Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2251: The commonest organism causing Empyema in a child under 2 years -

A. Pneumococcus (Correct Answer)
B. Staphylococcus
C. Klebsiella
D. E.Coli

Explanation: ***Pneumococcus*** - **Streptococcus pneumoniae (Pneumococcus)** is the most common bacterial cause of **empyema** in children across all age groups, including those under 2 years. - It accounts for **40-70% of pediatric empyema cases**, typically developing as a complication of pneumonia. - Even in the post-pneumococcal vaccine era, S. pneumoniae remains the leading cause, though serotype distribution has changed. *Staphylococcus* - **Staphylococcus aureus** is the **second most common** cause of empyema in children under 2 years, accounting for 10-30% of cases. - It causes more severe, rapidly progressing infections with significant **pleural effusion** and **necrotizing pneumonia**. - While historically prominent, it is no longer the most common cause in the modern antibiotic era. *Klebsiella* - **Klebsiella pneumoniae** is a less common cause of empyema in children, more frequently seen in immunocompromised patients or those with underlying chronic diseases. - It is not a typical cause of empyema in otherwise healthy children under 2 years old. *E. Coli* - **Escherichia coli** can cause empyema, particularly in neonates, immunocompromised patients, or as a complication of intra-abdominal infections. - It is a rare cause of empyema in the typical pediatric population under 2 years without specific predisposing factors.

Question 2252: Which of the following causes Melioidosis?

A. Burkholderia pseudomallei (Correct Answer)
B. Burkholderia cepacia
C. Burkholderia mallei
D. None of the options

Explanation: ***Burkholderia pseudomallei*** - **Melioidosis** is caused by the bacterium **_Burkholderia pseudomallei_**, a **Gram-negative rod** found in soil and water. - This organism is endemic in tropical and subtropical regions, particularly Southeast Asia and northern Australia, and can cause a wide spectrum of disease from asymptomatic infection to fulminant sepsis. *Burkholderia cepacia* - **_Burkholderia cepacia complex_** is a group of bacteria known to cause infections, particularly in individuals with **cystic fibrosis** or **immunocompromised states**. - While it can cause opportunistic infections, it is not the causative agent of melioidosis. *Burkholderia mallei* - **_Burkholderia mallei_** is the causative agent of **glanders**, a contagious disease primarily affecting **horses, donkeys, and mules**, which can be transmitted to humans. - It is related to **_B. pseudomallei_** but does not cause melioidosis. *None of the options* - This option is incorrect because **_Burkholderia pseudomallei_** is the specific and well-established causative agent of melioidosis.

Question 2253: Which does not cause skin involvement -

A. M. tuberculosis
B. M. Ulcerans
C. M. Kansasii (Correct Answer)
D. M. Marinum

Explanation: ***M. Kansasii*** - While *M. kansasii* is a **nontuberculous mycobacterium**, it primarily causes **pulmonary disease** similar to tuberculosis, especially in individuals with compromised immune systems. - Skin involvement is **rare** and not a typical presentation for *M. kansasii* infections. *M. tuberculosis* - **Cutaneous tuberculosis** can manifest in various forms, including **lupus vulgaris**, scrofuloderma, and tuberculosis verrucosa cutis. - Skin involvement occurs due to direct inoculation, contiguous spread, or lymphatic/hematogenous dissemination. *M. Ulcerans* - *M. ulcerans* is the causative agent of **Buruli ulcer**, a devastating skin infection characterized by large, necrotic ulcers. - It produces **mycolactone**, a toxin that causes tissue destruction and immunosuppression in the affected area. *M. Marinum* - *M. marinum* typically causes **"fish tank granuloma"** or "swimming pool granuloma," an infection acquired through exposure to contaminated water. - It results in chronic skin lesions, often granulomatous nodules or ulcers, primarily on exposed extremities.

Question 2254: "Chancroid" is caused by:

A. H. influenzae
B. H. ducreyi (Correct Answer)
C. All of the options
D. H. aphrophilus

Explanation: ***H. ducreyi*** - **Chancroid** is a sexually transmitted infection characterized by painful genital ulcers and is specifically caused by the bacterium **_Haemophilus ducreyi_**. - This organism creates soft chancres (ulcers) that often bleed easily and are frequently accompanied by painful swelling of the lymph nodes in the groin. *H. influenzae* - **_Haemophilus influenzae_** is a common cause of respiratory tract infections, such as otitis media, sinusitis, and pneumonia, and can also lead to meningitis. - It is not associated with sexually transmitted infections or genital ulcers, and therefore does not cause chancroid. *H. aprophilus* - **_Haemophilus aphrophilus_** (now reclassified as _Aggregatibacter aphrophilus_) is a rare cause of human infections, primarily associated with endocarditis and brain abscesses. - This bacterium is part of the **HACEK group** of gram-negative bacteria known to cause infective endocarditis and is not involved in chancroid. *All of the options* - While all listed organisms are types of _Haemophilus_ or former _Haemophilus_ species, only **_H. ducreyi_** specifically causes chancroid. - The other options are responsible for different types of infections and do not cause genital ulcers characteristic of chancroid.

Question 2255: Salmonella typhi is the causative agent of typhoid fever. The infective dose of S. typhi is -

A. 10^8-10^10 bacilli
B. 1-10 bacilli
C. One bacillus
D. 10^2-10^5 bacilli (Correct Answer)

Explanation: ***10^2-10^5 bacilli*** - The infective dose of **Salmonella typhi** is relatively low compared to other enteric pathogens, typically ranging from **100 to 100,000 bacteria**. - This low infective dose contributes to its high transmissibility and ability to cause widespread outbreaks. *10^8-10^10 bacilli* - This range represents a significantly **higher infective dose**, which is characteristic of some less virulent or more easily cleared enteric bacteria. - If **Salmonella typhi** required such a high dose, typhoid fever would be a much rarer disease. *1-10 bacilli* - An infective dose this low is characteristic of highly pathogenic organisms with extreme virulence or unique immune evasion mechanisms, such as **Shigella**. - While **Salmonella typhi** is virulent, its infective dose is not typically in this single-digit range. *One bacillus* - It is highly improbable for a single bacterium to consistently establish infection and cause a systemic disease like typhoid fever, even for highly virulent pathogens. - Effective infection usually requires a minimum threshold of bacteria to overcome host defenses and replicate.

Question 2256: Xanthogranulomatous pyelonephritis is most characteristically associated with:

A. E. coli
B. Proteus (Correct Answer)
C. Pseudomonas
D. Klebsiella

Explanation: ***Proteus*** - **Proteus mirabilis** is most characteristically associated with xanthogranulomatous pyelonephritis due to its ability to produce **urease**, which hydrolyzes urea to ammonia and carbon dioxide. - This leads to the formation of **struvite stones (magnesium ammonium phosphate)**, causing chronic obstruction and infection, which are key predisposing factors for XGPN. - The classic presentation involves **staghorn calculi** with Proteus infection. *E. coli* - While **E. coli** is the most frequently isolated organism in XGPN cases (found in up to 50% of cases), it is not as specifically associated with the characteristic pathophysiology. - E. coli is the most common cause of uncomplicated UTIs and acute pyelonephritis but lacks the urease production that drives struvite stone formation. *Pseudomonas* - **Pseudomonas aeruginosa** can cause severe and complicated UTIs, especially in catheterized or immunocompromised patients. - However, it is **not typically associated** with the urease activity and struvite stone formation that are hallmark features predisposing to XGPN. *Klebsiella* - **Klebsiella pneumoniae** can cause UTIs and may produce urease, leading to stone formation. - While it can be a cause of pyelonephritis, **Proteus species** are more consistently and strongly associated with the specific pathogenesis that characterizes XGPN.

Question 2257: "School of fish" appearance with bipolar staining is characteristically shown by?

A. Chlamydia
B. Donovani granulomatis
C. Mycoplasma
D. Hemophilus ducreyi (Correct Answer)

Explanation: ***Hemophilus ducreyi*** - This bacterium, the causative agent of **chancroid**, classically exhibits a **"school of fish" appearance** with prominent **bipolar staining** when viewed on Gram stain. - The organisms appear as pleomorphic Gram-negative coccobacilli arranged in chains or groups, with bipolar staining giving each bacterium a characteristic appearance. - Identification is typically done through **Gram stain** of a sample from the ulcer base, revealing this distinctive morphology. *Chlamydia* - **Chlamydia trachomatis** is an obligate intracellular bacterium and does not display this morphology; it appears as elementary or reticulate bodies within host cells. - It is known for causing sexually transmitted infections such as **chlamydia** and **lymphogranuloma venereum**. - Cannot be visualized by standard Gram staining. *Donovani granulomatis* - This bacterium, now reclassified as **Klebsiella granulomatis**, causes **granuloma inguinale (donovanosis)** and is characterized by the presence of **Donovan bodies** within macrophages. - Donovan bodies are encapsulated rod-shaped bacteria with a characteristic "closed safety pin" appearance found in the cytoplasm of host cells. - Does not show the "school of fish" arrangement. *Mycoplasma* - **Mycoplasma** species are the smallest free-living bacteria lacking a cell wall, which gives them a pleomorphic (variable) shape. - They do not exhibit bipolar staining or the "school of fish" appearance and cannot be visualized by Gram stain. - Associated with conditions like **atypical pneumonia** and **urethritis**.

Question 2258: Craigie tube is used to differentiate between:

A. Capsulated and non-capsulated strains
B. Virulent and avirulent strains
C. Motile and non-motile strains (Correct Answer)
D. Rough and smooth strains

Explanation: ***Motile and non-motile strains*** - The **Craigie tube method** is a specific microbiological technique designed to demonstrate bacterial motility. - A small tube is inserted into a semi-solid agar medium; motile bacteria will grow out of the inserted tube into the surrounding medium, while non-motile bacteria will only grow within the tube. *Capsulated and non-capsulated strains* - Differentiation between capsulated and non-capsulated strains typically involves **negative staining** (e.g., India ink) or specific **serological tests**, not the Craigie tube. - The presence of a capsule is a virulence factor, but the Craigie tube does not directly assess this structure. *Virulent and avirulent strains* - Distinguishing virulent from avirulent strains usually requires **animal inoculation studies**, assessment of specific **virulence factors**, or molecular methods. - The Craigie tube tests for motility, which is a characteristic that may or may not correlate with virulence depending on the specific bacterial species. *Rough and smooth strains* - Rough and smooth colony morphology is observed visually on agar plates and is related to the presence or absence of **O-antigen** in Gram-negative bacteria. - This characteristic is assessed by direct colony observation, often related to phase variation, and is not determined using a Craigie tube.

Question 2259: Following are true of Gram negative bacterial cell wall compared to Gram positive bacteria except:

A. Thinner
B. Presence of lipopolysaccharide
C. Presence of outer membrane
D. Presence of Teichoic acid (Correct Answer)

Explanation: ***Presence of Teichoic acid*** - **Teichoic acid** is a unique component of the cell wall in **Gram-positive bacteria**, playing a role in cell wall structure and antigenicity. - Its presence is **not a characteristic of Gram-negative bacteria**, making this statement the exception. *Thinner* - The cell wall of **Gram-negative bacteria** is indeed **thinner** than that of Gram-positive bacteria. - This **thin peptidoglycan layer** (2-3 nm) is much less substantial compared to the thick peptidoglycan layer (20-80 nm) of Gram-positive bacteria. *Presence of lipopolysaccharide* - **Lipopolysaccharide (LPS)**, or endotoxin, is a characteristic component of the **outer membrane** of Gram-negative bacteria. - LPS contributes to the **pathogenicity** of Gram-negative bacteria and is absent in Gram-positive bacteria. *Presence of outer membrane* - **Gram-negative bacteria** have a unique **outer membrane** that lies external to the thin peptidoglycan layer. - This outer membrane contains LPS and porins, and is a distinguishing feature **absent in Gram-positive bacteria**, which have only a single cytoplasmic membrane.

Question 2260: Which one of the following gram-positive organisms is the most common cause of UTI among sexually active women?

A. Staphylococcus aureus
B. Staphylococcus saprophyticus (Correct Answer)
C. Enterococcus
D. Staphylococcus epidermidis

Explanation: ***Staphylococcus saprophyticus*** - This organism is a common cause of **urinary tract infections (UTIs)**, particularly in sexually active young women. - It is known for its ability to adhere to uroepithelial cells and is **novobiocin-resistant**, distinguishing it from other coagulase-negative staphylococci. *Staphylococcus aureus* - While *Staphylococcus aureus* can cause UTIs, it's typically associated with **catheter-associated infections** or hematogenous spread, not primarily UTIs in sexually active women. - It is a **coagulase-positive** staphylococcus and often linked to more severe infections. *Enterococcus* - **Enterococci**, such as *Enterococcus faecalis*, are a common cause of UTIs, especially in hospitalized patients or those with urinary tract abnormalities. - However, they are less frequently the primary cause in otherwise healthy, sexually active women compared to *S. saprophyticus*. *Staphylococcus epidermidis* - *Staphylococcus epidermidis* is part of the normal skin flora and is a common **contaminant** in urine cultures. - It infrequently causes true UTIs and is generally only pathogenic in immunocompromised individuals or those with **indwelling medical devices**.

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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