Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2201: Infection occurring after human bite is?

A. Anaerobic streptococci (Correct Answer)
B. Spirochetes
C. Staphylococcus
D. Candida

Explanation: **Anaerobic streptococci** - Human bite infections are characteristically **polymicrobial**, with **anaerobic bacteria being the most predominant** organisms isolated from these wounds. - **Anaerobic streptococci** (including *Peptostreptococcus* species) are commonly found in the human oral flora and are frequently implicated in infections resulting from human bites. - These bacteria thrive in the **anaerobic (low oxygen)** environment created in deep puncture wounds caused by bites. - Other important anaerobes include *Fusobacterium*, *Prevotella*, and *Porphyromonas* species. *Spirochetes* - **Spirochetes** are spiral-shaped bacteria, and while some (*Treponema denticola*) are part of the normal oral flora, they are not the primary cause of acute infections following human bites. - They are more commonly associated with conditions like **syphilis** (*Treponema pallidum*) or **Lyme disease** (*Borrelia burgdorferi*), which are not typically transmitted via human bites in this manner. *Staphylococcus* - **Staphylococcus aureus** is commonly isolated from human bite wounds and can cause significant soft tissue infections. - However, human bite infections are characteristically **polymicrobial with predominance of anaerobic bacteria**, making anaerobic streptococci more characteristic than staphylococci alone. - Note: ***Eikenella corrodens*** is considered pathognomonic for human bite infections but is an **aerobic** organism. *Candida* - **Candida** is a genus of yeast (fungus) and a common cause of fungal infections (e.g., thrush, candidiasis). - While *Candida* can be found in the mouth, it is **not a primary pathogen** causing acute bacterial infection following a human bite wound.

Question 2202: Causative organism of rheumatic fever is:

A. Group B Streptococci
B. Group D Streptococci
C. Group A Streptococci (Correct Answer)
D. Staphylococci

Explanation: ***Group A Streptococci*** - **Rheumatic fever** is a serious inflammatory complication that can develop after an untreated or inadequately treated infection of the throat with **streptococcus pyogenes**, which is also known as **Group A Streptococcus (GAS)**. - The immune response to GAS infection mistakenly attacks the body's own tissues, particularly the heart, joints, brain, and skin. *Group B Streptococci* - **Group B Streptococcus (GBS)** is primarily known for causing serious infections in **newborns** (e.g., sepsis, meningitis) and can cause infections in pregnant women. - It is not associated with the development of rheumatic fever. *Group D Streptococci* - **Group D Streptococci** encompasses species such as *Enterococcus faecalis* and *Streptococcus gallolyticus* (formerly *Streptococcus bovis*). - These are typically associated with **urinary tract infections**, **endocarditis**, and **colorectal cancer** (in the case of *S. gallolyticus*), not rheumatic fever. *Staphylococci* - **Staphylococci**, such as *Staphylococcus aureus*, cause a wide range of infections including **skin infections**, **pneumonia**, **sepsis**, and **toxic shock syndrome**. - They are not the causative agents of rheumatic fever; this condition is specifically linked to streptococcal infections.

Question 2203: What is the etiological agent for buruli ulcer which usually occurs in the tropics?

A. Mycobacterium fortuitum
B. Mycobacterium marinum
C. Mycobacterium kansasii
D. Mycobacterium ulcerans (Correct Answer)

Explanation: ***Mycobacterium ulcerans*** - **Buruli ulcer** (also known as Bairnsdale ulcer or Daintree ulcer) is caused by infection with **Mycobacterium ulcerans**. - This bacterium produces a toxin called **mycolactone**, which destroys tissue and suppresses the immune system, leading to characteristic skin lesions. *Mycobacterium fortuitum* - *Mycobacterium fortuitum* is a **rapidly growing mycobacterium** that can cause skin and soft tissue infections, especially after trauma or surgery. - However, it is not the etiologic agent for **Buruli ulcer**, which is associated with a specific slow-growing mycobacterium. *Mycobacterium marinum* - *Mycobacterium marinum* is known to cause **"fish tank granuloma"** or "swimming pool granuloma," an infection acquired from contaminated water, typically presenting as skin lesions on extremities. - While it is a **nontuberculous mycobacterium** with environmental reservoirs, it is not responsible for **Buruli ulcer**. *Mycobacterium kansasii* - *Mycobacterium kansasii* is a **nontuberculous mycobacterium** primarily associated with pulmonary disease, resembling tuberculosis, particularly in individuals with compromised immune systems. - It is not the causative agent of **Buruli ulcer**, which has distinct clinical and epidemiological characteristics.

Question 2204: All of the following organisms are known to survive intracellularly except

A. Neisseria meningitidis (Correct Answer)
B. Legionella pneumophila
C. Streptococcus pyogenes
D. Salmonella Typhi

Explanation: ***Neisseria meningitidis*** - While *Neisseria meningitidis* can be found in the **phagosomes of neutrophils** in infected individuals, it is not considered a classic intracellular pathogen that replicates significantly or survives for extended periods within host cells. - Its primary mechanism of pathogenesis involves replication extracellularly and the production of a capsule to evade phagocytosis. *Legionella pneumophila* - *Legionella pneumophila* is a **facultative intracellular pathogen** known to replicate within **macrophages** and **amoebae** in aquatic environments. - Its ability to survive and multiply inside host cells is crucial for its virulence and persistence. *Streptococcus pyogenes* - *Streptococcus pyogenes* (Group A Streptococcus) can **invade and survive within non-phagocytic cells** such as epithelial cells, which may contribute to persistent infections and immune evasion. - This intracellular survival mechanism can make it more challenging for antibiotics and the immune system to eradicate the bacteria. *Salmonella Typhi* - *Salmonella Typhi* is a well-known **intracellular pathogen** that survives and replicates within **macrophages and other phagocytic cells** in the host. - This intracellular lifestyle allows it to evade immune surveillance and disseminate throughout the body, leading to systemic infections like typhoid fever.

Question 2205: Most common serotype of Listeria causing infections:

A. 1/2c
B. 1/3b
C. 1/2a (Correct Answer)
D. 1/3a

Explanation: ***1/2a*** - **Listeria monocytogenes serotype 1/2a** is one of the three major serotypes (1/2a, 1/2b, and 4b) responsible for the vast majority of human listeriosis cases. - **Serotype 1/2a** is the most frequently isolated serotype from food sources and is commonly associated with sporadic cases of invasive listeriosis. - Together with serotypes 1/2b and 4b, serotype 1/2a accounts for over 95% of clinical isolates worldwide. - While serotype 4b is more commonly associated with epidemic outbreaks and severe disease, **1/2a predominates in sporadic cases** and environmental/food contamination. *1/2c* - **Serotype 1/2c** is a relatively uncommon serotype in human listeriosis. - It accounts for a small minority of clinical cases compared to the major serotypes. - This serotype is less frequently isolated from both food products and clinical specimens. *1/3b* - **Serotype 1/3b** is a rare serotype of Listeria monocytogenes in human infections. - It represents a very small fraction of clinical isolates and is not considered a major pathogenic serotype. - This serotype is infrequently encountered in outbreak investigations. *1/3a* - **Serotype 1/3a** is another uncommon serotype with minimal clinical significance. - It accounts for a negligible proportion of human listeriosis cases. - This serotype is rarely isolated from clinical specimens or implicated in disease.

Question 2206: Mammalian reservoir for R. prowazekii ?

A. Rodents
B. Humans (Correct Answer)
C. Cattle
D. Dog

Explanation: ***Humans*** - Humans are the primary reservoir for **epidemic typhus**, caused by *Rickettsia prowazekii*. - The disease is typically transmitted among humans by the human **body louse** (*Pediculus humanus corporis*). *Rodents* - Rodents are common reservoirs for other rickettsial diseases, such as **murine typhus** (*R. typhi*), which is transmitted by **fleas**. - While some **flying squirrels** are noted reservoirs for sylvatic typhus caused by *R. prowazekii*, humans remain the main reservoir for epidemic forms. *Cattle* - Cattle are generally not considered a mammalian reservoir for *Rickettsia prowazekii*. - They are more commonly associated with diseases like **Q fever** caused by **Coxiella burnetii**, which is a related but distinct pathogen. *Dog* - Dogs are not a known reservoir for *Rickettsia prowazekii*. - They can be infected with other rickettsial species, such as *Rickettsia rickettsii* (Rocky Mountain spotted fever) or *Ehrlichia canis*, but not epidemic typhus.

Question 2207: Dienes' method is used for -

A. Chlamydiae
B. Mycoplasma (Correct Answer)
C. Plague
D. Diphtheria

Explanation: ***Mycoplasma*** - **Dienes' stain** (or Dienes' method) is a specialized stain used for the **direct microscopic detection of Mycoplasma species** in clinical samples or cultures. - It differentiates Mycoplasma colonies from other bacterial contaminants by staining their characteristic **"fried egg" appearance** blue. *Chlamydiae* - **Chlamydiae** are **obligate intracellular bacteria** and are typically identified using cell culture, immunofluorescence, or molecular methods like PCR. - Dienes' stain is **not effective** for visualizing Chlamydiae due to their small size and intracellular replication cycle. *Plague* - **Plague**, caused by *Yersinia pestis*, is primarily diagnosed by Gram staining (showing **Gram-negative bacilli** with bipolar staining) and culture. - Dienes' stain has **no role** in the diagnosis of plague. *Diphtheria* - **Diphtheria**, caused by *Corynebacterium diphtheriae*, is diagnosed by culture on selective media like tellurite agar and demonstration of toxin production. - While Gram staining is used (showing **Gram-positive bacilli**), Dienes' stain is **not relevant** for its identification.

Question 2208: Incubation period of Lymphogranuloma venereum is:

A. 60-90 days
B. 10-30 days (Correct Answer)
C. 5-7 days
D. 1-5 days

Explanation: ***10-30 days*** - The incubation period for **Lymphogranuloma venereum (LGV)**, caused by specific serovars of *Chlamydia trachomatis*, typically ranges from **10 to 30 days**. - This period aligns with the presentation of the primary lesion, often a transient, painless papule or ulcer. *60-90 days* - An incubation period of **60-90 days** is excessively long for the primary stage of LGV. - Such an extended period is more typical for diseases with a very slow progression or long latency, not the initial manifestation of LGV. *5-7 days* - While some sexually transmitted infections (STIs) have an incubation period of **5-7 days**, this is too short for LGV. - This shorter timeframe is more characteristic of diseases like **genital herpes** or **chancroid**. *1-5 days* - An incubation period of **1-5 days** is very short and not consistent with LGV. - This rapid onset suggests infections like **gonorrhea** or early stages of some bacterial infections.

Question 2209: Which among the following is the etiological agent of 'pneumonia alba'?

A. Staphylococci
B. Streptococci
C. Klebsiella
D. Treponema pallidum (Correct Answer)

Explanation: ***Treponema pallidum*** - **Pneumonia alba** is a specific manifestation of **congenital syphilis**, caused by the spirochete **Treponema pallidum**. - It presents as a diffuse **interstitial pneumonia** with thickened alveolar septa and edema, giving the lungs a pale, firm appearance. *Staphylococci* - **Staphylococcus aureus** can cause various forms of pneumonia, including necrotizing pneumonia, but it is not associated with the distinct pathology of **pneumonia alba**. - Staphylococcal pneumonia is typically characterized by **abscess formation** and **purulent exudate**, rather than diffuse interstitial changes. *Streptococci* - **Streptococcus pneumoniae** is the most common cause of community-acquired bacterial pneumonia, often leading to **lobar pneumonia**. - Streptococcal pneumonia presents with **consolidation** and **inflammatory infiltrates**, which are distinct from the pale, edematous appearance of pneumonia alba. *Klebsiella* - **Klebsiella pneumoniae** is known for causing **severe, necrotizing pneumonia**, particularly in immunocompromised individuals or those with alcoholism. - It is characterized by **thick, tenacious sputum** (currant jelly sputum) and tissue destruction, which is not consistent with the pathology of pneumonia alba.

Question 2210: Fermentation of glycerol is the basis for classification of:

A. Neisseria
B. Enterobacteriaceae
C. Streptococci
D. Yersinia (Correct Answer)

Explanation: ***Yersinia*** - Fermentation of **glycerol** to produce acid (seen as a color change in specific media) is a key biochemical test used to differentiate and classify species within the **Yersinia** genus. - This metabolic characteristic, along with other biochemical reactions, helps in identifying pathogenic species. **Yersinia pestis** is glycerol-**negative**, while **Yersinia pseudotuberculosis** is glycerol-**positive**, making this test crucial for species differentiation. *Neisseria* - **Neisseria** species are primarily classified by their ability to ferment various **sugars** (glucose, maltose, sucrose, lactose), not glycerol. - They are typically Gram-negative cocci that are oxidase-positive, and their sugar fermentation patterns are critical for species identification (e.g., *N. gonorrhoeae* ferments glucose, *N. meningitidis* ferments glucose and maltose). *Enterobacteriaceae* - The **Enterobacteriaceae** family is a large and diverse group of Gram-negative rods, and while they can ferment many carbohydrates, glycerol fermentation is not a primary or sole basis for their classification. - Classification within **Enterobacteriaceae** relies on a wide array of biochemical tests including lactose fermentation, indole production, citrate utilization, H2S production, and urease activity. *Streptococci* - **Streptococci** are Gram-positive cocci classified primarily by their **hemolytic patterns** on blood agar (alpha, beta, gamma) and by **Lancefield serogrouping** based on cell wall antigens. - While they ferment various carbohydrates, glycerol fermentation is not a standard or distinguishing feature used for their classification.

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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