Bacteriology — MCQs
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Which of the following statements about diphtheria toxin is true?
Which of the following statements is true about Campylobacter jejuni?
Primary mode of transmission of Listeria monocytogenes is:
What is the primary use of PLET medium in microbiology?
What is the most common type of Staphylococcus aureus causing human infections in community settings?
What is the mechanism of action in the pathogenesis of Pseudomembranous colitis caused by C. difficile?
Which strain of E. coli is primarily associated with hemolytic uremic syndrome?
Which of the following bacteria can cause enteric fever?
Double zone of hemolysis is seen in ?
On which medium is the earliest growth of diphtheria detected?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 2351: Which of the following statements about diphtheria toxin is true?
- A. It is heat labile.
- B. It consists of two fragments. (Correct Answer)
- C. It acts through cGMP.
- D. It has a special affinity for the brain.
Explanation: ***It consists of two fragments.*** - Diphtheria toxin is a **protein exotoxin** composed of an **A subunit (active)** and a **B subunit (binding)**, connected by a disulfide bond. - The **A subunit** inhibits protein synthesis by ADP-ribosylation of elongation factor-2 (EF-2), while the **B subunit** facilitates binding and entry into host cells. - This two-fragment structure is the **classic and most characteristic feature** of diphtheria toxin. *It is heat labile.* - While diphtheria toxin can be inactivated by heating at high temperatures (60°C for 10 minutes), it is relatively **more stable** compared to many other bacterial toxins under moderate conditions. - The **two-fragment structure** is considered the more definitively characteristic feature for exam purposes. *It acts through cGMP.* - Diphtheria toxin's mechanism of action involves **ADP-ribosylation of elongation factor-2 (EF-2)**, which inhibits host protein synthesis. - It does not exert its effects through the **cGMP signaling pathway** (cholera toxin acts through cAMP). *It has a special affinity for the brain.* - Diphtheria toxin primarily affects tissues with high protein synthesis turnover, such as the **heart (myocarditis)** and **nerves (peripheral neuropathy)**, rather than the brain. - Neurological complications are typically **peripheral neuropathies**, not central nervous system effects.
Question 2352: Which of the following statements is true about Campylobacter jejuni?
- A. Obligate aerobe
- B. Oxidase negative
- C. Grows at 42°C (Correct Answer)
- D. Non-motile
Explanation: ***Grows at 42°C*** - *Campylobacter jejuni* is thermophilic, meaning it **prefers and grows optimally at warmer temperatures**, specifically around 42°C. - This characteristic is often exploited in selective culture media used for its isolation from clinical samples. *Obligate aerobe* - *Campylobacter jejuni* is not an obligate aerobe, but rather a **microaerophilic** organism, meaning it requires a reduced oxygen atmosphere (typically 5-10% oxygen) for growth. - Obligate aerobes require normal atmospheric oxygen levels (around 21%) to survive and grow. *Oxidase negative* - *Campylobacter jejuni* is **oxidase-positive**, an important diagnostic characteristic that helps differentiate it from other enteric bacteria. - The oxidase test detects the presence of cytochrome c oxidase, an enzyme involved in the electron transport chain. *Non-motile* - *Campylobacter jejuni* is highly **motile** due to its **single polar flagellum**, which contributes to its ability to colonize and invade the intestinal mucosa. - Motility is a key virulence factor, allowing the bacterium to move through the viscous mucosal layer.
Question 2353: Primary mode of transmission of Listeria monocytogenes is:
- A. Ingestion (Correct Answer)
- B. Inhalation
- C. Skin inoculation
- D. None of the options
Explanation: ***Ingestion*** - *Listeria monocytogenes* is primarily transmitted through the **consumption of contaminated food**, making **ingestion** the main route. - This bacterium can adapt well to cold temperatures and grow in refrigerated foods, such as **deli meats**, **soft cheeses**, and **unpasteurized milk**. *Inhalation* - While possible in specific occupational settings (e.g., slaughterhouse workers exposed to aerosols), **inhalation** is not the primary mode of transmission for the general population. - Respiratory infections with *Listeria* are rare and typically occur secondary to severe systemic infection. *Skin inoculation* - **Skin inoculation** is an uncommon route of transmission for *Listeria monocytogenes* and is not considered a primary mode of spread. - It could theoretically occur through direct contact with infected animal tissues, but this is a rare event. *None of the options* - This option is incorrect because **ingestion** is a well-established and primary mode of transmission for *Listeria monocytogenes*. - The other options are either rare or not the main way this pathogen is acquired by humans.
Question 2354: What is the primary use of PLET medium in microbiology?
- A. Plague
- B. Anthrax (Correct Answer)
- C. Typhoid
- D. Cholera
Explanation: ***Anthrax*** - **Polymyxin-Lysozyme-EDTA-Thallous Acetate (PLET)** medium is a selective medium primarily used for the isolation of *Bacillus anthracis* from clinical or environmental samples. - The selective agents in PLET medium inhibit the growth of most contaminating bacteria while allowing for the growth of *Bacillus anthracis*. *Plague* - The causative agent of plague, *Yersinia pestis*, is typically isolated using specialized media like **Cefsulodin-Irgasan-Novobiocin (CIN) agar**, not PLET medium. - CIN agar is selective for *Yersinia* species, and *Y. pestis* grows as "bull's eye" colonies on it. *Typhoid* - The isolation of *Salmonella Typhi*, the causative agent of typhoid fever, commonly uses selective media such as **Bismuth Sulfite Agar (BSA)** or **Salmonella-Shigella (SS) Agar**. - These media are designed to inhibit coliforms and other enteric bacteria while allowing for the growth of *Salmonella*. *Cholera* - *Vibrio cholerae*, the bacterium responsible for cholera, is typically isolated using **Thiosulfate Citrate Bile Salts Sucrose (TCBS) agar**. - TCBS agar is a highly selective and differential medium that allows for the rapid identification of *Vibrio* species.
Question 2355: What is the most common type of Staphylococcus aureus causing human infections in community settings?
- A. VRSA (Vancomycin-Resistant S. aureus)
- B. MSSA (Methicillin-Sensitive S. aureus) (Correct Answer)
- C. MRSA (Methicillin-Resistant S. aureus)
- D. VISA (Vancomycin-Intermediate S. aureus)
Explanation: ***MSSA (Methicillin-Sensitive S. aureus)*** - **Methicillin-sensitive *Staphylococcus aureus*** is still the most prevalent type causing human infections in community settings globally. - While MRSA receives significant attention due to antibiotic resistance, a larger proportion of *S. aureus* infections in the community are caused by MSSA. *MRSA (Methicillin-Resistant S. aureus)* - **Methicillin-resistant *Staphylococcus aureus*** is a significant public health concern due to its resistance to common antibiotics like methicillin and other beta-lactams. - Although prevalent in healthcare settings and an increasing cause of community-associated infections, it does not yet account for the majority of all *S. aureus* infections in the community. *VISA (Vancomycin-Intermediate S. aureus)* - **Vancomycin-intermediate *Staphylococcus aureus*** strains have reduced susceptibility to vancomycin, making them harder to treat. - These strains are relatively rare and primarily associated with healthcare settings or patients with prolonged vancomycin exposure, rather than being common in the general community. *VRSA (Vancomycin-Resistant S. aureus)* - **Vancomycin-resistant *Staphylococcus aureus*** strains are extremely rare and have complete resistance to vancomycin. - They are typically found in patients with severe underlying conditions and extensive antibiotic exposure, making them an uncommon cause of community infections.
Question 2356: What is the mechanism of action in the pathogenesis of Pseudomembranous colitis caused by C. difficile?
- A. Due to invasiveness
- B. Due to endotoxin
- C. Due to exotoxin (Correct Answer)
- D. Due to NM blockade
Explanation: ***Due to exotoxin*** - *Clostridioides difficile* produces two major **exotoxins**, Toxin A (enterotoxin) and Toxin B (cytotoxin), which are responsible for the pathogenesis of **pseudomembranous colitis**. - These toxins disrupt the **cytoskeleton** of intestinal epithelial cells, leading to fluid secretion, inflammation, and the formation of characteristic **pseudomembranes**. *Due to invasiveness* - While some pathogens cause disease through direct invasion of host tissues, *C. difficile* primarily acts through its **secreted toxins** rather than cellular invasion. - The organism itself typically remains in the **intestinal lumen**, colonizing the gut. *Due to endotoxin* - **Endotoxins** are lipopolysaccharides (LPS) found in the outer membrane of **Gram-negative bacteria** and are released upon bacterial lysis. - *C. difficile* is a **Gram-positive bacterium**, and its pathogenesis is not mediated by endotoxins. *Due to NM blockade* - **Neuromuscular (NM) blockade** refers to the inhibition of acetylcholine signaling at the **neuromuscular junction**, leading to muscle paralysis. - This mechanism is characteristic of certain neurotoxins, such as those produced by *Clostridium botulinum* (botulinum toxin), and is not involved in *C. difficile* pathogenesis.
Question 2357: Which strain of E. coli is primarily associated with hemolytic uremic syndrome?
- A. Enteropathogenic
- B. Enterotoxigenic
- C. Enteroinvasive
- D. Enterohemorrhagic (Correct Answer)
Explanation: ***Enterohemorrhagic*** - **Enterohemorrhagic E. coli (EHEC)**, particularly the O157:H7 serotype, produces **Shiga toxins** (Stx1 and Stx2). - These toxins damage endothelial cells in the kidneys, leading to **hemolytic uremic syndrome (HUS)**, characterized by hemolytic anemia, thrombocytopenia, and acute kidney injury. *Enteropathogenic* - **Enteropathogenic E. coli (EPEC)** causes **diarrhea** in infants by attaching to intestinal cells and effacing microvilli, leading to malabsorption. - It does not produce Shiga toxins and is not associated with HUS. *Enterotoxigenic* - **Enterotoxigenic E. coli (ETEC)** is a common cause of **traveler's diarrhea**, producing **heat-labile (LT)** and **heat-stable (ST)** toxins. - These toxins stimulate fluid and electrolyte secretion in the small intestine, but do not cause HUS. *Enteroinvasive* - **Enteroinvasive E. coli (EIEC)** invades and destroys the colonic epithelium, causing **dysentery** with fever and bloody stools. - It is similar to Shigella in its pathogenic mechanism but is not associated with the development of HUS.
Question 2358: Which of the following bacteria can cause enteric fever?
- A. Salmonella paratyphi A
- B. Salmonella paratyphi B
- C. Salmonella typhi
- D. All of the above (Correct Answer)
Explanation: ***All of the above*** - **Enteric fever** (typhoid fever and paratyphoid fever) is a systemic infection caused by specific serotypes of *Salmonella enterica*. - *Salmonella typhi*, *Salmonella paratyphi A*, and *Salmonella paratyphi B* are all known to cause different forms of enteric fever. *Salmonella typhi* - This is the primary causative agent of **typhoid fever**, the most severe form of enteric fever. - It is responsible for the classic presentation of prolonged fever, headache, abdominal pain, and rose spots rash. *Salmonella paratyphi A* - This serotype is a well-established cause of **paratyphoid fever**, which is clinically similar to typhoid but often milder. - It specifically refers to infections caused by *Salmonella enterica* serovar Paratyphi A. *Salmonella paratyphi B* - Similar to *Salmonella paratyphi A*, this serotype also causes **paratyphoid fever**. - Infections by *Salmonella enterica* serovar Paratyphi B represent another cause of enteric fever with clinical presentation similar to but milder than typhoid.
Question 2359: Double zone of hemolysis is seen in ?
- A. Streptococcus pyogenes
- B. Clostridium perfringens (Correct Answer)
- C. Corynebacterium diphtheriae
- D. Staphylococcus aureus
Explanation: ***Clostridium perfringens*** - **Double zone of hemolysis** is a classic characteristic seen when *Clostridium perfringens* is grown on **blood agar**. - This is due to the synergistic action of two different toxins: the inner zone of complete hemolysis (**beta-hemolysis**) is caused by **alpha-toxin** (lecithinase), and the outer, less complete zone is caused by **theta-toxin**. *Staphylococcus aureus* - *Staphylococcus aureus* typically exhibits a **single zone of beta-hemolysis** (complete hemolysis) on blood agar, not a double zone. - It produces various hemolysins, but their combined effect does not result in the distinct double-zone phenomenon. *Streptococcus pyogenes* - *Streptococcus pyogenes* is known for producing **clear, complete beta-hemolysis** on blood agar due to the action of streptolysin O and S. - It does not produce a double zone of hemolysis; the zone of clearing is uniform around the colonies. *Corynebacterium diphtheriae* - *Corynebacterium diphtheriae* is generally **non-hemolytic** or shows **alpha-hemolysis** (partial hemolysis), which appears as a green discoloration around the colonies. - It is commonly identified by its growth on selective media like tellurite agar, where it forms black colonies.
Question 2360: On which medium is the earliest growth of diphtheria detected?
- A. Potassium tellurite medium with iron
- B. MacConkey agar
- C. Dorset egg medium
- D. Loeffler serum slope (Correct Answer)
Explanation: ***Loeffler serum slope*** - This medium provides rapid early growth of *Corynebacterium diphtheriae*, facilitating quick diagnosis. - It enhances the **pleomorphism** and typical **metachromatic granules** (Babes-Ernst bodies) of diphtheria bacilli, making them observable within **6-8 hours**. *Potassium tellurite medium with iron* - This medium is **selective** for *Corynebacterium diphtheriae* and differentiates it from other bacteria. - While useful for isolation, the earliest growth and colonial morphology are observed later, providing a result in **18-24 hours**. *MacConkey agar* - This is a **selective and differential medium** primarily used for **Gram-negative bacteria**, particularly enteric bacilli. - *Corynebacterium diphtheriae* is a **Gram-positive bacterium** and would not grow well or at all on this medium. *Dorset egg medium* - This is a solid medium used for the **transport and cultivation of various bacteria**, including some mycobacteria. - It is not specifically optimized for the rapid or earliest growth of *Corynebacterium diphtheriae* compared to Loeffler's serum slope.
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