is. *Corynebacterium* - **Corynebacterium** species are generally **aerobic** or facultative aerobes. - They grow best in the presence of oxygen and are not characteristic of organisms that survive well in anaerobic environments.
Question 2338: Species of Shigella most commonly causing arthritis
- A. Shigella flexneri (Correct Answer)
- B. Shigella boydii
- C. Shigella dysenteriae
- D. Shigella sonnei
Explanation: ***Shigella flexneri*** - This species is most commonly associated with **post-infectious reactive arthritis**, especially in individuals with the **HLA-B27 genotype**. - The arthritis typically develops days to weeks after the diarrheal illness and can affect large joints. *Shigella boydii* - While *Shigella boydii* can cause **shigellosis**, it is less frequently implicated in reactive arthritis compared to *Shigella flexneri*. - Its infections are more common in some developing regions but have a lower association with rheumatological sequelae. *Shigella dysenteriae* - *Shigella dysenteriae* causes severe forms of shigellosis, including **dysentery with potential hemolytic-uremic syndrome**, particularly serotype 1. - Although it can trigger reactive arthritis, its prevalence as a cause of this complication is lower than *Shigella flexneri*. *Shigella sonnei* - *Shigella sonnei* is the most common cause of **shigellosis in industrialized countries** and often presents with milder symptoms. - While reactive arthritis can theoretically follow any *Shigella* infection, *S. sonnei* is less commonly linked to this specific complication compared to *S. flexneri*.
Question 2339: Which organism is most commonly associated with a fishy odour on growth?
- A. Proteus species (Correct Answer)
- B. Escherichia coli
- C. Klebsiella species
- D. Pseudomonas species
Explanation: ***Proteus species*** - **Proteus species** are classically known for their characteristic **putrid or ammonia-like odor**, NOT typically described as "fishy" - This odor is due to their production of **urease enzyme**, which breaks down urea to ammonia, creating a strong alkaline smell - The ammonia smell is particularly noticeable in urine samples infected with Proteus - **Note**: While "fishy odor" in microbiology is classically associated with **Gardnerella vaginalis** and certain anaerobes, among the options listed, this may be a legacy question with outdated terminology *Escherichia coli* - **E. coli** produces a characteristic **fecal or indolic odor** due to tryptophan metabolism - This is described as musty or fecal-smelling, distinctly different from fishy - Common cause of UTIs but with different odor profile *Klebsiella species* - **Klebsiella** produces a **sweet, bread-like or musty odor** - Their mucoid colonies (due to polysaccharide capsule) have a characteristic appearance - The odor is described as sweet or yeasty, not fishy *Pseudomonas species* - **Pseudomonas aeruginosa** has a distinctive **grape-like, fruity, or corn tortilla-like odor** - This is due to production of **2-aminoacetophenone** and other aromatic compounds - The odor is sweet/fruity and easily recognizable, not fishy - Also produces pyocyanin (blue-green pigment) as an additional identifying feature
Question 2340: The Griffith classification is based on which of the following?
- A. M, T, R antigens
- B. Types of hemolysis
- C. Oxygen requirement
- D. Capsular polysaccharides (Correct Answer)
Explanation: ***Capsular polysaccharides*** - The **Griffith classification** system categorizes *Streptococcus pneumoniae* strains based on the biochemical and antigenic differences in their **polysaccharide capsules**. - These capsules are crucial **virulence factors** that protect bacteria from phagocytosis and are targeted by vaccines. *M, T, R antigens* - These antigens are associated with the **Lancefield grouping system** for beta-hemolytic streptococci, primarily *Streptococcus pyogenes*, and distinguish different serotypes based on cell wall carbohydrate antigens. - They are not the basis for the Griffith classification, which focuses specifically on *Streptococcus pneumoniae*. *Types of hemolysis* - Hemolysis patterns (alpha, beta, gamma) are used as a general preliminary classification for many streptococcal species cultured on **blood agar**, indicating their ability to lyse red blood cells. - While *Streptococcus pneumoniae* typically exhibits **alpha-hemolysis**, this characteristic is not specific enough to differentiate between the numerous serotypes defined by the Griffith system. *Oxygen requirement* - Oxygen requirement defines a microorganism's growth conditions (e.g., aerobic, anaerobic, facultative anaerobic) and is a fundamental characteristic for bacterial identification but is **not used for serotyping**. - *Streptococcus pneumoniae* is a **facultative anaerobe**, but this physiological trait does not differentiate its serotypes.
Practice by Chapter
Staphylococci
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Streptococci and Enterococci
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Neisseria and Moraxella
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Corynebacterium and Listeria
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Bacillus and Clostridium
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Enterobacteriaceae
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Vibrio, Aeromonas, and Plesiomonas
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Pseudomonas and Related Bacteria
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Haemophilus and HACEK Group
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Bordetella and Brucella
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Mycobacteria
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Spirochetes
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