Bacteriology Practice Questions

Q: Shiga toxin acts by which mechanism?

Inhibition of protein synthesis. **Explanation:** The Shiga toxin (produced by *Shigella dysenteriae* type 1) and Shiga-like toxins (produced by Enterohemorrhagic *E. coli* or EHEC) are classic **AB5 toxins**. The 'B' subunit binds to the host cell receptor (Gb3), while the 'A' subunit is internalized. Once inside, the A subunit acts as an **N-glycosidase** that cleaves a specific adenine residue from the **28S rRNA** of the **60S ribosomal subunit**. This irreversible damage prevents the binding of aminoacyl-tRNA, thereby **inhibiting protein synthesis** and leading to cell death. **Analysis of Incorrect Options:** * **Options A & B (cAMP/cGMP):** These mechanisms are associated with secretory diarrheas. **Cholera toxin** and *E. coli* **Heat-Labile (LT)** toxin increase **cAMP**, while *E. coli* **Heat-Stable (ST)** toxin increases **cGMP**. These lead to electrolyte imbalance without direct cell death. * **Option C (Leakage of enterocytes):** While *Shigella* causes mucosal destruction, "leakage" is a non-specific term. The primary molecular pathology is the arrest of translation (protein synthesis) which leads to the characteristic bloody diarrhea (dysentery). **High-Yield Pearls for NEET-PG:** * **Target:** 60S ribosomal subunit (specifically 28S rRNA). * **Clinical Correlation:** Shiga toxin is a major trigger for **Hemolytic Uremic Syndrome (HUS)** because Gb3 receptors are highly expressed on renal glomerular endothelium. * **Similar Toxin:** The **Vero toxin** of EHEC (O157:H7) is structurally and functionally identical to Shiga toxin. * **Diphtheria Toxin vs. Shiga Toxin:** Both inhibit protein synthesis, but Diphtheria toxin targets **Elongation Factor-2 (EF-2)** via ADP-ribosylation.

Q: All of the following Salmonellae are motile except?

S. gallinarum pullorum. **Explanation:** The genus *Salmonella* belongs to the family Enterobacteriaceae. A defining characteristic of this genus is that almost all members are **motile** via **peritrichous flagella**. **1. Why Option C is Correct:** *Salmonella gallinarum* and *Salmonella pullorum* (often grouped as the Gallinarum-Pullorum serotypes) are the **only two primary exceptions** to the rule of motility in the genus *Salmonella*. These are non-motile because they lack functional flagella. They are primarily avian pathogens causing fowl typhoid and pullorum disease, respectively. **2. Analysis of Incorrect Options:** * **Option A (S. typhi):** This is the causative agent of Typhoid fever. It is highly motile and possesses the H antigen (flagellar antigen), which is used in the Widal test for diagnosis. * **Option B (S. enteritidis):** A common cause of salmonella food poisoning (gastroenteritis). Like most non-typhoidal salmonellae, it is motile. * **Option C (S. chester):** This is a less common serotype of *Salmonella enterica*, but it follows the general genus characteristic of being motile. **Clinical Pearls for NEET-PG:** * **Exceptions to Motility:** In the Enterobacteriaceae family, the mnemonic **"KYS"** (Klebsiella, Yersinia pestis, and Shigella) represents the non-motile genera. *Salmonella gallinarum-pullorum* is the specific exception within the otherwise motile Salmonella genus. * **Yersinia Exception:** *Yersinia enterocolitica* is unique because it is motile at 25°C but non-motile at 37°C. * **H-Antigen:** Motility in Salmonella is linked to the **H-antigen**. Non-motile strains lack this antigen, which is a critical point for serotyping using the Kauffman-White scheme.

Q: Panton-Valentine leucocidin is seen in infection with which bacterium?

Staphylococci. **Explanation:** **Panton-Valentine Leucocidin (PVL)** is a potent pore-forming cytotoxin produced by certain strains of ***Staphylococcus aureus***. It specifically targets and destroys human polymorphonuclear leukocytes (neutrophils) and macrophages by creating pores in their cell membranes, leading to cell lysis and tissue necrosis. **Why Staphylococci is correct:** PVL is a key virulence factor primarily associated with **Community-Acquired Methicillin-Resistant *Staphylococcus aureus* (CA-MRSA)**. Clinically, PVL-producing strains are notorious for causing severe, recurrent skin and soft tissue infections (like furunculosis) and a highly fatal, necrotizing pneumonia in young, otherwise healthy individuals. **Why other options are incorrect:** * **Streptococci:** While they produce various toxins like Streptolysin O and S, they do not produce PVL. * **Gonococci (*N. gonorrhoeae*):** Their primary virulence factors include pili, Opa proteins, and LOS (lipooligosaccharide), not leucocidins. * **Pneumococci (*S. pneumoniae*):** The hallmark virulence factor is the polysaccharide capsule and the toxin Pneumolysin, which facilitates alveolar damage. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** PVL is a "Bicomponent Toxin" consisting of two proteins: **LukS-PV and LukF-PV**. * **Genetic Basis:** The genes for PVL are carried by a **bacteriophage** (phiSLT), which integrates into the *S. aureus* chromosome. * **Exam Association:** If a clinical vignette describes a young patient with **necrotizing pneumonia** following an influenza-like illness or **recurrent skin abscesses**, always look for PVL-positive *S. aureus*.

Q: A 2-year-old child presents with fever and vomiting, exhibiting neck rigidity. Cerebrospinal fluid analysis reveals over 2000 polymorphs/mm³, protein of 100 mg/dL, and glucose of 10 mg/dL. Gram stain shows Gram-negative coccobacilli, and culture demonstrates growth only on chocolate agar, not on blood agar. What is the causative agent?

Haemophilus influenzae. ### Explanation The clinical presentation of fever, neck rigidity, and CSF findings (high polymorphs, high protein, and very low glucose) is diagnostic of **Acute Bacterial Meningitis**. **Why Haemophilus influenzae is correct:** The key diagnostic clue is the growth pattern: **growth on chocolate agar but not on blood agar**. *H. influenzae* is a fastidious Gram-negative coccobacillus that requires two specific growth factors: **Factor X (Hemin)** and **Factor V (NAD)**. * **Blood Agar:** Contains Factor X but lacks Factor V (as NAD is sequestered inside RBCs and inactivated by blood enzymes). Hence, *H. influenzae* cannot grow alone on blood agar (unless "satellitism" occurs with *S. aureus*). * **Chocolate Agar:** Prepared by heating blood, which lyses RBCs to release both Factor X and V and inactivates the NAD-destroying enzymes, allowing *H. influenzae* to thrive. **Analysis of Incorrect Options:** * **Neisseria meningitidis (A):** While a common cause of meningitis, it is a Gram-negative **diplococcus** (kidney-bean shaped) and can grow on both blood agar and chocolate agar. * **Branhamella (Moraxella) catarrhalis (C):** This is a Gram-negative diplococcus primarily associated with otitis media and respiratory infections, not typically meningitis. It grows well on routine blood agar. * **Legionella pneumophila (D):** This causes pneumonia (Legionnaires' disease), not meningitis. It requires **BCYE (Buffered Charcoal Yeast Extract) agar** for growth, not chocolate agar. **High-Yield Clinical Pearls for NEET-PG:** * **Satellitism:** *H. influenzae* grows on blood agar only near colonies of *Staphylococcus aureus*, which provide the necessary Factor V. * **Quellung Reaction:** Positive for *H. influenzae* type b (Hib) due to its polysaccharide capsule. * **Vaccination:** The Hib conjugate vaccine has significantly reduced the incidence of meningitis in children under 5.

Q: The enzyme glucosyl transferase secreted by Streptococcus mutans synthesizes glucans from what substrate?

Sucrose. **Explanation:** **Streptococcus mutans** is the primary etiological agent of dental caries. Its pathogenicity is largely attributed to its ability to produce an extracellular polysaccharide matrix called **glucan** (dextran). 1. **Why Sucrose is Correct:** *S. mutans* secretes the enzyme **glucosyltransferase (GTF)**. This enzyme specifically utilizes **sucrose** as its substrate. It cleaves the disaccharide sucrose into glucose and fructose. The energy released from breaking the high-energy glycosidic bond of sucrose is used to polymerize the glucose units into long-chain, sticky **glucans**. These glucans act as a "biological glue," allowing the bacteria to adhere firmly to the tooth enamel and form dental plaque (biofilm). 2. **Why Other Options are Incorrect:** * **Glucose and Fructose:** While these are the constituent monosaccharides of sucrose, glucosyltransferase cannot utilize them individually to synthesize glucans. The reaction requires the specific energy-rich bond found only in the disaccharide sucrose. * **Lactose:** This is the sugar found in milk. While oral bacteria can ferment lactose to produce acid, it is not a substrate for the glucosyltransferase enzyme and does not contribute to the formation of sticky dextran matrices. **High-Yield Clinical Pearls for NEET-PG:** * **Dental Caries Mechanism:** *S. mutans* ferments the remaining fructose into **lactic acid**, which demineralizes tooth enamel (critical pH < 5.5). * **Viridans Group:** *S. mutans* belongs to the Viridans group of Streptococci, which are typically $\alpha$-hemolytic and Optochin resistant. * **Infective Endocarditis:** *S. mutans* and *S. sanguinis* can enter the bloodstream during dental procedures, leading to Subacute Bacterial Endocarditis (SBE) on damaged heart valves.

Q: An 18-year-old woman presents with diplopia, dysarthria, and dysphagia 18 hours after consuming homemade preserves. Examination reveals cranial nerve palsies affecting extraocular movements, difficulty swallowing, and voice changes. Muscle strength is 4/5 in the arms and 5/5 in the legs, with normal reflexes. What is the most likely causative organism?

Clostridium botulinum toxin. The clinical presentation describes the classic triad of **Botulism**: symmetric descending paralysis, clear sensorium, and absence of fever [2], [3]. **1. Why Clostridium botulinum is correct:** The patient exhibits the "4 Ds" of botulism: **Diplopia, Dysarthria, Dysphagia, and Dyspnea** [2]. The toxin produced by *C. botulinum* irreversibly binds to the presynaptic nerve terminal, preventing the release of **Acetylcholine (ACh)** at the neuromuscular junction. The history of consuming **homemade preserves** (canned foods) is a classic trigger, as the anaerobic environment allows the spores to germinate and produce toxins. The paralysis is characteristically **descending** (starting with cranial nerves and moving to limbs) and **symmetric** [3]. **2. Why the other options are incorrect:** * **Staphylococcal toxin:** Causes rapid-onset (1–6 hours) nausea, vomiting, and abdominal cramps (food poisoning) but does not cause neurological deficits or paralysis [1]. * **Salmonellosis:** Typically presents with inflammatory diarrhea, fever, and abdominal pain. It does not cause descending paralysis [1]. * **Brucellosis:** A chronic granulomatous disease presenting with undulant fever, arthralgia, and hepatosplenomegaly, usually linked to unpasteurized dairy. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Proteolysis of SNARE proteins, inhibiting ACh release. * **Reflexes:** Unlike Guillain-Barré Syndrome (GBS), which features ascending paralysis and *absent* reflexes, Botulism features descending paralysis and often *preserved* reflexes initially. * **Infant Botulism:** Caused by ingestion of **honey** (spores), leading to "Floppy Baby Syndrome" [3]. * **Treatment:** Immediate administration of Equine Antitoxin (Heptavalent) and supportive respiratory care [3]. Do not wait for lab confirmation.

Q: Which type of E. coli is characterized by the presence of LT (heat-labile) and ST (heat-stable) proteins?

Enterotoxigenic (ETEC). ### Explanation **Enterotoxigenic *E. coli* (ETEC)** is the correct answer because its pathogenicity is primarily driven by two plasmid-encoded enterotoxins: **Heat-Labile (LT)** and **Heat-Stable (ST)**. * **LT (Heat-Labile):** Acts similarly to the Cholera toxin by activating **Adenylate cyclase**, increasing cAMP, which leads to the secretion of water and electrolytes into the intestinal lumen. * **ST (Heat-Stable):** Activates **Guanylate cyclase**, increasing cGMP, which also results in fluid loss. ETEC is the most common cause of **Traveler’s Diarrhea** and childhood diarrhea in developing countries. **Why the other options are incorrect:** * **EIEC (Enteroinvasive):** Pathogenesis involves direct invasion and destruction of the colonic epithelium (similar to *Shigella*), causing inflammatory diarrhea with blood and mucus (dysentery). It does not produce LT/ST. * **EHEC (Enterohemorrhagic):** Characterized by **Shiga-like toxins (Verotoxins)**. It is associated with O157:H7, causing Hemorrhagic Colitis and Hemolytic Uremic Syndrome (HUS). * **EPEC (Enteropathogenic):** Pathogenesis involves **Attachment and Effacement (A/E)** lesions. It lacks toxins and instead uses a bundle-forming pilus (BFP) to disrupt microvilli, leading to malabsorption and infantile diarrhea. **High-Yield NEET-PG Pearls:** * **Mnemonic for Toxins:** **"Labile in the Air (cAMP), Stable on the Ground (cGMP)."** * ETEC diarrhea is typically **watery** (non-inflammatory) and does not show RBCs or WBCs on stool microscopy. * **Colonization Factor Antigens (CFAs)** are the fimbriae ETEC uses to attach to the small intestine. * **EHEC** is the only *E. coli* that **does not ferment Sorbitol** (tested on Sorbitol MacConkey Agar).

Q: Which of the following is true about the cholera vibrio?

It has marked tolerance of alkaline pH.. **Explanation:** *Vibrio cholerae*, the causative agent of cholera, has distinct biochemical and physiological characteristics that are frequently tested in NEET-PG. **1. Why Option C is Correct:** *Vibrio cholerae* is a **strongly halophilic and alkaliphilic** organism. It has a marked tolerance for high pH, growing optimally at a pH of **8.2**, but can survive in environments with a pH as high as 9.2–9.6. This unique physiological trait is exploited in the laboratory using selective enrichment media like **Alkaline Peptone Water (APW)** and solid media like **TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar**, which inhibit most other intestinal commensals that cannot tolerate such alkalinity. **2. Why the Other Options are Incorrect:** * **Option A:** *Vibrio cholerae* is a **strongly aerobic** organism; growth is poor under anaerobic conditions. * **Option B:** The optimum temperature for growth is **37°C** (human body temperature), not 25°C. * **Option D:** While it grows over a wide range, it is **highly sensitive to acid**. Gastric acidity is a primary host defense against cholera; patients with achlorhydria or those taking antacids are at a much higher risk of infection. **3. High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Described as "comma-shaped" (Vibrio) with a single polar flagellum showing **darting motility** (shooting star appearance). * **Culture:** On TCBS agar, it produces distinctive **yellow colonies** due to sucrose fermentation. * **String Test:** Used for biochemical identification; colonies become mucoid and form a "string" when mixed with 0.5% sodium deoxycholate. * **Transport Media:** Venkatraman-Ramakrishnan (VR) medium or Cary-Blair medium are used for stool samples.

Q: Which is the most common causative agent of gas gangrene?

Clostridium perfringens. **Explanation:** **Clostridium perfringens (Option D)** is the correct answer as it is the most common causative agent of gas gangrene (clostridial myonecrosis), isolated in approximately **80–95%** of clinical cases. The pathogenesis is primarily driven by the production of the **Alpha toxin (Lecithinase)**, which causes extensive tissue necrosis, hemolysis, and vascular damage, creating an anaerobic environment conducive to bacterial spread. **Analysis of Incorrect Options:** * **Clostridium novyi (Option B):** This is the second most common cause (approx. 40% of cases). It is often associated with more profound edema and is frequently seen in soil-contaminated wounds. * **Clostridium septicum (Option C):** While it can cause gas gangrene, it is uniquely associated with **atraumatic/spontaneous gas gangrene**, often linked to underlying gastrointestinal malignancies (e.g., colon cancer) or neutropenia. * **Clostridium histolyticum (Option A):** This species produces potent proteolytic enzymes (collagenases) and is a rare cause of gas gangrene, usually occurring in polymicrobial infections. **High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** A biochemical test used for rapid identification of *C. perfringens* based on its lecithinase activity (opalescence on egg yolk agar, inhibited by antitoxin). * **Morphology:** It is a Gram-positive, boxcar-shaped anaerobic bacillus. Notably, it is **non-motile** and rarely shows spores in clinical samples. * **Clinical Sign:** Presence of **crepitus** (gas bubbles in tissue) and a "mousy" or "sweetly putrid" odor. * **Double Zone of Hemolysis:** On blood agar, *C. perfringens* produces a characteristic double zone (inner complete hemolysis by theta toxin, outer partial hemolysis by alpha toxin).

Question 1

Shiga toxin acts by which mechanism?

Accepted Answer

Inhibition of protein synthesis

Answer

Producing cAMP

Answer

Producing cGMP

Answer

Leakage of enterocytes

Question 2

All of the following Salmonellae are motile except?

Accepted Answer

S. gallinarum pullorum

Answer

S. typhi

Answer

S. enteridis

Answer

S. chester

Question 3

Panton-Valentine leucocidin is seen in infection with which bacterium?

Accepted Answer

Staphylococci

Answer

Streptococci

Answer

Gonococci

Answer

Pneumococci

Question 4

A 2-year-old child presents with fever and vomiting, exhibiting neck rigidity. Cerebrospinal fluid analysis reveals over 2000 polymorphs/mm³, protein of 100 mg/dL, and glucose of 10 mg/dL. Gram stain shows Gram-negative coccobacilli, and culture demonstrates growth only on chocolate agar, not on blood agar. What is the causative agent?

Accepted Answer

Haemophilus influenzae

Answer

Neisseria meningitidis

Answer

Branhamella catarrhalis

Answer

Legionella pneumophila

Question 5

The enzyme glucosyl transferase secreted by Streptococcus mutans synthesizes glucans from what substrate?

Accepted Answer

Sucrose

Answer

Glucose

Answer

Fructose

Answer

Lactose

Question 6

An 18-year-old woman presents with diplopia, dysarthria, and dysphagia 18 hours after consuming homemade preserves. Examination reveals cranial nerve palsies affecting extraocular movements, difficulty swallowing, and voice changes. Muscle strength is 4/5 in the arms and 5/5 in the legs, with normal reflexes. What is the most likely causative organism?

Accepted Answer

Clostridium botulinum toxin

Answer

Staphylococcal toxin

Answer

Salmonellosis

Answer

Brucellosis

Question 7

Which type of E. coli is characterized by the presence of LT (heat-labile) and ST (heat-stable) proteins?

Accepted Answer

Enterotoxigenic (ETEC)

Answer

Enteroinvasive (EIEC)

Answer

Enterohemorrhagic (EHEC)

Answer

Enteropathogenic (EPEC)

Question 8

Which of the following is true about the cholera vibrio?

Accepted Answer

It has marked tolerance of alkaline pH.

Answer

It is strongly anaerobic.

Answer

It grows best at 25°C.

Answer

It grows over a wide range but best at a slight acid pH.

Question 9

Which is the most common causative agent of gas gangrene?

Accepted Answer

Clostridium perfringens

Answer

Clostridium histolyticum

Answer

Clostridium novyi

Answer

Clostridium septicum

Question 10

All statements are true except,

Accepted Answer

Enteroinvasive Escherichia coli (EIEC) produce disease similar to salmonellosis.

Answer

Enteroaggregative Escherichia coli (EAEC) is associated with persistent diarrhea.

Answer

Enterohemorrhagic Escherichia coli (EHEC) can cause hemolytic uremic syndrome (HUS).

Answer

Enterotoxigenic Escherichia coli (ETEC) is a common cause of traveler's diarrhea.

Bacteriology — MCQs

Bacteriology — MCQs

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