Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 651: Why is Streptococcus referred to as a flesh-eating bacteria?

A. Streptolysin A
B. Streptolysin O
C. Pyrogenic exotoxin (Correct Answer)
D. Hyaluronidase

Explanation: **Explanation:** *Streptococcus pyogenes* (Group A Streptococcus) is termed "flesh-eating bacteria" due to its ability to cause **Necrotizing Fasciitis**, a rapidly progressive destruction of subcutaneous tissue and fascia. **Why Pyrogenic Exotoxin is correct:** The primary virulence factors responsible for this severe manifestation are **Streptococcal Pyrogenic Exotoxins (SPE types A, B, and C)**. These act as **superantigens**, non-specifically activating a massive number of T-cells. This triggers a "cytokine storm" (release of IL-1, IL-6, and TNF-α), leading to systemic shock and extensive tissue necrosis. SPE-B, specifically, is a cysteine protease that directly degrades host proteins, facilitating the rapid spread of the infection through tissue planes. **Analysis of Incorrect Options:** * **Streptolysin A:** This is a distractor; the two main hemolysins are Streptolysin O and S. * **Streptolysin O:** An oxygen-labile hemolysin that creates pores in cell membranes. While it aids in immune evasion, it is primarily used clinically for diagnosis (ASO titer) rather than being the primary driver of necrotizing fasciitis. * **Hyaluronidase:** Known as the "spreading factor," it breaks down hyaluronic acid in connective tissue. While it helps the bacteria invade, it does not cause the massive tissue death and systemic toxicity characteristic of "flesh-eating" disease. **NEET-PG High-Yield Pearls:** * **M-Protein:** The most important virulence factor for *S. pyogenes*; it is anti-phagocytic and mediates adhesion. * **Toxic Shock-Like Syndrome (TSLS):** Also mediated by Pyrogenic Exotoxins (Superantigens). * **Drug of Choice:** Penicillin G is the mainstay, but **Clindamycin** is added in necrotizing fasciitis because it inhibits protein synthesis, thereby shutting down exotoxin production (the "Eagle Effect").

Question 652: Which of the following organisms does NOT produce a heat-stable enterotoxin causing food poisoning?

A. Bacillus cereus
B. Staphylococcus
C. Yersinia enterocolitica
D. Clostridium perfringens (Correct Answer)

Explanation: **Explanation:** The correct answer is **Clostridium perfringens**. The distinction lies in the nature of the enterotoxin and the mechanism of food poisoning. **1. Why Clostridium perfringens is correct:** *Clostridium perfringens* (Type A) causes food poisoning via a **heat-labile** enterotoxin. Crucially, the toxin is not pre-formed in food; instead, vegetative cells are ingested, and the toxin is released in the small intestine during **sporulation**. Because the toxin is destroyed by heating (at 74°C), it does not fit the "heat-stable" category. **2. Why the other options are incorrect:** * **Bacillus cereus:** Produces two types of toxins. The **emetic type** (associated with fried rice) is a **heat-stable** enterotoxin (cereulide) that survives reheating. * **Staphylococcus aureus:** Produces several **heat-stable** enterotoxins (Type A-E). These toxins are pre-formed in contaminated food (like creamy salads or meats) and can withstand boiling for 30 minutes. * **Yersinia enterocolitica:** Some strains produce a **heat-stable enterotoxin (STa)**, similar to that of ETEC, which increases cGMP levels. **High-Yield Clinical Pearls for NEET-PG:** * **Short Incubation (1–6 hours):** Think *S. aureus* or *B. cereus* (emetic type) due to pre-formed, heat-stable toxins. * **Long Incubation (8–16 hours):** Think *C. perfringens* or *B. cereus* (diarrheal type) as the toxin is produced *in vivo*. * **C. perfringens** is classically associated with reheated meat dishes/gravies. * **Mechanism:** *C. perfringens* enterotoxin acts by inserting into the brush border of intestinal epithelial cells, leading to altered permeability.

Question 653: Gram staining differentiates most bacteria into gram-positive and gram-negative types. Which of the following stains is NOT used in the Gram staining procedure?

A. Methylene blue (Correct Answer)
B. Crystal violet
C. Iodine
D. Safranin

Explanation: **Explanation:** The Gram stain is a differential staining technique developed by Hans Christian Gram that categorizes bacteria based on the structural differences in their cell walls. **Why Methylene Blue is the correct answer:** Methylene blue is **not** a component of the standard Gram staining procedure. It is primarily used as a simple stain or as a counterstain in the **Ziehl-Neelsen (Acid-fast) stain** and the Albert stain (for *Corynebacterium diphtheriae*). In Gram staining, the counterstain used is Safranin or Dilute Carbol Fuchsin. **Analysis of incorrect options (Steps of Gram Staining):** * **Crystal Violet (Option B):** This is the **Primary Stain**. It colors all cells purple by binding to the peptidoglycan layer. * **Iodine (Option C):** This acts as the **Mordant**. It forms a Crystal Violet-Iodine (CV-I) complex that gets trapped within the thick peptidoglycan layer of Gram-positive bacteria. * **Safranin (Option D):** This is the **Counterstain**. After decolorization with alcohol/acetone (which removes the CV-I complex from thin-walled Gram-negative bacteria), Safranin stains the now-colorless Gram-negative cells pink/red. **High-Yield Clinical Pearls for NEET-PG:** * **The Decolorizer:** This is the most critical step. Acetone or 95% Ethyl alcohol is used. Over-decolorization can make Gram-positive cells appear Gram-negative. * **Cell Wall Difference:** Gram-positive bacteria have a thick peptidoglycan layer and contain **Teichoic acid**. Gram-negative bacteria have a thin peptidoglycan layer and a lipid-rich **Outer Membrane**. * **Gram-variable organisms:** Old cultures or bacteria treated with antibiotics may show inconsistent staining. * **Non-Gram staining organisms:** Remember the mnemonic **"These Microbes May Lack Real Color"** (Treponema, Mycobacteria, Mycoplasma, Legionella, Rickettsia, Chlamydia).

Question 654: What is the causative organism of Bacillary Angiomatosis?

A. Brucella
B. Bartonella (Correct Answer)
C. Listeria
D. Borrelia

Explanation: **Explanation:** **Bacillary Angiomatosis (BA)** is a vascular proliferative disease caused by Gram-negative, fastidious, pleomorphic bacilli of the genus **Bartonella**. It primarily affects immunocompromised individuals, particularly those with advanced HIV/AIDS (CD4 count <100 cells/mm³). 1. **Why Bartonella is Correct:** The two primary species involved are ***Bartonella henselae*** (transmitted via cat scratches/fleas) and ***Bartonella quintana*** (transmitted via human body lice). These bacteria induce endothelial cell proliferation, leading to the formation of characteristic "mulberry-like" red-to-purple skin nodules that can mimic Kaposi Sarcoma. Diagnosis is confirmed via biopsy showing lobular vascular proliferation and **Warthin-Starry silver stain**, which highlights the bacteria. 2. **Why Other Options are Incorrect:** * **Brucella:** Causes Brucellosis (Undulant fever), characterized by drenching sweats and hepatosplenomegaly, but does not cause vascular skin lesions. * **Listeria:** A Gram-positive rod causing meningitis or sepsis, especially in neonates and the elderly; it is not associated with angiomatous lesions. * **Borrelia:** Causes Lyme disease (*B. burgdorferi*) or Relapsing fever (*B. recurrentis*). While Lyme presents with a rash (*Erythema migrans*), it is not a vascular proliferative lesion. **High-Yield Clinical Pearls for NEET-PG:** * **Differential Diagnosis:** Must distinguish BA from **Kaposi Sarcoma** (caused by HHV-8). BA shows neutrophilic infiltrate on histology, whereas Kaposi shows spindle cells and slit-like spaces. * **Treatment:** The drug of choice for Bacillary Angiomatosis is **Erythromycin** or Doxycycline. * **Other Bartonella Diseases:** *B. henselae* also causes Cat Scratch Disease (in immunocompetent hosts), and *B. quintana* causes Trench Fever.

Question 655: What is true about H. influenza?

A. Grown on sheep blood agar and CO2
B. It is not capsulated
C. Invasive strains are most common (Correct Answer)
D. Gram positive

Explanation: **Explanation:** *Haemophilus influenzae* is a small, pleomorphic Gram-negative coccobacillus. Understanding its classification and growth requirements is high-yield for NEET-PG. **1. Why Option C is Correct:** *H. influenzae* is divided into **capsulated (typable)** and **non-capsulated (non-typable)** strains. While non-typable strains cause localized mucosal infections (like otitis media), the **capsulated strains** (specifically **Type b**) are highly invasive. The capsule (polyribosylribitol phosphate - PRP) allows the bacteria to resist phagocytosis and enter the bloodstream, leading to severe systemic conditions like meningitis, epiglottitis, and septic arthritis. In the context of clinically significant, severe disease, these invasive strains are the primary focus. **2. Why Other Options are Incorrect:** * **Option A:** *H. influenzae* **cannot** grow on plain sheep blood agar because it requires both **Factor X (Hemin)** and **Factor V (NAD)**. Sheep blood contains NADase which destroys Factor V. It grows on **Chocolate Agar**, where heat lyses the RBCs, releasing both factors and inactivating NADase. * **Option B:** While non-typable strains exist, the most clinically significant and virulent strains are **capsulated**. * **Option D:** *H. influenzae* is a **Gram-negative** organism (stains pink/red). **Clinical Pearls for NEET-PG:** * **Satellitism:** *H. influenzae* grows on blood agar only when streaked alongside *Staphylococcus aureus*, which provides the necessary Factor V. * **Vaccine:** The Hib vaccine targets the **Type b capsule (PRP)**. * **Culture:** Best grown on Chocolate agar in 5–10% $CO_2$. * **Drug of Choice:** Ceftriaxone for invasive disease; Amoxicillin-clavulanate for mucosal infections.

Question 656: A positive Schick's test indicates that the person is:

A. Immune to diphtheria
B. Hypersensitive to diphtheria
C. Susceptible to diphtheria (Correct Answer)
D. Carrier of diphtheria

Explanation: **Explanation:** The **Schick’s test** is an intradermal skin test used to determine the immune status of an individual against *Corynebacterium diphtheriae*. It relies on the principle of toxin-antitoxin neutralization. **Why Option C is Correct:** A small amount of purified diphtheria toxin is injected intradermally. If the person lacks protective antibodies (antitoxins), the toxin causes local tissue damage, resulting in **erythema and edema** (a positive reaction) at the injection site within 4–7 days. Therefore, a **positive test indicates susceptibility** to diphtheria. **Analysis of Incorrect Options:** * **Option A (Immune):** If a person is immune, circulating antitoxins neutralize the injected toxin, resulting in no reaction (Negative Schick’s test). * **Option B (Hypersensitive):** Hypersensitivity is identified by a "Pseudoreaction," which disappears within 48–72 hours. This is caused by a reaction to the bacterial proteins rather than the toxin itself. * **Option D (Carrier):** Schick’s test measures humoral immunity (antitoxin levels), not the presence of the bacteria in the nasopharynx. Carrier status is diagnosed via throat swabs and culture (e.g., Loeffler’s serum slope). **High-Yield Clinical Pearls for NEET-PG:** * **Control Injection:** To differentiate between a true positive and a pseudoreaction, heat-inactivated toxin is injected into the other arm as a control. * **Interpretation Summary:** * **Positive:** Susceptible (Reaction on test arm only). * **Negative:** Immune (No reaction on either arm). * **Pseudo-reaction:** Immune but hypersensitive (Reaction on both arms, fading by day 3). * **Combined reaction:** Susceptible and hypersensitive (Reaction on both arms, but test arm reaction persists longer). * **Current Relevance:** Schick’s test is now largely obsolete in clinical practice, replaced by ELISA to measure antitoxin titers, but remains a classic "favorite" for competitive exams.

Question 657: Which organism is most commonly associated with food poisoning having the shortest incubation period?

A. Salmonella
B. Clostridium perfringens
C. Vibrio cholerae
D. Bacillus cereus (Correct Answer)

Explanation: **Explanation:** The incubation period of food poisoning is primarily determined by whether the illness is caused by an **ingested pre-formed toxin** (intoxication) or by the **growth of bacteria** within the gut (infection). **Why Bacillus cereus is correct:** *Bacillus cereus* produces two types of toxins. The **emetic (vomiting) type** is caused by a pre-formed heat-stable toxin (cereulide) usually found in contaminated **reheated fried rice**. Because the toxin is already present in the food, symptoms manifest rapidly, typically within **1–6 hours**. This represents one of the shortest incubation periods in clinical microbiology, alongside *Staphylococcus aureus*. **Why the other options are incorrect:** * **Salmonella:** This is an invasive infection. The bacteria must colonize the intestines and multiply, leading to an incubation period of **12–72 hours**. * **Clostridium perfringens:** Though it produces an enterotoxin, the toxin is produced *in vivo* after the ingestion of spores. This results in a longer incubation period of **8–16 hours**, characterized primarily by watery diarrhea. * **Vibrio cholerae:** This requires bacterial colonization and the subsequent production of cholera toxin. The incubation period is typically **1–3 days**. **High-Yield Clinical Pearls for NEET-PG:** * **Shortest Incubation (1–6 hours):** Think *Staphylococcus aureus* (creamy foods/salads) and *Bacillus cereus* (emetic type/rice). * **Intermediate Incubation (8–16 hours):** Think *Clostridium perfringens* (reheated meat/gravy) and *Bacillus cereus* (diarrheal type/meat and vegetables). * **Bacillus cereus Dual Presentation:** 1. **Emetic type:** Short incubation (1–6h), heat-stable toxin, associated with rice. 2. **Diarrheal type:** Long incubation (8–16h), heat-labile toxin, associated with meat/veg.

Question 658: Which of the following is false regarding Q fever?

A. Causative agent is Coxiella burnetii
B. Incubation period is 2-3 weeks
C. Vector is louse (Correct Answer)
D. Mode of transmission is inhalation of infected dust

Explanation: **Explanation:** **Q Fever** is a unique zoonotic disease caused by **Coxiella burnetii**. Understanding its transmission and characteristics is high-yield for NEET-PG. **Why Option C is False (The Correct Answer):** Unlike other Rickettsial diseases, **Q fever does not require an arthropod vector** for human transmission. While ticks maintain the infection cycle in wild animals, humans are primarily infected through the inhalation of contaminated aerosols or dust. Therefore, the statement "Vector is louse" is incorrect (Louse is the vector for Epidemic typhus, caused by *Rickettsia prowazekii*). **Analysis of Other Options:** * **Option A:** *Coxiella burnetii* is the confirmed causative agent. It was formerly classified under Rickettsiae but is now placed in the Gammaproteobacteria. * **Option B:** The incubation period typically ranges from **2 to 3 weeks** (average 18–20 days), fitting the clinical profile. * **Option C:** The primary mode of transmission is the **inhalation of infected dust** or aerosols from birth products (placenta), feces, or urine of infected livestock (cattle, sheep, goats). **High-Yield Clinical Pearls for NEET-PG:** * **Resistance:** *C. burnetii* forms **spore-like structures**, making it highly resistant to environmental heat, drying, and many common disinfectants. * **Diagnosis:** It is diagnosed serologically (Weil-Felix test is **negative**). * **Clinical Features:** Presents as an interstitial pneumonia or hepatitis. **Culture-negative endocarditis** is the most common manifestation of chronic Q fever. * **Antigenic Variation:** It undergoes **Phase variation**. Phase I is highly infectious (isolated from animals), while Phase II is less infectious (subcultured in labs).

Question 659: What is the most common causative organism in acute emphysematous cholecystitis?

A. Pseudomonas aeruginosa
B. Staphylococcus Aureus
C. Clostridium perfringens (Correct Answer)
D. Streptococcus pyogenes

Explanation: **Explanation:** **Acute Emphysematous Cholecystitis** is a severe, life-threatening variant of acute cholecystitis characterized by the presence of gas within the gallbladder wall, lumen, or pericholecystic tissues. **1. Why Clostridium perfringens is correct:** The underlying pathophysiology involves ischemia of the gallbladder wall (often due to cystic artery compromise), which creates an anaerobic environment. This allows for the proliferation of **gas-forming organisms**. **_Clostridium perfringens_** is the most frequently isolated pathogen in this condition, followed by *Escherichia coli* and *Klebsiella*. These bacteria ferment glucose and other substrates, leading to the production of carbon dioxide and hydrogen gas within the gallbladder tissues. **2. Why the other options are incorrect:** * **A. Pseudomonas aeruginosa:** While it can cause healthcare-associated infections, it is an aerobic organism and not a primary gas-producer in the biliary tract. * **B. Staphylococcus aureus:** This is a common cause of skin and soft tissue infections or bacteremia, but it is rarely implicated in emphysematous cholecystitis. * **D. Streptococcus pyogenes:** This is a Group A Strep primarily associated with pharyngitis and necrotizing fasciitis; it does not typically involve the gallbladder. **Clinical Pearls for NEET-PG:** * **Risk Factor:** Strongly associated with **Diabetes Mellitus** (found in >30-50% of cases) and is more common in elderly males. * **Diagnosis:** The investigation of choice is a **CT Scan**, which highly sensitively detects intramural or intraluminal gas. * **Radiology Sign:** "Champagne glass sign" (gas bubbles rising in the gallbladder). * **Management:** It is a surgical emergency requiring urgent cholecystectomy due to the high risk of perforation and gangrene.

Question 660: Toxin production by Corynebacterium diphtheriae is due to which mechanism?

A. Chromosomal mutation
B. Presence of a plasmid
C. Lysogenic conversion (Correct Answer)
D. Transformation

Explanation: **Explanation:** The pathogenicity of *Corynebacterium diphtheriae* is primarily due to the production of the **Diphtheria Toxin (DT)**. This toxin is not encoded on the bacterial chromosome itself, but rather by the **tox gene**, which is introduced into the bacterium by a temperate bacteriophage (specifically the **Beta-phage**). This process, where a bacteriophage integrates its DNA into the bacterial genome and confers new phenotypic properties (like toxin production), is called **Lysogenic conversion**. Only strains of *C. diphtheriae* that are "lysogenized" by this phage can cause clinical diphtheria. **Analysis of Options:** * **A. Chromosomal mutation:** While mutations can lead to antibiotic resistance, they do not account for the acquisition of the diphtheria toxin gene. * **B. Presence of a plasmid:** Plasmids often carry genes for antibiotic resistance (R-factors), but the *tox* gene in *C. diphtheriae* is specifically phage-mediated, not plasmid-mediated. * **D. Transformation:** This involves the uptake of naked DNA from the environment. While a mechanism of horizontal gene transfer, it is not the natural mechanism for toxigenicity in Diphtheria. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Diphtheria toxin inhibits protein synthesis by **ADP-ribosylation of Elongation Factor-2 (EF-2)**. * **Regulation:** Toxin production is regulated by iron levels. High iron inhibits toxin production via the **DtxR** (Diphtheria toxin receptor) protein. * **Diagnosis:** The **Elek’s Gel Precipitation Test** is the gold standard for detecting toxigenicity (immunodiffusion). * **Other Lysogenic Conversion Examples:** Remember the mnemonic **ABCDES**: **A**dherence (Group A Strep), **B**otulinum toxin, **C**holera toxin, **D**iphtheria toxin, **E**rythrogenic toxin (Strep pyogenes), and **S**higa toxin.

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