Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2331: Which of the following diseases is caused by Borrelia?

A. Vincent angina
B. Weil's disease
C. Bejel
D. Lyme disease (Correct Answer)

Explanation: ***Lyme disease*** - **Lyme disease** is caused by the spirochete bacterium **_Borrelia burgdorferi_**, transmitted primarily by ticks. - Its classic early symptom is an **erythema migrans** rash, often described as a "bull's-eye" appearance. *Weil's disease* - **Weil's disease** is a severe form of **leptospirosis**, caused by the bacterium **_Leptospira interrogans_**, not _Borrelia_. - It presents with **jaundice**, **renal failure**, and **hemorrhage**. *Bejel* - **Bejel** (also known as endemic syphilis) is caused by **_Treponema pallidum subspecies endemicum_**, a bacterium closely related to the one causing syphilis, but not _Borrelia_. - It primarily affects the **skin**, **mucous membranes**, and **bones**, particularly in children in arid regions. *Vincent angina* - **Vincent angina** (also known as necrotizing ulcerative gingivitis or "trench mouth") is caused by a polymicrobial infection involving **anaerobic bacteria**, particularly **_Fusobacterium nucleatum_** and **_Treponema vincentii_**, not _Borrelia_. - It is characterized by **painful**, bleeding gums, and **halitosis**.

Question 2332: The following statements are true regarding botulism except:

A. Clostridium botulinum A, B, E and F cause human disease.
B. The gene for botulinum toxin is encoded by a bacteriophage.
C. Clostridium baratii may cause botulism.
D. Infant botulism is caused by ingestion of preformed toxin. (Correct Answer)

Explanation: ***Infant botulism is caused by ingestion of preformed toxin.*** - This statement is incorrect because **infant botulism** is caused by the *in vivo* production of **botulinum toxin** in the infant's intestine after *Clostridium botulinum* spores are ingested and colonize the gut. - Unlike foodborne botulism in adults, it is **not** due to consuming preformed toxin, as the infant's immature gut flora allows spore germination. *Clostridium botulinum A, B, E and F cause human disease.* - This statement is true; **neurotoxin types A, B, E, and F** are the most common causes of human botulism, with type A and B being responsible for the majority of cases worldwide. - These types produce potent **neurotoxins** that block acetylcholine release at neuromuscular junctions, leading to flaccid paralysis. *The gene for botulinum toxin is encoded by a bacteriophage.* - This statement is true. The genes for **botulinum neurotoxins** (BoNT) in *C. botulinum* are often found on **bacteriophages** or plasmids, which can transfer these virulence genes between bacteria. - This horizontal gene transfer contributes to the diversity of *C. botulinum* and the spread of toxin production capability. *Clostridium baratii may cause botulism.* - This statement is true. While *Clostridium botulinum* is the most common species, **atypical clostridia** such as *Clostridium baratii* and *Clostridium butyricum* have also been identified as rare causes of botulism, particularly infant botulism. - These species also produce **botulinum neurotoxins** (specifically type F in *C. baratii* and type E in *C. butyricum*), leading to similar clinical manifestations.

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: 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 2335: Clinical classification of staphylococcus is based on -

A. Catalase test
B. Coagulase test (Correct Answer)
C. Mannitol fermentation
D. Optochin sensitivity

Explanation: ***Coagulase test*** - The **coagulase test** is the primary method used to differentiate pathogenic *Staphylococcus aureus* from non-pathogenic coagulase-negative staphylococci. - *S. aureus* produces **coagulase**, an enzyme that clots plasma, while other staphylococci do not. *Catalase test* - The **catalase test** differentiates staphylococci from streptococci (staphylococci are catalase-positive, streptococci are catalase-negative). - It does not help in differentiating between different *Staphylococcus* species. *Mannitol fermentation* - **Mannitol fermentation** is a characteristic used to distinguish *Staphylococcus aureus* (ferments mannitol) from some other *Staphylococcus* species (e.g., *Staphylococcus epidermidis*). - While helpful, it is not the primary test for broad classification of the genus. *Optochin sensitivity* - **Optochin sensitivity** is a test used to identify *Streptococcus pneumoniae* (sensitive) from other alpha-hemolytic streptococci. - This test is not used for the classification or differentiation of *Staphylococcus* species.

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: 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 2338: 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 2339: Which organism uses the Iron-regulated surface determinant (Isd) system for acquiring iron from hemoglobin as a key virulence mechanism?

A. Staphylococcus aureus (Correct Answer)
B. Streptococcus pneumoniae
C. Haemophilus influenzae
D. Escherichia coli

Explanation: ***Staphylococcus aureus*** - *Staphylococcus aureus* possesses a unique **iron-regulated surface determinant (Isd) system** that allows it to extract iron directly from **hemoglobin and heme-containing proteins**. - The Isd system includes cell wall-anchored proteins (IsdA, IsdB, IsdC) that capture hemoglobin, extract heme, and transport iron across the cell wall and membrane. - This specialized iron acquisition system is a **critical virulence factor** enabling *S. aureus* to thrive in iron-limited host environments, promoting bacterial growth, biofilm formation, and resistance to oxidative stress. - The Isd system is particularly important during deep tissue infections where hemoglobin from damaged red blood cells is the primary iron source. *Streptococcus pneumoniae* - *S. pneumoniae* uses different iron acquisition mechanisms, primarily **ABC transporters** (PiaA, PiuA) for ferric iron uptake from transferrin and lactoferrin. - Does not possess the specific Isd system characteristic of *S. aureus*. - Its virulence relies more on **capsular polysaccharides** and **pneumolysin** rather than specialized hemoglobin-based iron acquisition. *Haemophilus influenzae* - Acquires iron through **transferrin and lactoferrin binding proteins** but lacks the Isd system. - Cannot utilize hemoglobin directly without the Isd system machinery. - Virulence factors include **capsule** (type b strains) and **IgA protease**, with iron obtained from iron-binding glycoproteins rather than hemoglobin. *Escherichia coli* - Uses **siderophores** (enterobactin, aerobactin) to chelate ferric iron from the environment. - Does not possess the Isd system and cannot efficiently extract iron from hemoglobin. - Pathogenic strains have diverse virulence mechanisms (adhesins, toxins) with iron acquisition via siderophore-mediated pathways, not hemoglobin extraction.

Question 2340: 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

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

Practice Questions

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