Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2281: Superantigen is produced by which of the following?

A. Clostridium perfringens
B. Vibrio cholerae
C. Staphylococcus epidermidis
D. Streptococcus pyogenes (Correct Answer)

Explanation: ***Streptococcus pyogenes*** - *Streptococcus pyogenes* produces **superantigens** such as **streptococcal pyrogenic exotoxins (SPE-A, SPE-B, SPE-C)**. - These superantigens cause massive non-specific T-cell activation, leading to **toxic shock syndrome** and **scarlet fever**. - Superantigens bypass normal antigen processing by directly binding MHC class II molecules to T-cell receptors. *Clostridium perfringens* - *Clostridium perfringens* primarily produces **alpha-toxin** (phospholipase C) and other toxins causing **gas gangrene** and **food poisoning**. - It does not produce superantigens. *Vibrio cholerae* - *Vibrio cholerae* produces **cholera toxin**, an **enterotoxin** that causes massive fluid secretion in the intestine, leading to profuse watery diarrhea. - Cholera toxin is not a superantigen; it acts by activating adenylate cyclase through ADP-ribosylation. *Staphylococcus epidermidis* - *Staphylococcus epidermidis* is a common skin commensal that can cause infections related to **medical devices** and **biofilm formation**. - Unlike *Staphylococcus aureus* (which produces TSST-1 and enterotoxins), *S. epidermidis* does not produce superantigens.

Question 2282: Major bacterial enzyme responsible for putrefaction:

A. Protease (Correct Answer)
B. Metalloproteinase
C. Hyaluronidase
D. Collagenase

Explanation: ***Protease*** - **Proteases** break down **proteins** into smaller peptides and amino acids, a key process in **putrefaction** (decomposition of organic matter by microorganisms). - Bacterial proteases are essential for the degradation of **tissue proteins** post-mortem, contributing to the characteristic odor and liquefaction of decaying bodies. *Metalloproteinase* - **Metalloproteinases** are a specific class of proteases that require a **metal ion** (usually zinc) for their catalytic activity. - While they contribute to protein degradation, the general term **protease** encompasses all enzymes involved in protein breakdown during putrefaction, making it a broader and more accurate answer. *Hyaluronidase* - **Hyaluronidase** breaks down **hyaluronic acid**, a main component of the **extracellular matrix**, aiding in bacterial invasion and spread. - Its primary role is in **tissue penetration** rather than the bulk decomposition of proteins characteristic of putrefaction. *Collagenase* - **Collagenase** specifically targets **collagen**, the most abundant protein in the body, breaking it down into smaller peptides. - While important for degrading a major structural protein, it is a specific type of protease, and the general term **protease** better describes the overall enzymatic activity responsible for putrefaction.

Question 2283: Gram stain of CSF shows gram-negative diplococci. Culture grows on Thayer-Martin medium. Most likely organism?

A. L. monocytogenes
B. S. pneumoniae
C. N. meningitidis (Correct Answer)
D. H. influenzae

Explanation: ***N. meningitidis*** - The presence of **gram-negative diplococci** in CSF is a hallmark feature of *Neisseria meningitidis*. - **Thayer-Martin medium** is a selective medium specifically designed for the isolation and growth of *Neisseria* species, inhibiting other bacterial and fungal contaminants. *L. monocytogenes* - *Listeria monocytogenes* is a **gram-positive rod**, which contradicts the gram stain finding of gram-negative diplococci. - It typically causes meningitis in neonates, elderly, and immunocompromised individuals, but its morphology is distinct. *S. pneumoniae* - *Streptococcus pneumoniae* is a **gram-positive coccus**, often appearing in pairs (diplococci) or short chains, but it is gram-positive, not gram-negative. - It is a common cause of bacterial meningitis but does not grow readily on Thayer-Martin medium. *H. influenzae* - *Haemophilus influenzae* is a **pleomorphic gram-negative coccobacillus**, not a diplococcus, and it requires specific growth factors (X and V factors) found in chocolate agar. - While it can cause meningitis, its characteristic morphology and growth requirements differ from the description.

Question 2284: A neonate develops sepsis with organism showing CAMP test positive. Likely organism?

A. S. aureus
B. E. coli
C. Listeria
D. Group B Streptococcus (Correct Answer)

Explanation: ***Group B Streptococcus*** - **Group B Streptococcus (GBS)**, or *Streptococcus agalactiae*, is the **most common cause of neonatal sepsis** and is the **classic organism** associated with a **positive CAMP test**. - The **CAMP test** (Christie-Atkins-Munch-Petersen) detects synergistic hemolysis between the CAMP factor produced by GBS and *Staphylococcus aureus* beta-lysin, resulting in an **arrowhead-shaped zone of enhanced hemolysis**. - GBS is strongly associated with **early-onset neonatal sepsis** (within first 7 days), transmitted vertically during delivery. - When the CAMP test is mentioned in the context of neonatal sepsis, **GBS is the intended answer** due to its classical association and epidemiological importance. *S. aureus* - *Staphylococcus aureus* can cause **sepsis** in neonates but is **CAMP test negative**. - It provides the beta-lysin used in the CAMP test to detect other organisms but does not produce the CAMP factor itself. *E. coli* - *Escherichia coli* is a **Gram-negative rod** and a frequent cause of **neonatal sepsis** and meningitis. - As a Gram-negative bacterium, *E. coli* is **CAMP test negative**. The CAMP test is specific for certain Gram-positive bacteria. *Listeria* - *Listeria monocytogenes* is **also CAMP test positive**, which can cause diagnostic confusion. - However, it causes a distinct clinical pattern: **granulomatosis infantiseptica**, meningoencephalitis, and is associated with **maternal ingestion of contaminated food**. - Listeria is **less common** than GBS as a cause of neonatal sepsis and is not the classic teaching association for CAMP positivity. - The CAMP positivity of Listeria is **weaker** and shows a different pattern (reverse CAMP) compared to the strong, characteristic arrowhead pattern of GBS.

Question 2285: A sex worker presents with purulent urethral discharge and gram-negative diplococci. Diagnosis?

A. N. gonorrhoeae (Correct Answer)
B. T. pallidum
C. C. trachomatis
D. H. ducreyi

Explanation: ***N. gonorrhoeae*** - The presence of **purulent urethral discharge** and **Gram-negative diplococci** on microscopy is highly characteristic of **gonorrhea**. - **Neisseria gonorrhoeae** is a common cause of sexually transmitted infections, particularly in individuals with a higher risk profile like sex workers. *T. pallidum* - **Treponema pallidum** causes syphilis, which typically presents with a **chancre** (primary stage) or a rash (secondary stage), not purulent urethral discharge. - It is a **spirochete** and cannot be visualized as Gram-negative diplococci. *C. trachomatis* - **Chlamydia trachomatis** causes **nongonococcal urethritis**, which often presents with a less purulent or mucoid discharge, and may be asymptomatic. - It is an **obligate intracellular bacterium** and does not appear as Gram-negative diplococci on Gram stain. *H. ducreyi* - **Haemophilus ducreyi** causes **chancroid**, characterized by painful genital ulcers with regional lymphadenopathy. - It would not typically present with purulent urethral discharge and is a **Gram-negative rod**, not diplococci.

Question 2286: Which venereal disease is associated with the 'school of fish' appearance in gram stain?

A. Donovanosis
B. Gonorrhea
C. Chancroid (Correct Answer)
D. Syphilis

Explanation: ***Chancroid*** - **Chancroid**, caused by *Haemophilus ducreyi*, is classically associated with the **"school of fish" appearance** on Gram stain due to the characteristic arrangement of short, plump, gram-negative rods. - This venereal disease presents with **painful genital ulcers** and often **inguinal lymphadenopathy**. *Donovanosis* - Donovanosis, caused by *Klebsiella granulomatis*, is characterized by **Donovan bodies** (large macrophages packed with bacteria) on microscopy, not a "school of fish" pattern. - It presents with **painless, progressive ulcerative lesions** that can be highly vascular. *Gonorrhea* - Gonorrhea, caused by *Neisseria gonorrhoeae*, typically shows **intracellular gram-negative diplococci within neutrophils** on Gram stain. - It commonly causes urethritis, cervicitis, and other mucosal infections, but not the "school of fish" pattern. *Syphilis* - Syphilis is caused by the spirochete *Treponema pallidum*, which is **not visible on Gram stain** and requires darkfield microscopy for visualization. - It presents in stages, with primary syphilis characterized by a **painless chancre**.

Question 2287: A 50-year-old diabetic presents with a foot ulcer. Which pathogen is most likely?

A. Pseudomonas aeruginosa
B. Escherichia coli
C. Candida albicans
D. Staphylococcus aureus (Correct Answer)

Explanation: ***Staphylococcus aureus*** - **_Staphylococcus aureus_** is the most common pathogen isolated from **diabetic foot ulcers** due to its prevalence on the skin and ability to infect compromised tissues. - Diabetic patients are particularly susceptible to **_S. aureus_** infections due to **impaired immune function** and **poor circulation**. *Pseudomonas aeruginosa* - While _Pseudomonas aeruginosa_ can cause foot infections, it is typically associated with **chronic, wet wounds** or those exposed to water, and is less common as a primary pathogen than _S. aureus_. - Infections by _Pseudomonas_ often result in a **greenish discharge** and a characteristic fruity odor, which are not mentioned here. *Escherichia coli* - **_Escherichia coli_** is primarily a cause of **urinary tract infections** and **gastrointestinal infections**. - While it can be found in polymicrobial wound infections, it is not the most likely single pathogen to initiate a diabetic foot ulcer infection. *Candida albicans* - **_Candida albicans_** is a **fungus** that can cause infections, particularly in immunocompromised individuals and in moist areas. - While **fungal infections** can complicate diabetic foot ulcers, it is not the primary bacterial pathogen typically responsible for the initial presentation of such ulcers.

Question 2288: Which of the following bacteria produces a toxin that inhibits protein synthesis?

A. Corynebacterium diphtheriae (Correct Answer)
B. Bacillus anthracis
C. Staphylococcus aureus
D. Vibrio cholerae

Explanation: ***Corynebacterium diphtheriae*** - This bacterium produces the **diphtheria toxin**, which is an **A-B toxin**. - The **A subunit** of the toxin inhibits **protein synthesis** by inactivating elongation factor-2 (EF-2), leading to cell death. *Bacillus anthracis* - Produces **anthrax toxin**, which consists of three components: protective antigen, edema factor, and lethal factor. - The **lethal factor** cleaves specific protein kinases, while the **edema factor** acts as an adenylate cyclase, disturbing cell water balance, but neither directly inhibits protein synthesis in the same manner as diphtheria toxin. *Staphylococcus aureus* - Produces numerous toxins, including **superantigens** (like toxic shock syndrome toxin-1) and **exfoliative toxins**, which cause scaly skin, and **alpha-toxin**, which forms pores in cell membranes. - These toxins have diverse mechanisms of action, but none primarily inhibit **protein synthesis**. *Vibrio cholerae* - Produces **cholera toxin**, an **A-B toxin**, which acts on intestinal cells. - The **A subunit** irreversibly activates **adenylate cyclase**, leading to increased cyclic AMP (cAMP) levels and massive efflux of water and electrolytes, causing severe diarrhea, but it does not inhibit protein synthesis.

Question 2289: A patient presents with bloody diarrhea after eating undercooked meat. Which bacterial pathogen is most likely?

A. Salmonella typhi
B. Shigella dysenteriae
C. Vibrio cholerae
D. Escherichia coli O157:H7 (Correct Answer)

Explanation: ***Escherichia coli O157:H7*** - **Enterohemorrhagic E. coli (EHEC)** is the classic cause of **bloody diarrhea** following consumption of **undercooked ground beef** or hamburgers, producing **Shiga toxins** that cause **hemorrhagic colitis**. - Can lead to serious complications like **hemolytic uremic syndrome (HUS)**, particularly in children and elderly patients. *Salmonella typhi* - **Salmonella typhi** specifically causes **typhoid fever** with sustained fever, headache, and rose spots, not acute bloody diarrhea from undercooked meat. - While non-typhoidal Salmonella can cause gastroenteritis, it typically produces **non-bloody diarrhea** and is more associated with poultry and eggs. *Shigella dysenteriae* - Although it causes **bloody diarrhea** and severe **dysentery**, transmission is primarily **person-to-person** through the fecal-oral route or contaminated water. - Not typically associated with **undercooked meat consumption** but rather with poor sanitation and contaminated produce. *Vibrio cholerae* - Causes **cholera** with characteristic profuse, **watery diarrhea** ("rice-water stools") leading to severe dehydration, not bloody diarrhea. - Transmission occurs through contaminated **water and seafood**, not undercooked meat.

Question 2290: Which of the following organisms causes paroxysmal coughing and is prevented by DPT vaccine?

A. Neisseria meningitidis
B. Corynebacterium diphtheriae
C. Bordetella pertussis (Correct Answer)
D. Haemophilus influenzae

Explanation: ***Bordetella pertussis*** - This bacterium is the causative agent of **pertussis**, commonly known as whooping cough, characterized by severe, **paroxysmal coughing fits** followed by a characteristic \"whooping\" sound. - The **DPT vaccine** (Diphtheria, Pertussis, Tetanus) provides immunity against *Bordetella pertussis*, preventing this highly contagious respiratory infection. *Neisseria meningitidis* - This organism causes **meningitis** and **meningococcemia**, which are characterized by fever, headache, stiff neck, and rash, not paroxysmal coughing. - While there are vaccines for *Neisseria meningitidis*, it is not covered by the DPT vaccine. *Corynebacterium diphtheriae* - This bacterium causes **diphtheria**, a disease marked by a **pseudomembrane** formation in the throat, which can lead to airway obstruction and systemic toxin effects like myocarditis. - Although the 'D' in DPT stands for Diphtheria and this organism is prevented by the DPT vaccine, **diphtheria does not present with paroxysmal coughing** as its characteristic feature. *Haemophilus influenzae* - This bacterium can cause a variety of infections, including **meningitis**, **epiglottitis**, and **otitis media**, but not typically paroxysmal coughing. - The **Hib vaccine** specifically targets *Haemophilus influenzae* type b, not the DPT vaccine.

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