Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2391: Most common site for Staphylococcus carriage?

A. Skin
B. Nose (Correct Answer)
C. Oropharynx
D. Perineum

Explanation: ***Nose*** - The **anterior nares** are the most common site for **Staphylococcus aureus** colonization in healthy individuals. - Nasal carriage is a significant risk factor for subsequent **Staphylococcus aureus infections**, both self-acquired and transmitted to others. *Skin* - While Staphylococcus aureus can colonize the skin, especially in areas like the axillae and groin, it is **less common** as a primary carrier site compared to the nose. - Skin colonization often occurs due to spread from **nasal carriage**. *Oropharynx* - The oropharynx can be colonized by various bacteria, but it is **not the primary or most common site** for Staphylococcus aureus carriage. - Other bacteria like **Streptococcus species** are more prevalent colonizers of the oropharynx. *Perineum* - The perineum can harbor Staphylococcus aureus, particularly in specific populations or in individuals with certain skin conditions, but it is **not the most common or primary site** of colonization. - Colonization here is often **secondary** to nasal carriage or contact with contaminated surfaces.

Question 2392: Primary complex of M bovis involves:

A. Tonsil and skin
B. Tonsil and intestine (Correct Answer)
C. Tonsil and lung
D. Skin and Intestine

Explanation: ***Tonsil and intestine*** - *Mycobacterium bovis* is primarily transmitted through **consumption of contaminated milk and dairy products**, making the **alimentary tract** the main route of infection - The primary complex (Ghon complex) involves the **initial site of infection plus regional lymph nodes** - In alimentary tuberculosis, the organisms enter through the **intestinal mucosa** (Peyer's patches) or **tonsillar tissue**, creating foci with associated mesenteric or cervical lymphadenopathy - Both tonsils and intestines are part of the **alimentary system**, representing the typical primary complex for M. bovis in humans *Tonsil and lung* - This incorrectly combines **two different routes of entry** (alimentary and respiratory) - A primary complex involves a **single portal of entry**, not multiple unrelated organ systems - While M. bovis can rarely cause pulmonary TB through inhalation, this would create a separate lung + hilar node complex, not a combined tonsil-lung complex *Tonsil and skin* - **Skin involvement** requires direct inoculation through cuts or abrasions and does not form a primary complex with tonsillar infection - These represent different portals of entry and would not occur together as a primary complex *Skin and Intestine* - **Skin infection** by M. bovis is rare and requires occupational exposure with direct inoculation (e.g., veterinarians, butchers) - This incorrectly pairs two different routes of infection that would not form a single primary complex

Question 2393: Listeria escapes from phagosomes (phagolysosomes) due to:

A. Opacity associated protein (OAP)
B. Caspases
C. Cell membrane adhesion molecules
D. beta-hemolysin (Correct Answer)

Explanation: ***beta-hemolysin*** - *Listeria monocytogenes* produces **listeriolysin O (LLO)**, a **beta-hemolysin**, which is a pore-forming toxin that lyses the phagosomal membrane. - This allows the bacteria to escape into the host cell cytoplasm, thus **avoiding destruction** by lysosomal enzymes and neutralizing the microbicidal environment. *Caspases* - **Caspases** are a family of proteases that play essential roles in programmed cell death (apoptosis) and inflammation. - While bacteria can manipulate host cell apoptosis, caspases themselves are **host enzymes** and do not directly confer resistance to phagosome destruction. *Cell membrane adhesion molecules* - **Adhesion molecules** facilitate bacterial attachment to host cells and internalisation, but they do not directly provide a mechanism for **escaping the phagosome** once internalised. - Examples include internalins, which are involved in bacterial entry into cells. *Opacity associated protein (OAP)* - **Opacity associated proteins (OAPs)** are primarily associated with *Neisseria gonorrhoeae* and are involved in adherence to host cells and evasion of the immune response. - They are not a mechanism for **phagosomal escape** or directly related to *Listeria's* intracellular survival.

Question 2394: Granulomatosis infantiseptica is caused by:

A. Pseudomonas
B. Chlamydia trachomatis
C. Group D streptococci
D. Listeria (Correct Answer)

Explanation: ***Listeria*** - **Granulomatosis infantiseptica** is a severe manifestation of congenital **listeriosis**, caused by *Listeria monocytogenes*. - This condition is characterized by widespread **granulomas** and **microabscesses** in various organs of the infected newborn. *Pseudomonas* - *Pseudomonas aeruginosa* is a common cause of healthcare-associated infections but is not typically associated with **granulomatosis infantiseptica**. - It can cause severe infections in immunocompromised individuals, including **pneumonia**, **sepsis**, and wound infections. *Chlamydia trachomatis* - *Chlamydia trachomatis* is a common cause of **conjunctivitis** and **pneumonia** in neonates, acquired during passage through the birth canal. - It does not cause **granulomatosis infantiseptica**. *Group D streptococci* - While Group D streptococci (e.g., *Enterococcus faecalis*) can cause neonatal infections like **sepsis** and **meningitis**, they are not the causative agents of **granulomatosis infantiseptica**. - This condition is specifically linked to **Listeria**.

Question 2395: Bartonella quintana causes:

A. Trench fever (Correct Answer)
B. Scrub typhus
C. Epidemic typhus
D. Endemic typhus

Explanation: ***Trench fever*** - **Bartonella quintana** is the causative agent of **trench fever**, a louse-borne disease historically common during wartime. - Symptoms include **recurrent fevers**, headache, bone pain (especially in the shins), and rash. *Scrub typhus* - Scrub typhus is caused by **Orientia tsutsugamushi**, a rickettsial bacterium, not Bartonella. - It is transmitted by **chiggers** and characterized by a rash, fever, and eschar at the bite site. *Epidemic typhus* - Epidemic typhus is caused by **Rickettsia prowazekii** and is also louse-borne. - It presents with sudden high fever, severe headache, and a maculopapular rash that spares the face, palms, and soles. *Endemic typhus* - Endemic typhus (murine typhus) is caused by **Rickettsia typhi** and is transmitted by **rat fleas**. - Its symptoms are generally milder than epidemic typhus, including fever, headache, and a truncal rash.

Question 2396: Anaplasma phagocytophilum mainly affects -

A. Neutrophils (white blood cells) (Correct Answer)
B. Red Blood Cells (RBCs)
C. Macrophages (immune cells)
D. Platelets (thrombocytes)

Explanation: ***Neutrophils (white blood cells)*** - *Anaplasma phagocytophilum* specifically infects and replicates within **neutrophils**, leading to the characteristic morulae observed inside these cells. - This infection causes **human granulocytic anaplasmosis (HGA)**, primarily affecting white blood cells. *Red Blood Cells (RBCs)* - While other tick-borne diseases like **babesiosis** infect RBCs, *Anaplasma phagocytophilum* does not target red blood cells. - RBCs are responsible for oxygen transport and lack the necessary cellular machinery for *Anaplasma* replication. *Macrophages (immune cells)* - Macrophages are significant immune cells but are not the primary target cell for *Anaplasma phagocytophilum*. - Infections that primarily target macrophages include some **bacterial** and **parasitic infections** like *Leishmania*. *Platelets (thrombocytes)* - Platelets are crucial for **blood clotting** and are not directly infected by *Anaplasma phagocytophilum*. - Although **thrombocytopenia** (low platelet count) is a common finding in anaplasmosis, it is an indirect effect of the infection, not due to direct platelet invasion.

Question 2397: All are true about anaerobic infections except:

A. Specimen for UTI is suprapubic aspiration
B. They are found normally on skin and GIT
C. Exudates and swabs are ideal for culture (Correct Answer)
D. Most infections are endogenous

Explanation: ***Exudates and swabs are ideal for culture.*** - **Swabs exposed to air** are generally **not ideal** for anaerobic culture because oxygen exposure can kill obligate anaerobes, leading to false-negative results. - Optimal anaerobic specimen collection requires techniques that **minimize oxygen exposure**, such as aspirates or tissue biopsies placed in anaerobic transport media. *Most infections are endogenous* - Anaerobic infections commonly arise from **one's own commensal flora**, which becomes pathogenic under specific conditions like tissue damage or impaired blood supply. - These bacteria are part of the normal microbiota of various body sites, including the gastrointestinal tract, oral cavity, and skin. *Specimen for UTI is suprapubic aspiration* - For **suspected anaerobic urinary tract infections (UTIs)**, suprapubic aspiration is considered the gold standard for specimen collection. - This method bypasses potential contamination from urethral flora and ensures a sterile, oxygen-free sample for accurate anaerobic culture. *They are found normally on skin and GIT* - **Anaerobic bacteria** are a significant component of the normal flora of the **skin and gastrointestinal tract (GIT)**, as well as the oral cavity and genitourinary tract. - Their presence in these sites is crucial for maintaining normal physiological functions and preventing the overgrowth of pathogens.

Question 2398: "Citron bodies" are boat- or leaf-shaped pleomorphic organisms found in exudates. This is a characteristic feature of which organism?

A. Cl. edematiens
B. Cl. septicum (Correct Answer)
C. Cl. tetani
D. Cl. welchii

Explanation: ***Cl. septicum*** - *Clostridium septicum* characteristically forms **"Citron bodies"** - boat-shaped or leaf-shaped pleomorphic organisms in exudates - These spindle-shaped, cigar-like forms are a **distinctive morphological feature** of this organism - Associated with **gas gangrene** and spontaneous myonecrosis, particularly in patients with underlying malignancy or neutropenia - The pleomorphic morphology distinguishes it from other clostridial species *Cl. welchii* - *Clostridium perfringens* (formerly *Cl. welchii*) appears as **large, rectangular, "boxcar-shaped" Gram-positive rods** - Does **not** form citron bodies or show the characteristic boat/leaf-shaped pleomorphism - Most common cause of gas gangrene but has different morphological appearance *Cl. edematiens* - *Clostridium oedematiens* (now *Clostridium novyi*) appears as **large, Gram-positive rods** with subterminal spores - Does not form citron bodies - Associated with gas gangrene but lacks the pleomorphic morphology described *Cl. tetani* - *Clostridium tetani* has characteristic **terminal spores** giving a "drumstick" or "tennis racket" appearance - Does not form citron bodies or pleomorphic shapes - Morphologically distinct with its terminal spore appearance

Question 2399: Bacteria most commonly involved in bowel decomposition after death is?

A. Streptococcus pyogenes
B. Pseudomonas aeruginosa
C. Clostridium perfringens (Correct Answer)
D. Escherichia coli

Explanation: ***Clostridium perfringens*** - This bacterium is a ubiquitous **anaerobe** in the gut and is known for its rapid proliferation after death, producing gases that contribute to **bloating and decomposition**. - It is a primary cause of **gas gangrene** in living individuals, reflecting its tissue-destructive capabilities, which extend to post-mortem changes. - Produces large amounts of **hydrogen and CO2**, making it the most significant contributor to post-mortem gas formation and putrefaction. *Streptococcus pyogenes* - While a significant pathogen in life, causing conditions like **strep throat** and **necrotizing fasciitis**, it is not the primary agent of putrefaction. - Its role in post-mortem decomposition is generally less prominent compared to anaerobic gut flora. *Pseudomonas aeruginosa* - This bacterium is an opportunistic pathogen often associated with infections in immunocompromised individuals or in healthcare settings. - It is not typically identified as the most common or primary bacterium involved in the initial stages of post-mortem **bowel decomposition**, though it can be present in later stages. *Escherichia coli* - While *E. coli* is abundant in the bowel and participates in post-mortem decomposition, it is not the **most common** agent responsible for gas production and tissue decomposition. - *Clostridium perfringens* proliferates more rapidly and produces significantly more gas, making it the predominant bacterium in bowel putrefaction.

Question 2400: A patient presents with purulent urethral discharge and laboratory findings of gram-negative diplococci on modified Thayer-Martin media. What is the most likely diagnosis?

A. Treponema pallidum
B. Neisseria gonorrhoeae (Correct Answer)
C. Haemophilus ducreyi
D. Chlamydia trachomatis

Explanation: ***Neisseria gonorrhoeae*** - **Purulent urethral discharge** and identification of **gram-negative diplococci** on microscopy from a urethral swab are highly characteristic of *Neisseria gonorrhoeae* infection. - *N. gonorrhoeae* grows well on **modified Thayer-Martin media**, a selective medium specifically designed for *Neisseria* species. - This organism is the causative agent of **gonorrhea**, a common sexually transmitted infection. - The combination of clinical presentation (purulent discharge) and laboratory findings (gram-negative diplococci on Thayer-Martin media) makes this the definitive diagnosis. *Haemophilus ducreyi* - Causes **chancroid**, which presents with **painful genital ulcers** and **lymphadenopathy**, NOT purulent urethral discharge. - *H. ducreyi* is a gram-negative **bacillus** (rod-shaped), NOT diplococci. - It requires special media supplemented with X and V factors for growth, not primarily Thayer-Martin medium. *Treponema pallidum* - Causes **syphilis**, which typically presents with a **chancre** (painless ulcer) in primary stage and systemic symptoms in later stages. - Cannot be cultured on routine media and is identified by **darkfield microscopy** or serological tests. - Does not appear as gram-negative diplococci. *Chlamydia trachomatis* - Can cause urethritis with **mucopurulent or watery discharge**, but not typically purulent. - *Chlamydia* are **obligate intracellular bacteria** that cannot be visualized as gram-negative diplococci on gram stain. - Cannot be cultured on standard bacteriologic media like Thayer-Martin medium.

Practice by Chapter

Staphylococci

Practice Questions

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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