Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2401: What is the most common cause of acute tonsillitis?

A. Streptococcus pneumoniae
B. H. influenzae
C. Staphylococcus aureus
D. Group A beta-hemolytic streptococci (Correct Answer)

Explanation: ***Group A beta-hemolytic streptococci*** - **Group A Streptococcus (GAS)**, specifically *Streptococcus pyogenes*, is the most frequent bacterial cause of **acute tonsillitis** and pharyngitis, especially in school-aged children. - Infection can lead to complications such as **rheumatic fever** and **post-streptococcal glomerulonephritis** if not appropriately treated. *Streptococcus pneumoniae* - While *S. pneumoniae* is a common cause of **otitis media**, **sinusitis**, and **pneumonia**, it is less commonly the primary cause of acute tonsillitis. - It typically causes respiratory infections involving the lower airways or adjacent structures rather than primarily tonsillar inflammation. *H. Influenza* - *Haemophilus influenzae* is a significant pathogen for **otitis media**, **epiglottitis**, and **meningitis**, particularly in unvaccinated children. - It is not a common primary cause of acute tonsillitis, which is predominantly bacterial or viral. *Staphylococcus aureus* - *Staphylococcus aureus* is often associated with **skin and soft tissue infections**, as well as more serious conditions like **sepsis** and **endocarditis**. - Although it can cause pharyngeal infections, it is an infrequent cause of acute tonsillitis compared to Group A Streptococcus.

Question 2402: What is the most common cause of a peritonsillar abscess?

A. Streptococcus pneumoniae
B. Staphylococcus aureus
C. H. influenzae
D. Group A beta-hemolytic streptococcus (Correct Answer)

Explanation: ***Group A beta-hemolytic streptococcus*** - This bacterium, also known as **_Streptococcus pyogenes_**, is the most common cause of **streptococcal pharyngitis**, which is the precursor to most peritonsillar abscesses. - Peritonsillar abscesses are typically a complication of untreated or inadequately treated **tonsillitis**. *Streptococcus pneumoniae* - While **_Streptococcus pneumoniae_** can cause upper respiratory infections like sinusitis and otitis media, it is not the primary cause of peritonsillar abscesses. - Its infections are more commonly associated with **pneumonia** and **meningitis**. *Staphylococcus aureus* - **_Staphylococcus aureus_** is a common cause of skin and soft tissue infections, but it is less frequently isolated as the primary pathogen in peritonsillar abscesses. - It can be involved in **polymicrobial infections** or as a secondary invader. *H. influenzae* - **_Haemophilus influenzae_** can cause various infections, particularly in children, such as otitis media and epiglottitis. - However, it is not considered the most common cause of **peritonsillar abscesses**.

Question 2403: An adult male presents with chronic atrophic gastritis. Growth on Skirrow's medium and a positive rapid urease test were observed. What is the most likely diagnosis?

A. H. pylori (Correct Answer)
B. V. cholerae
C. H. influenzae
D. K. pneumoniae

Explanation: ***H pylori*** - The combination of **chronic atrophic gastritis**, growth on **Skirrow's medium**, and a **positive rapid urease test** is highly characteristic of *Helicobacter pylori* infection. - *H. pylori* is a known cause of **gastritis**, peptic ulcers, and is the only bacterium that produces large amounts of **urease**, which is detected by the rapid urease test. *H. influenzae* - This bacterium is primarily associated with **respiratory tract infections**, meningitis, and otitis media, not gastric conditions. - It does not typically grow on Skirrow's medium and is not known to produce significant urease for a positive rapid urease test. *K. pneumoniae* - *Klebsiella pneumoniae* is a common cause of **pneumonia**, urinary tract infections, and sepsis, with no direct involvement in chronic atrophic gastritis or urease production in the stomach. - It does not typically grow on selective media like Skirrow's, which is designed for microaerophilic organisms. *V. cholerae* - *Vibrio cholerae* is the causative agent of **cholera**, characterized by severe watery diarrhea, and is not associated with gastric inflammation or positive urease tests in this context. - It grows on specific media like TCBS agar and does not cause chronic atrophic gastritis.

Question 2404: Empyema thoracis is most commonly caused by which organism

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

Explanation: ***Streptococcus pneumoniae*** - This is the **most common cause of empyema thoracis**, accounting for **40-70% of all cases** in most published series - It is the leading cause of **community-acquired pneumonia** that progresses to parapneumonic effusion and empyema - The pneumococcal infection leads to **inflammatory exudation** into the pleural space, which can progress through exudative, fibrinopurulent, and organizing phases to form frank empyema - Remains the most frequent causative organism across all age groups in community-acquired empyema *Staphylococcus aureus* - An important but **less common cause** of empyema, accounting for approximately 10-20% of cases - More frequently seen in **specific clinical contexts**: post-influenza pneumonia, hospital-acquired infections, hematogenous spread, and pediatric empyema - Causes **severe, necrotizing infections** with abscess formation and is associated with higher morbidity - While clinically significant, it is **not the most common** overall cause *Pseudomonas* - *Pseudomonas aeruginosa* is primarily associated with **nosocomial empyema** in hospitalized patients - Risk factors include **ventilator-associated pneumonia**, immunosuppression, and **cystic fibrosis** - Accounts for a small percentage of empyema cases overall *Mycobacterium* - *Mycobacterium tuberculosis* causes **tuberculous empyema**, a chronic condition with characteristic **thick pleural fluid** and caseous material - Important in regions with high TB prevalence but represents a distinct entity from acute bacterial empyema - Not a common cause of acute pyogenic empyema in most settings

Question 2405: Which of the following is a saccharolytic species of Clostridium?

A. Clostridium tetani
B. Clostridium septicum (Correct Answer)
C. Clostridium cochlearium
D. None of the options

Explanation: ***Clostridium septicum*** - *Clostridium septicum* is a **saccharolytic** species, meaning it ferments carbohydrates to produce gas, which contributes to the rapid tissue destruction seen in **gas gangrene**. - Its ability to rapidly consume sugars in tissues fuels its invasive growth and toxin production, which is characteristic of its pathogenic mechanism. *Clostridium tetani* - *Clostridium tetani* is a **proteolytic** species, primarily deriving its energy from **protein degradation**. - It does not ferment carbohydrates, and its pathogenicity is mainly due to the production of **tetanospasmin**, a neurotoxin. *Clostridium cochlearium* - *Clostridium cochlearium* is a **proteolytic** species that metabolizes proteins and amino acids. - It is not known for significant saccharolytic activity and is typically found in environments rich in protein. *None of the options* - This option is incorrect because *Clostridium septicum* is indeed a saccharolytic species, as detailed above.

Question 2406: Which of the following bacteria is known to exhibit antigenic variation?

A. Yersinia
B. Bordetella
C. Brucella
D. Borrelia (Correct Answer)

Explanation: ***Borrelia*** - *Borrelia* species, particularly *Borrelia burgdorferi* (causing **Lyme disease**), are known for extensive **antigenic variation** of their outer surface proteins (Osps), especially OspC. - This variation helps the bacteria evade the host's immune response, contributing to persistent infection. *Yersinia* - While *Yersinia* species produce various virulence factors, including proteins that interfere with immune cell function, they are not primarily known for the type of rapid and extensive **antigenic variation**seen in *Borrelia*. - Their immune evasion strategies often involve modifying host cell signaling pathways and resisting phagocytosis. *Bordetella* - *Bordetella pertussis*, causative agent of **whooping cough**, varies its expression of adhesins and toxins through **phase variation**, which is a form of phenotypic switching. - However, this is distinct from the frequent and sequential changes in surface antigens (antigenic variation) observed in *Borrelia*. *Brucella* - *Brucella* species are **intracellular pathogens** that primarily evade the immune system by surviving and replicating within host cells. - They do not typically engage in significant **antigenic variation** of their surface components as a primary immune evasion mechanism.

Question 2407: Which is the most common bacterial organism causing bacterial upper respiratory tract infections (including sinusitis, otitis media, and pharyngitis) in adults?

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

Explanation: ***Streptococcus pneumoniae*** - *Streptococcus pneumoniae* is the **most common bacterial pathogen** causing upper respiratory tract infections overall, including **bacterial sinusitis**, **otitis media**, and **community-acquired pneumonia**. - It is a frequent colonizer of the nasopharynx and leads to infection when host immunity is compromised. - Accounts for the highest burden of bacterial URTIs when considering all anatomical sites. *Haemophilus influenzae* - *Haemophilus influenzae* (particularly non-typeable strains) is the **second most common** cause of bacterial sinusitis and otitis media in adults. - While significant, it is less prevalent overall than *S. pneumoniae* across all URTI types. *Staphylococcus aureus* - *Staphylococcus aureus* primarily causes **skin and soft tissue infections** and device-related infections. - It is **not a common primary pathogen** in typical acute bacterial URTIs, though it may cause secondary infections or colonize the anterior nares. *Streptococcus pyogenes* - *Streptococcus pyogenes* (Group A Streptococcus) is the **most common cause of bacterial pharyngitis** (strep throat) in adults. - However, when considering the **full spectrum of bacterial URTIs** (pharyngitis, sinusitis, otitis media), *S. pneumoniae* has a broader overall impact and higher prevalence across multiple sites.

Question 2408: Color of granules in mycetoma caused by Actinomadura pelletierrii -

A. Black
B. Yellow
C. Red (Correct Answer)
D. Brown

Explanation: ***Red*** - *Actinomadura pelletierrii* is a common cause of **actinomycetoma** and is characterized by producing **red granules** in the lesions. - The color of these granules is a key diagnostic feature, helping differentiate it from other causes of mycetoma. *Black* - **Black granules** are typically associated with **eumycetoma** caused by **dematiaceous fungi** such as *Madurella mycetomatis*. - This color indicates the presence of **melanin**, a pigment produced by these fungal species. *Yellow* - **Yellow granules** are commonly seen in actinomycetoma caused by organisms such as *Nocardia brasiliensis* or sometimes *Actinomadura madurae*. - This color helps differentiate these specific actinomycetes from other types that produce different granule colors. *Brown* - While varying shades of color can occur, **brown granules** are not the characteristic color produced by *Actinomadura pelletierrii*. - Some *Streptomyces* species or certain eumycetoma agents can occasionally produce brownish granules, but red is specific for *A. pelletierrii*.

Question 2409: Who discovered the bacterium Mycobacterium tuberculosis, which causes tuberculosis?

A. Jenner
B. Louis Pasteur
C. Robert Koch (Correct Answer)
D. Lister

Explanation: ***Robert Koch*** - **Robert Koch** discovered the bacterium *Mycobacterium tuberculosis* in **1882**, identifying its role as the causative agent of tuberculosis. - His work was crucial in establishing the germ theory of disease and earned him the Nobel Prize in Physiology or Medicine in 1905. *Louis Pasteur* - **Louis Pasteur** was a pioneer in microbiology, known for his work on **pasteurization**, **vaccinations** (e.g., rabies and anthrax), and disproving spontaneous generation. - While he made significant contributions to understanding infectious diseases, he did not discover *Mycobacterium tuberculosis*. *Lister* - **Joseph Lister** was a British surgeon who revolutionized surgery by introducing **antiseptic techniques** using carbolic acid to prevent infections. - His contributions were fundamental to reducing mortality rates from surgical infections, but he did not discover the tuberculosis bacterium. *Jenner* - **Edward Jenner** is famous for developing the **smallpox vaccine**, which was a groundbreaking achievement in immunology and public health. - His work, though vital for preventing infectious diseases, predates and is unrelated to the discovery of *Mycobacterium tuberculosis*.

Question 2410: Inclusion body containing glycogen is seen in which of the following organisms?

A. Chlamydia trachomatis (Correct Answer)
B. Chlamydia pneumoniae
C. Chlamydia psittaci
D. None of the options

Explanation: ***Chlamydia trachomatis*** - This species is known to form **intracytoplasmic inclusion bodies** that contain **glycogen**. - The presence of this glycogen allows these inclusions to be stained by **iodine**, aiding in laboratory identification. *Chlamydia pneumoniae* - Inclusion bodies of *Chlamydia pneumoniae* are typically **round or pear-shaped** and **do not contain glycogen**. - They are often **vacuolated** and stain poorly with iodine, unlike those of *C. trachomatis*. *Chlamydia psittaci* - The inclusion bodies of *Chlamydia psittaci* are usually **dense and pleiomorphic**, but they **do not contain glycogen**. - They tend to be **larger** and are often found near the host cell nucleus. *None of the options* - This option is incorrect because *Chlamydia trachomatis* specifically forms **glycogen-containing inclusion bodies**. - The presence of glycogen within inclusions is a key distinguishing feature of this species.

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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