Bacteriology Practice Questions

Q: Which mycobacteria produces an exotoxin?

M. ulcerans. **Explanation:** The correct answer is **C. M. ulcerans**. **1. Why M. ulcerans is correct:** *Mycobacterium ulcerans* is unique among mycobacteria because it is the only species known to produce a potent exotoxin called **Mycolactone**. This lipid-like toxin is polyketide-derived and possesses potent **necrotizing and immunosuppressive** properties. It causes extensive destruction of the skin and soft tissue by inducing apoptosis in mammalian cells. Clinically, this results in **Buruli ulcer**, characterized by large, painless, necrotic ulcers with deeply undermined edges. **2. Why the other options are incorrect:** * **M. tuberculosis (A) and M. bovis (D):** These are members of the *M. tuberculosis* complex. Their pathogenicity is primarily due to their ability to survive intracellularly within macrophages and the resulting delayed-type hypersensitivity (Type IV) immune response. They do **not** produce exotoxins. * **M. marinum (B):** This is a photochromogen associated with "fish tank granuloma." While it causes skin lesions, its pathogenesis involves intracellular growth and granuloma formation rather than toxin production. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mycolactone:** Remember this specific name; it is the key virulence factor for *M. ulcerans*. * **Buruli Ulcer:** It is the third most common mycobacterial disease in immunocompetent hosts (after TB and Leprosy). * **Undermined Edges:** A classic descriptive term for the ulcers produced by *M. ulcerans*. * **Temperature Sensitivity:** Like *M. leprae*, *M. ulcerans* prefers cooler temperatures (28–33°C), which is why it primarily affects the skin of the extremities.

Q: Which of the following bacteria are bounded by a membrane containing sterols?

Mycoplasma. **Explanation:** The correct answer is **Mycoplasma**. **1. Why Mycoplasma is correct:** The defining characteristic of the genus *Mycoplasma* (and *Ureaplasma*) is the **complete absence of a peptidoglycan cell wall**. To compensate for the lack of structural rigidity provided by a cell wall, their cell membrane is uniquely enriched with **sterols** (cholesterol). These sterols provide osmotic stability and mechanical strength, preventing the bacteria from bursting. It is important to note that *Mycoplasma* cannot synthesize sterols themselves; they must acquire them from the external environment (e.g., serum-enriched growth media). **2. Why the other options are incorrect:** * **Klebsiella (A) & Staphylococcus (B):** These are typical Gram-negative and Gram-positive bacteria, respectively. They possess a rigid cell wall. Their cytoplasmic membranes consist of phospholipids and proteins but **do not contain sterols**. * **Mycobacteria (C):** While *Mycobacteria* have a unique, complex cell wall rich in **mycolic acids** (long-chain fatty acids), their underlying plasma membrane lacks sterols. **3. NEET-PG High-Yield Clinical Pearls:** * **Antibiotic Resistance:** Because they lack a cell wall, *Mycoplasma* are **innately resistant** to all beta-lactam antibiotics (Penicillins, Cephalosporins) which target cell wall synthesis. * **Pleomorphism:** The lack of a cell wall makes them highly pleomorphic (variable shapes) and allows them to pass through filters (0.45 μm) that typically trap other bacteria. * **Gram Staining:** They do not stain with Gram stain; **Giemsa or Wright stains** are preferred. * **Culture:** They produce characteristic **"fried-egg" colonies** on specialized media (e.g., PPLO agar/Eaton’s agar).

Q: Streptococcal toxic shock syndrome is due to the liberation of which substance?

Pyrogenic exotoxin. **Explanation:** Streptococcal Toxic Shock Syndrome (STSS) is a severe, life-threatening condition primarily caused by **Group A Streptococcus (Streptococcus pyogenes)**. The pathogenesis is driven by the release of **Streptococcal Pyrogenic Exotoxins (SpeA, SpeB, and SpeC)**. These toxins act as **superantigens**, which bypass normal antigen processing by directly binding to the MHC class II molecules on antigen-presenting cells and the Vβ region of T-cell receptors. This leads to a massive, non-specific activation of T-cells and a "cytokine storm" (IL-1, IL-6, TNF-α, and IFN-γ), resulting in capillary leak, hypotension, and multi-organ failure. **Analysis of Options:** * **A. TSS-1 toxin:** Also known as TSST-1, this is the superantigen responsible for Toxic Shock Syndrome caused by *Staphylococcus aureus*, not Streptococcus. * **B. Enterotoxin:** These are heat-stable toxins produced by *Staphylococcus aureus* that cause food poisoning; they are not the primary cause of STSS. * **D. Endotoxin:** These are Lipopolysaccharides (LPS) found in the cell walls of **Gram-negative bacteria**. *Streptococcus* is a Gram-positive organism and does not possess endotoxins. **High-Yield Clinical Pearls for NEET-PG:** * **STSS vs. Staph TSS:** STSS (Streptococcal) is often associated with **necrotizing fasciitis** and bacteremia, whereas Staph TSS is frequently associated with tampon use or focal infections and is less commonly bacteremic. * **M-Protein:** The most important virulence factor of *S. pyogenes* that inhibits phagocytosis; strains with M-types 1 and 3 are most commonly linked to STSS. * **Drug of Choice:** While Penicillin is used for the bacteria, **Clindamycin** is added in STSS because it inhibits protein synthesis, thereby shutting down toxin production.

Q: Vibrio cholera is able to colonize the gastrointestinal tract due to which of the following mechanisms?

Motility. **Explanation:** *Vibrio cholerae* is a highly motile, Gram-negative bacterium characterized by a single polar flagellum. Its ability to colonize the small intestine is primarily dependent on its **darting motility**. 1. **Why Motility is Correct:** To establish infection, *V. cholerae* must penetrate the thick mucus layer covering the intestinal epithelium. Its rapid motility, often described as "darting," allows the organism to swim through this mucus barrier to reach the underlying enterocytes. Once it reaches the surface, it uses **Toxin-Coregulated Pili (TCP)** to adhere and form microcolonies. Without motility, the bacteria would be swept away by intestinal peristalsis before they could attach. 2. **Why Other Options are Incorrect:** * **Acid resistance:** *Vibrio cholerae* is highly sensitive to gastric acid (acid-labile). This is why a high infectious dose ($10^8$ organisms) is required to cause disease, as most are killed in the stomach. * **Bile resistance:** While *Vibrios* can tolerate bile (and are often grown on TCBS agar which contains bile salts), this is a survival trait rather than the primary mechanism for active colonization of the mucosal surface. * **Binding to specific receptors:** While adhesion is necessary, it occurs *after* the bacteria have successfully navigated the mucus layer via motility. **High-Yield Clinical Pearls for NEET-PG:** * **Hanging Drop Preparation:** Used to demonstrate the characteristic "darting motility" of *V. cholerae*. * **TCBS Agar:** The selective medium of choice; *V. cholerae* produces yellow colonies due to sucrose fermentation. * **Cholera Toxin (Choleragen):** An $A_5B$ toxin that increases intracellular **cAMP**, leading to the classic "rice-water stools." * **String Test:** Used for rapid biochemical identification of *Vibrio* species.

Q: Which of the following is not a rapidly growing atypical mycobacteria causing lung infections?

M. kansasii. **Explanation:** The classification of Non-Tuberculous Mycobacteria (NTM) is primarily based on the **Runyon Classification**, which categorizes them into four groups based on their growth rate and pigment production. **Why M. kansasii is the correct answer:** *M. kansasii* belongs to **Runyon Group I (Photochromogens)**. These are **slow-growing** mycobacteria (taking >7 days to form colonies) that produce pigment only when exposed to light. While it is a common cause of chronic lung disease resembling tuberculosis, it is not a rapid grower. **Analysis of Incorrect Options (Rapid Growers):** Options A, B, and C all belong to **Runyon Group IV (Rapid Growers)**. These organisms typically produce visible colonies on solid media within **3 to 7 days**. * **M. fortuitum:** Often associated with post-surgical wound infections and skin/soft tissue infections, but can cause pulmonary disease. * **M. chelonae:** Frequently implicated in skin, bone, and joint infections following trauma or medical procedures. * **M. abscessus:** The most pathogenic of the rapid growers; it is highly resistant to antibiotics and is a significant cause of chronic lung disease, especially in patients with cystic fibrosis or bronchiectasis. **High-Yield Clinical Pearls for NEET-PG:** * **Runyon Group IV Mnemonic:** Remember **"FAC"** (Fortuitum, Abscessus, Chelonae) for Rapid growers. * **M. kansasii Treatment:** It is unique among NTMs because it is highly sensitive to **Rifampin**, making the treatment regimen similar to standard TB therapy. * **M. marinum:** Known as the "Fish tank granuloma" agent (Group I). * **M. scrofulaceum:** Common cause of cervical lymphadenitis in children (Group II - Scotochromogen).

Question 1

Rice watery stool is indicative of which organism?

Accepted Answer

Vibrio cholerae O1

Answer

Vibrio cholerae O139

Answer

Vibrio vulnificus

Answer

Shigella

Question 2

All of the following are TRUE about ASO titre, EXCEPT?

Accepted Answer

ASO titre is included in the major criteria of the Jones criteria.

Answer

ASO titre can be increased in school children.

Answer

ASO titre may be negative in post-streptococcal glomerulonephritis.

Answer

ASO titre may not be elevated in 20% of cases of carditis.

Question 3

Which mycobacteria produces an exotoxin?

Accepted Answer

M. ulcerans

Answer

M. tuberculosis

Answer

M. ajarinum

Answer

M. bovis

Question 4

Which of the following bacteria are bounded by a membrane containing sterols?

Accepted Answer

Mycoplasma

Answer

Klebsiella

Answer

Staphylococcus

Answer

Mycobacteria

Question 5

Which of the following is TRUE regarding Neisseria meningitidis infection?

Accepted Answer

It is the most common cause of meningitis in children.

Answer

All strains are uniformly sensitive to sulfonamides.

Answer

Vaccines are contraindicated in immunosuppressed individuals.

Answer

In India, serotype B is the most common cause.

Question 6

A 22-year-old software engineer was brought in an unconscious state to the casualty. Clinical examination revealed features suggestive of shock, disseminated intravascular coagulation (DIC), and multisystem failure. Cerebrospinal fluid (CSF) and petechial rashes yielded gram-negative diplococci, which subsequently grew on modified Thayer-Martin medium. This isolation is known to be associated with?

Accepted Answer

Waterhouse-Friderichsen syndrome

Answer

Fitz-Hugh-Curtis syndrome

Answer

Job's syndrome

Answer

Toxic shock syndrome

Question 7

Streptococcal toxic shock syndrome is due to the liberation of which substance?

Accepted Answer

Pyrogenic exotoxin

Answer

TSS-1 toxin

Answer

Enterotoxin

Answer

Endotoxin

Question 8

Which of the following statements about Pneumococcus is false?

Accepted Answer

The virulence of pneumococci is dependent only on the production of the capsular polysaccharide.

Answer

Pneumolysin, a thiol-activated toxin, exerts a variety of effects on ciliary and PMNs.

Answer

Autolysin may contribute to the pathogenesis of pneumococcal disease by lysing the bacteria.

Answer

Anticapsular antibodies are serotype-specific.

Question 9

Vibrio cholera is able to colonize the gastrointestinal tract due to which of the following mechanisms?

Accepted Answer

Motility

Answer

Acid resistance

Answer

Bile resistance

Answer

Binding to specific receptors

Question 10

Which of the following is not a rapidly growing atypical mycobacteria causing lung infections?

Accepted Answer

M. kansasii

Answer

M. chelonae

Answer

M. fortuitum

Answer

M. abscessus

Bacteriology — MCQs

Bacteriology — MCQs

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