Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2481: Which of the following is a true statement regarding Enterobacteriaceae?

A. Glucose is fermented by most members of the family
B. Motility in Enterobacteriaceae is via peritrichous flagella
C. Nitrate reduction positive
D. All members are oxidase negative (Correct Answer)

Explanation: ***All members are oxidase negative*** - This is the **definitive and universally true** statement for all Enterobacteriaceae, with **no exceptions**. - The **negative oxidase test** is a key defining characteristic that distinguishes Enterobacteriaceae from other Gram-negative bacteria such as *Pseudomonas*. - This enzymatic characteristic is crucial for basic laboratory identification and is **consistently used as a primary screening test** for this family. *Glucose is fermented by most members of the family* - While the fermentation of glucose is indeed a characteristic of Enterobacteriaceae, the statement is **imprecise** because it says "most" when in fact **ALL members** ferment glucose. - The use of "most" makes this statement technically less accurate than the oxidase-negative statement, which correctly uses "all." *Motility in Enterobacteriaceae is via peritrichous flagella* - This statement is **incomplete and potentially misleading** because **not all members are motile**. - While motile members do have **peritrichous flagella** (distributed over the entire cell surface), important members like *Klebsiella* species are **non-motile**. - The statement implies universal motility, which is incorrect. *Nitrate reduction positive* - While all Enterobacteriaceae do reduce nitrates to nitrites, this statement **lacks specificity** compared to the oxidase test. - The oxidase-negative characteristic is more **diagnostically distinctive** and is the **primary screening test** used to differentiate Enterobacteriaceae from other Gram-negative bacteria. - Nitrate reduction is confirmatory but not as definitive for initial identification.

Question 2482: Which of the following organisms is an obligate intracellular bacterium that commonly causes sexually transmitted infections?

A. Mycoplasma
B. Chlamydia (Correct Answer)
C. Rickettsia
D. Prion

Explanation: ***Chlamydia*** - **Chlamydia trachomatis** is a classic example of an **obligate intracellular bacterium** that is a leading cause of sexually transmitted infections (STIs). - It has a unique biphasic developmental cycle, alternating between an infectious **elementary body** and a replicative **reticulate body** within host cells. *Rickettsia* - **Rickettsia** species are also **obligate intracellular bacteria** but are primarily transmitted via **arthropod vectors** (e.g., ticks, fleas, lice), causing diseases like **Rocky Mountain spotted fever** and **typhus**. - They are not typically associated with **sexually transmitted infections** in humans. *Mycoplasma* - **Mycoplasma** species are bacteria characterized by the **absence of a cell wall**, but they are generally **extracellular** or **intracellular but not obligate**. - While some *Mycoplasma* species can cause STIs (e.g., *Mycoplasma genitalium*), they do not fit the description of an **obligate intracellular bacterium** in the same way *Chlamydia* does (which requires host cell machinery for energy production). *Prion* - A **prion** is an **infectious protein particle** that lacks genetic material (DNA or RNA) and causes transmissible spongiform encephalopathies (TSEs), such as **Creutzfeldt-Jakob disease**. - It is not a bacterium and is not associated with **sexually transmitted infections**.

Question 2483: A 26-year-old male patient presented with a granulomatous penile ulcer. On Wright Giemsa staining, tiny organisms of 2 microns are seen engulfed by macrophages. What is the causative organism?

A. Lymphogranuloma venereum (LGV)
B. Donovanosis (Calymmatobacterium granulomatis) (Correct Answer)
C. Gonorrhea (Neisseria gonorrhoeae)
D. Syphilis (Treponema pallidum)

Explanation: ***Donovanosis (Calymmatobacterium granulomatis)*** - The presence of **granulomatous penile ulcers** and characteristic **Donovan bodies** (2-micron organisms engulfed by macrophages on Wright-Giemsa stain) are pathognomonic for donovanosis. - This condition is caused by *Calymmatobacterium granulomatis* (now reclassified as *Klebsiella granulomatis*). *Lymphogranuloma venereum (LGV)* - While LGV also causes genital ulcers and lymphadenopathy, the causative agent is *Chlamydia trachomatis* serovars L1, L2, or L3, and it does not present with **Donovan bodies** in macrophages. - Histologically, LGV often shows **stellate abscesses** in lymph nodes, not intracellular organisms in a granulomatous ulcer. *Gonorrhea (Neisseria gonorrhoeae)* - Gonorrhea typically presents as **urethritis** with purulent discharge in males or cervicitis in females, not commonly as a granulomatous penile ulcer. - Microscopic examination would show **Gram-negative intracellular diplococci** within neutrophils, not macrophage-engulfed organisms like Donovan bodies. *Syphilis (Treponema pallidum)* - Primary syphilis manifests as a **painless chancre**, which is typically a clean-based, indurated ulcer, not usually granulomatous. - Diagnosis is made by **darkfield microscopy** to visualize spirochetes or serological tests (e.g., RPR, VDRL, FTA-ABS), not by finding intracellular organisms on Wright-Giemsa stain.

Question 2484: Causative agent of trench fever is which of the following?

A. Coxiella burnetii
B. R. tsutsugamushi
C. R. prowazekii
D. Bartonella quintana (Correct Answer)

Explanation: ***Bartonella Quintana*** - **Bartonella quintana** is the bacterium responsible for causing **trench fever**, a disease historically prevalent during wartime in crowded and unsanitary conditions. - It is transmitted by the **human body louse** (*Pediculus humanus corporis*), which acts as a vector for the bacteria. *Coxiella burnetii* - **Coxiella burnetii** is the causative agent of **Q fever**, a zoonotic disease typically transmitted to humans through contact with infected animals or their products. - Its clinical presentation often includes acute fever, pneumonia, and hepatitis, which differs from the characteristic relapsing fever of trench fever. *R. tsutsugamushi* - **R. tsutsugamushi** (also known as *Orientia tsutsugamushi*) causes **scrub typhus**, a mite-borne disease common in rural areas of Asia and Australia. - The disease is characterized by a distinctive eschar, lymphadenopathy, and maculopapular rash, which are not typical features of trench fever. *R. prowazekii* - **R. prowazekii** is the bacterium responsible for **epidemic typhus** (louse-borne typhus), which shares the human body louse as a vector with trench fever. - However, epidemic typhus presents with a sustained high fever, rash that spreads centrifugally, and severe systemic symptoms, differentiating it from the relapsing fever patterns of trench fever.

Question 2485: Which of the following statements about Enterohemorrhagic E. coli (EHEC) is false?

A. Ferments sorbitol (Correct Answer)
B. Causes HUS
C. Elaborates shiga like exotoxin
D. Does not invade intestinal epithelium

Explanation: ***Ferments sorbitol*** - Most **EHEC** strains, particularly O157:H7, are **unable to ferment sorbitol**, a characteristic used in laboratory identification. - This inability to ferment sorbitol helps differentiate EHEC from most other *E. coli* strains which typically ferment sorbitol. - Sorbitol-MacConkey (SMAC) agar is used for selective identification of EHEC O157:H7. *Causes HUS* - **EHEC** is a major cause of **hemolytic uremic syndrome (HUS)**, especially in children, due to its production of Shiga toxins. - HUS is characterized by **hemolytic anemia**, **thrombocytopenia**, and **acute kidney injury**. *Elaborates shiga like exotoxin* - **EHEC** produces potent **Shiga-like toxins (Stx1 and Stx2)**, also known as Verotoxins, which are responsible for its virulence. - These toxins damage endothelial cells, particularly in the gastrointestinal tract and kidneys, leading to symptoms like **hemorrhagic colitis** and **HUS**. *Does not invade intestinal epithelium* - **EHEC** causes disease primarily through **toxin production and adherence**, not through invasive infection of host cells. - Unlike *Shigella* or enteroinvasive *E. coli* (EIEC), EHEC remains extracellular and does not invade the intestinal mucosa. - The Sereny test, which assesses bacterial invasiveness, is **negative** for EHEC.

Question 2486: Most common organism in acute osteomyelitis is

A. Staphylococcus aureus (Correct Answer)
B. Salmonella
C. Pseudomonas aeruginosa
D. Streptococcus pneumoniae

Explanation: ***Staphylococcus aureus*** - **_Staphylococcus aureus_** is the most common causative organism in cases of **acute osteomyelitis** across all age groups and is the primary pathogen isolated in most osteomyelitis cases. - Its ability to adhere to bone, form **biofilms**, and produce various virulence factors contributes to its dominance in bone infections. *Salmonella* - **_Salmonella_** species are a common cause of osteomyelitis, particularly in patients with **sickle cell disease** due to their impaired splenic function. - While significant in specific populations, it is not the most common cause of osteomyelitis in the general population. *Pseudomonas aeruginosa* - **_Pseudomonas aeruginosa_** is frequently associated with osteomyelitis in patients with a history of **intravenous drug use**, **puncture wounds** (particularly through athletic shoes), healthcare-associated infections, or those with compromised immunity. - It is an important pathogen in these specific contexts but not generally the most common cause. *Streptococcus pneumoniae* - **_Streptococcus pneumoniae_** can cause osteomyelitis, especially in **neonates** and **young children**, and in individuals with **immunosuppression**. - However, it is not as prevalent as **_Staphylococcus aureus_** as the overall leading cause of acute osteomyelitis.

Question 2487: Which of the following statements about FTA-ABS in syphilis is false?

A. Present in secondary syphilis
B. May be positive in Lyme's disease
C. It is a specific test
D. FTA - ABS becomes negative after treatment (Correct Answer)

Explanation: ***FTA - ABS becomes negative after treatment*** - The **FTA-ABS test** detects **treponemal antibodies**, which typically remain detectable for life even after successful treatment for syphilis. - A positive FTA-ABS test in a treated individual indicates **past infection** and not necessarily active disease. *Present in secondary syphilis* - The **FTA-ABS test** is highly sensitive and typically becomes positive early in the disease, including during **secondary syphilis**. - It is used to confirm a diagnosis when **non-treponemal tests** (like RPR or VDRL) are reactive. *It is a specific test* - The **FTA-ABS test** is considered a **treponemal-specific test**, meaning it detects antibodies directed against *Treponema pallidum* itself. - This specificity helps differentiate syphilis from other conditions that might produce false positives on non-treponemal tests. *May be positive in Lyme's disease* - While generally specific, cross-reactivity with other spirochetal infections like **Lyme disease** (caused by *Borrelia burgdorferi*) is a known, though rare, possibility. - This underscores the importance of **clinical context** and potentially other diagnostic tests in ambiguous cases.

Question 2488: Which organism is most commonly associated with causing bacterial meningitis in adults?

A. Streptococcus agalactiae (Group B Strep)
B. Staphylococcus aureus
C. Mycoplasma pneumoniae
D. Streptococcus pneumoniae (Pneumococcus) (Correct Answer)

Explanation: ***Streptococcus pneumoniae (Pneumococcus)*** - *Streptococcus pneumoniae* is the most common bacterial cause of **meningitis** in adults and children worldwide. - It often follows a primary infection like **otitis media**, sinusitis, or pneumonia. *Staphylococcus aureus* - While *S. aureus* can certainly cause meningitis, it is more commonly associated with **post-neurosurgical**, **post-traumatic**, or **hematogenous meningitis** from distant foci. - It is not the most frequent cause of community-acquired meningitis in the general population. *Streptococcus agalactiae (Group B Strep)* - *Streptococcus agalactiae* is a leading cause of **neonatal meningitis** (in newborns). - It is an important pathogen in infants but much less common in older children and adults. *Mycoplasma pneumoniae* - *Mycoplasma pneumoniae* is primarily known for causing **atypical pneumonia** and tracheobronchitis. - While neurological complications can occur, such as encephalitis or Guillain-Barré syndrome, it is a very rare cause of meningitis itself.

Question 2489: A female patient presents with signs of meningitis and cerebrospinal fluid showing gram-positive coccobacilli that exhibit characteristic tumbling motility. What is the most likely diagnosis?

A. Listeria (Correct Answer)
B. Meningococcus
C. H. influenzae
D. Pneumococcus

Explanation: ***Listeria*** - **Listeria monocytogenes** is a gram-positive coccobacillus that causes meningitis, especially in immunocompromised individuals, pregnant women, and neonates. - Its presence in CSF, combined with characteristic **tumbling motility**, is a key diagnostic feature. *Meningococcus* - **Neisseria meningitidis** is a gram-negative diplococcus, not a gram-positive coccobacillus, and does not exhibit tumbling motility. - While it is a common cause of bacterial meningitis, its microscopic appearance and motility are inconsistent with the description. *H. influenzae* - **Haemophilus influenzae** is a gram-negative coccobacillus and does not show tumbling motility. - It was a common cause of meningitis before widespread vaccination, but its gram stain characteristics differ from the described pathogen. *Pneumococcus* - **Streptococcus pneumoniae** is a gram-positive coccus often seen in pairs (diplococci) or short chains, and it does not exhibit motility. - Although it is a leading cause of bacterial meningitis, its microscopic morphology and lack of motility differentiate it from the presented case.

Question 2490: Which organism is associated with the Ehrlich phenomenon, particularly in relation to diphtheria toxin?

A. Proteus
B. Mycobacterium TB
C. Corynebacterium (Correct Answer)
D. Staphylococcus

Explanation: ***Corynebacterium*** - The Ehrlich phenomenon is historically linked to the standardization of **diphtheria antitoxin**, which targets the toxin produced by *Corynebacterium diphtheriae*. - This bacterium is the causative agent of **diphtheria**, a severe throat infection characterized by a pseudomembrane and systemic complications due to its potent exotoxin. *Proteus* - **Proteus** species are commonly associated with **urinary tract infections** and are known for their motility and swarming growth. - They are not known to produce diphtheria toxin or be involved in the Ehrlich phenomenon. *Mycobacterium TB* - *Mycobacterium tuberculosis* causes **tuberculosis**, a chronic infectious disease primarily affecting the lungs. - This bacterium is acid-fast and has a waxy cell wall, but it is not relevant to diphtheria toxin or the Ehrlich phenomenon. *Staphylococcus* - **Staphylococcus** species, such as *Staphylococcus aureus*, cause a wide range of infections, including **skin infections**, food poisoning, and toxic shock syndrome. - While they produce various toxins, none are diphtheria toxin, and they are unrelated to the Ehrlich phenomenon.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free