Bacteriology — MCQs
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Which of the following toxins produced by Staphylococcus is synergohymenotropic?
An outbreak of gastroenteritis occurred in a youth group camp. Water at the camp, which was not chlorinated or filtered, was obtained from a spring on the premises. The farmland near the camp was grazed by cattle and sheep, and run-off from the pasture entered the camp spring. The isolated microorganism required an atmosphere containing reduced oxygen and increased carbon dioxide for its growth. In most cases, the gastroenteritis was self-limiting, and those requiring antibiotic treatment responded to erythromycin. Which is the most likely causative agent?
Chancroid is caused by which bacterium?
Which of the following is transmitted by rat urine?
Which of the following conditions is NOT commonly associated with group A beta-hemolytic streptococci infection?
Blood in stool is seen in all of the following conditions except?
Tularemia belongs to which category of bioterrorism agents?
All of the following are true about Clostridium perfringens, EXCEPT:
Which of the following conditions is directly caused by Neisseria gonorrhoeae?
'Spider' colonies are produced by:
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 271: Which of the following toxins produced by Staphylococcus is synergohymenotropic?
- A. alpha toxin
- B. leucocidin (Correct Answer)
- C. beta toxin
- D. delta toxin
Explanation: **Explanation:** The term **"synergohymenotropic"** refers to toxins that act through the synergistic action of two non-associated protein components to damage the cellular membranes (hymeno-) of host cells, particularly leukocytes and erythrocytes. **1. Why Leucocidin is correct:** The **Panton-Valentine Leucocidin (PVL)** and the **gamma-hemolysin** produced by *Staphylococcus aureus* are the classic examples of synergohymenotropic toxins. They consist of two distinct polypeptide chains: the **S (slow)** component and the **F (fast)** component. These two subunits act together to form pores in the membranes of polymorphonuclear leukocytes, monocytes, and macrophages, leading to cell lysis. PVL is a major virulence factor in community-acquired MRSA (CA-MRSA) infections. **2. Why other options are incorrect:** * **Alpha toxin:** This is a single-component pore-forming toxin. It is the most important cytotoxin of *S. aureus* but does not require two separate protein subunits to function synergistically. * **Beta toxin:** Also known as sphingomyelinase C, it acts enzymatically by degrading sphingomyelin in the cell membrane. It is characterized by the "Hot-Cold" phenomenon on blood agar. * **Delta toxin:** This is a small, surfactant-like polypeptide that acts like a detergent to disrupt host cell membranes. It is not a two-component synergistic toxin. **High-Yield Clinical Pearls for NEET-PG:** * **PVL Association:** Strongly associated with **necrotizing pneumonia** and severe skin/soft tissue infections (furunculosis). * **Genetic Link:** The genes for PVL are carried on a **bacteriophage**. * **Gamma-hemolysin:** Unlike PVL, gamma-hemolysin is produced by almost all strains of *S. aureus* and can lyse both RBCs and WBCs.
Question 272: An outbreak of gastroenteritis occurred in a youth group camp. Water at the camp, which was not chlorinated or filtered, was obtained from a spring on the premises. The farmland near the camp was grazed by cattle and sheep, and run-off from the pasture entered the camp spring. The isolated microorganism required an atmosphere containing reduced oxygen and increased carbon dioxide for its growth. In most cases, the gastroenteritis was self-limiting, and those requiring antibiotic treatment responded to erythromycin. Which is the most likely causative agent?
- A. Campylobacter jejuni (Correct Answer)
- B. Enteroinvasive Escherichia coli (EIEC)
- C. Enteropathogenic Escherichia coli (EPEC)
- D. Vibrio cholerae
Explanation: ### Explanation The clinical presentation and laboratory characteristics point definitively to **Campylobacter jejuni**. **Why Campylobacter jejuni is the correct answer:** 1. **Epidemiology:** *C. jejuni* is a common cause of zoonotic gastroenteritis. It is frequently transmitted via contaminated water (run-off from pastures) or undercooked poultry. The mention of cattle and sheep grazing near the water source is a classic "trigger" for this diagnosis. 2. **Growth Requirements:** This is the most specific clue. *Campylobacter* is **microaerophilic** (requires reduced oxygen, ~5%) and **capnophilic** (requires increased $CO_2$, ~10%). It also grows best at $42^\circ C$ (thermophilic). 3. **Treatment:** While most cases are self-limiting, **Macrolides (Erythromycin/Azithromycin)** are the first-line treatment for severe or persistent infections. **Why the other options are incorrect:** * **EIEC:** Causes a dysentery-like illness similar to Shigella. It is a facultative anaerobe and does not require specific microaerophilic conditions. * **EPEC:** Primarily causes watery diarrhea in infants (pediatric age group). It does not have the specific atmospheric requirements mentioned. * **Vibrio cholerae:** Causes profuse "rice-water" stools. It is halophilic (requires salt) and grows well in alkaline media (TCBS agar), but it is not microaerophilic. The treatment of choice is typically rehydration and Doxycycline. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Described as "seagull-wing" shaped, Gram-negative curved bacilli. * **Motility:** Exhibits characteristic **"darting motility."** * **Complications:** *C. jejuni* is the most common antecedent infection associated with **Guillain-Barré Syndrome (GBS)** due to molecular mimicry (anti-GM1 antibodies). * **Culture Media:** Skirrow’s medium or Butzler’s medium.
Question 273: Chancroid is caused by which bacterium?
- A. Haemophilus ducreyi (Correct Answer)
- B. Chlamydia species
- C. Mycoplasma species
- D. Viruses
Explanation: **Explanation:** **Chancroid** is a sexually transmitted infection (STI) characterized by painful genital ulcers and associated regional lymphadenopathy (buboes). 1. **Why Haemophilus ducreyi is correct:** * *Haemophilus ducreyi* is a fastidious, Gram-negative coccobacillus. It is the causative agent of Chancroid. * **Pathogenesis:** The bacterium enters the skin through micro-abrasions during sexual contact, leading to the formation of a **"soft chancre"**—an ulcer that is characteristically **painful**, non-indurated, and has ragged, undermined edges. 2. **Why the other options are incorrect:** * **Chlamydia species:** Specifically *C. trachomatis* (serotypes L1-L3), cause **Lymphogranuloma Venereum (LGV)**. While it involves lymph nodes, the primary lesion is usually a painless, transient vesicle. * **Mycoplasma species:** *M. genitalium* is associated with urethritis and pelvic inflammatory disease, not ulcerative chancroid. * **Viruses:** The most common viral cause of genital ulcers is **Herpes Simplex Virus (HSV-2)**. Unlike the single large ulcer of chancroid, HSV typically presents with multiple, small, grouped vesicles on an erythematous base. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "You **do cry** with *H. **ducreyi***" (because the ulcer is **painful**), whereas Syphilis (*T. pallidum*) is painless. * **Microscopy:** On Gram stain, *H. ducreyi* shows a classic **"School of fish"** or **"Railroad track"** appearance. * **Culture:** Requires enriched media like **Chocolate agar** with IsoVitaleX or Vancomycin. * **Buboes:** In 50% of cases, patients develop painful, fluctuant inguinal lymph nodes (buboes) which may rupture spontaneously. * **Treatment:** Azithromycin (1g orally, single dose) or Ceftriaxone.
Question 274: Which of the following is transmitted by rat urine?
- A. Leptospira (Correct Answer)
- B. Listeria
- C. Legionella
- D. Mycoplasma
Explanation: **Explanation:** **Leptospira interrogans** is the correct answer. It is a spirochete that causes **Leptospirosis**, a classic zoonotic disease. The bacteria colonize the renal tubules of reservoir animals, most commonly **rats**, and are excreted in their **urine**. Humans typically acquire the infection through direct contact with infected urine or indirectly via contaminated water or soil (e.g., during floods or farming) through skin abrasions or mucous membranes. **Why other options are incorrect:** * **Listeria monocytogenes:** Primarily transmitted through the ingestion of contaminated **food** (unpasteurized milk, soft cheeses, deli meats). It is known for its ability to grow at refrigeration temperatures. * **Legionella pneumophila:** Transmitted via the **inhalation of aerosols** from contaminated water sources like air conditioning cooling towers, showers, or humidifiers. There is no person-to-person or animal-to-human transmission. * **Mycoplasma pneumoniae:** Transmitted via **respiratory droplets** from person to person. It is a common cause of "walking pneumonia." **High-Yield Clinical Pearls for NEET-PG:** * **Weil’s Disease:** The severe form of Leptospirosis characterized by the triad of **jaundice, renal failure, and hemorrhage**. * **Occupational Hazard:** Common in sewage workers, rice field farmers, and veterinarians. * **Diagnosis:** Gold standard is the **Microscopic Agglutination Test (MAT)**. Culture requires specialized media like **EMJH** or **Fletcher’s medium**. * **Biphasic illness:** Starts with a septicemic phase (fever, conjunctival suffusion) followed by an immune phase (meningitis, uveitis).
Question 275: Which of the following conditions is NOT commonly associated with group A beta-hemolytic streptococci infection?
- A. Scarlet fever
- B. Toxic shock syndrome
- C. Pharyngitis
- D. Endocarditis (Correct Answer)
Explanation: **Explanation:** The correct answer is **Endocarditis**. While Group A Streptococcus (GAS), or *Streptococcus pyogenes*, is a highly virulent pathogen, it is **not** a common cause of infective endocarditis. Native valve endocarditis is more typically associated with *Viridans group streptococci* (subacute) or *Staphylococcus aureus* (acute). Although GAS can cause Acute Rheumatic Fever (ARF) which leads to valvular damage (pancarditis), the damage is **immunologically mediated** rather than a direct bacterial infection of the endocardium. **Analysis of Options:** * **Scarlet Fever:** Caused by GAS strains that produce **Streptococcal Pyrogenic Exotoxins (SPE)** A, B, or C. It is characterized by a "sandpaper" rash and a strawberry tongue. * **Toxic Shock Syndrome (STSS):** Similar to staphylococcal TSS, this is mediated by superantigens (SPE-A and C) that cause massive cytokine release, leading to hypotension and multi-organ failure. * **Pharyngitis:** GAS is the most common bacterial cause of acute pharyngitis ("Strep throat"). If left untreated, it can lead to nonsuppurative complications like ARF. **High-Yield Clinical Pearls for NEET-PG:** * **M Protein:** The chief virulence factor of GAS; it is anti-phagocytic and shares antigenic determinates with cardiac myosin (molecular mimicry), leading to Rheumatic Heart Disease. * **ASO Titre:** Used to document recent streptococcal pharyngeal infection; **Anti-DNase B** is more reliable for skin infections (impetigo). * **Jones Criteria:** Used for the diagnosis of Acute Rheumatic Fever. * **PYR Test:** GAS is **PYR positive**, which helps differentiate it from other beta-hemolytic streptococci.
Question 276: Blood in stool is seen in all of the following conditions except?
- A. Vibrio cholerae infection (Correct Answer)
- B. Entamoeba infection
- C. Campylobacter infection
- D. Shigella infection
Explanation: **Explanation:** The presence of blood in stool (dysentery) typically indicates an **invasive or inflammatory process** that damages the intestinal mucosa. **1. Why Vibrio cholerae is the correct answer:** *Vibrio cholerae* causes a non-invasive, secretory diarrhea mediated by the **Cholera Toxin (Choleragen)**. The toxin increases intracellular cAMP, leading to the hypersecretion of water and electrolytes into the intestinal lumen without causing mucosal destruction or inflammation. The characteristic stool is **"Rice Water Stool,"** which is watery, voluminous, and contains mucus flecks but **no blood or pus cells.** **2. Analysis of Incorrect Options:** * **Entamoeba histolytica:** Causes amoebic dysentery. The parasite invades the colonic mucosa, creating "flask-shaped ulcers" that lead to blood and mucus in the stool. * **Campylobacter jejuni:** A common cause of inflammatory diarrhea. It invades the intestinal epithelium and produces cytolethal distending toxins, resulting in bloody stools. * **Shigella species:** The classic cause of bacillary dysentery. It invades the M cells of the Peyer's patches and produces the Shiga toxin, causing extensive mucosal ulceration, hemorrhage, and stools containing blood, mucus, and pus (neutrophils). **Clinical Pearls for NEET-PG:** * **Rice Water Stool:** Pathognomonic for Cholera. * **Darting Motility:** Seen in *Vibrio cholerae* (hanging drop preparation). * **Comma-shaped/S-shaped:** *Vibrio* is comma-shaped; *Campylobacter* is S-shaped/seagull-winged. * **Invasive vs. Enterotoxigenic:** If blood is present, think "Invasive" (Shigella, EIEC, Salmonella, Campylobacter, E. histolytica). If watery, think "Enterotoxigenic" (Vibrio, ETEC).
Question 277: Tularemia belongs to which category of bioterrorism agents?
- A. Category A (Correct Answer)
- B. Category B
- C. Category C
- D. Category D
Explanation: **Explanation:** The Centers for Disease Control and Prevention (CDC) classifies bioterrorism agents into three categories (A, B, and C) based on their potential for public health impact, ease of dissemination, and requirement for special public health preparedness. **1. Why Category A is Correct:** **Tularemia** (caused by *Francisella tularensis*) is classified as a **Category A** agent. These are high-priority agents because they: * Can be easily disseminated or transmitted from person to person (though Tularemia is primarily highly infectious via aerosol). * Result in high mortality rates and have the potential for major public health impact. * Might cause public panic and social disruption. * Require special action for public health preparedness. **2. Why other options are incorrect:** * **Category B:** These are second-highest priority agents. They are moderately easy to disseminate, result in moderate morbidity rates, and low mortality rates (e.g., *Coxiella burnetii* (Q fever), *Brucella* species, *Staphylococcal* enterotoxin B, and food safety threats like *Salmonella*). * **Category C:** These are third-priority agents and include emerging pathogens that could be engineered for mass dissemination in the future because of availability and ease of production (e.g., Nipah virus, Hantavirus, and Multi-drug resistant TB). * **Category D:** There is no official "Category D" in the CDC bioterrorism classification system. **Clinical Pearls for NEET-PG:** * **Infectivity:** *Francisella tularensis* is extremely virulent; as few as **10–50 organisms** can cause disease. * **Vector:** Often transmitted by ticks, deer flies, or contact with infected rabbits ("Rabbit Fever"). * **Clinical Forms:** Ulceroglandular (most common) and Pneumonic (most lethal, the form used in bioterrorism). * **Diagnosis:** Requires Biosafety Level 3 (BSL-3) labs. * **Treatment:** Streptomycin or Gentamicin are the drugs of choice.
Question 278: All of the following are true about Clostridium perfringens, EXCEPT:
- A. It is the commonest cause of gas gangrene.
- B. Serologic tests are not useful.
- C. The food poisoning spores of Clostridium perfringens are heat resistant.
- D. The major enzyme secreted is hyaluronidase. (Correct Answer)
Explanation: **Explanation:** The correct answer is **D** because the major virulence factor and enzyme secreted by *Clostridium perfringens* is **Alpha (α) toxin**, not hyaluronidase. Alpha toxin is a **lecithinase (phospholipase C)** that splits lecithin into diglyceride and phosphorylcholine, leading to cell membrane destruction, hemolysis, and tissue necrosis. While *C. perfringens* does produce hyaluronidase as a spreading factor, it is considered a minor toxin/enzyme compared to the lethal Alpha toxin. **Analysis of other options:** * **Option A:** *C. perfringens* (Type A) is indeed the **most common cause** of gas gangrene (clostridial myonecrosis), responsible for 80–90% of cases. * **Option B:** Serologic tests are **not useful** for diagnosis because the infection is acute and rapidly progressive; diagnosis relies on clinical presentation and Gram stain (showing "box-car" shaped bacilli with a lack of inflammatory cells). * **Option C:** Food poisoning is caused by **Type A** strains. These spores are notably **heat-resistant** (surviving 100°C for over an hour), allowing them to survive initial cooking and germinate if food is cooled slowly. **High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** A rapid biochemical test used to detect Alpha toxin (lecithinase activity) on egg yolk agar, inhibited by specific antitoxin. * **Morphology:** It is a Gram-positive, anaerobic, non-motile bacillus. It is unique among Clostridia for being **capsulated** and rarely showing spores in clinical samples. * **Target Hemolysis:** On blood agar, it produces a characteristic double zone of hemolysis (inner complete zone due to Theta toxin; outer partial zone due to Alpha toxin).
Question 279: Which of the following conditions is directly caused by Neisseria gonorrhoeae?
- A. Urethritis (Correct Answer)
- B. Salpingitis
- C. Cervicitis
- D. Vaginitis
Explanation: **Explanation:** *Neisseria gonorrhoeae* is a Gram-negative diplococcus that primarily infects **columnar or cuboidal epithelium**. **1. Why Urethritis is the correct answer:** Urethritis is the most common clinical manifestation of gonorrhea in men and a frequent presentation in women. The organism directly invades the urethral mucosa, leading to an acute inflammatory response characterized by a purulent discharge and dysuria. In the context of "direct" primary infection sites, the urethra is the hallmark site for *N. gonorrhoeae*. **2. Why the other options are incorrect:** * **Cervicitis:** While *N. gonorrhoeae* is a major cause of cervicitis, it specifically infects the **endocervix** (columnar epithelium). It does not infect the ectocervix. * **Salpingitis:** This is a **complication** (part of Pelvic Inflammatory Disease) resulting from the *ascending* spread of the bacteria from the lower genital tract to the fallopian tubes. It is considered a secondary sequela rather than the primary site of inoculation. * **Vaginitis:** This is the **incorrect** choice because the adult vagina is lined by **stratified squamous epithelium**, which is resistant to gonococcal infection. *N. gonorrhoeae* does not cause vaginitis in post-pubertal females (except in rare cases of vulvovaginitis in prepubertal girls where the epithelium is thin). **High-Yield Clinical Pearls for NEET-PG:** * **Thayer-Martin Medium:** The selective medium of choice (contains Vancomycin, Colistin, and Nystatin). * **Pili:** The most important virulence factor for attachment. * **Treatment:** Ceftriaxone (IM) is the drug of choice. Always co-treat for *Chlamydia* (Azithromycin or Doxycycline) unless ruled out. * **Fitz-Hugh-Curtis Syndrome:** A complication involving perihepatitis ("violin-string" adhesions).
Question 280: 'Spider' colonies are produced by:
- A. Bordetella pertussis
- B. Vibrio cholerae
- C. Actinomyces israelii (Correct Answer)
- D. Mycobacterium leprae
Explanation: **Explanation:** The correct answer is **Actinomyces israelii**. **Why Actinomyces israelii is correct:** *Actinomyces israelii* is a Gram-positive, anaerobic, filamentous bacterium. When grown on solid media (like Brain Heart Infusion agar) for a short duration (24–48 hours), the young colonies appear as **'Spider' colonies** due to the radiating filamentous growth. As the colonies mature (7–14 days), they develop a characteristic heaped-up, white, irregular appearance known as **'Molar Tooth' colonies**. This morphology is a classic high-yield diagnostic feature in microbiology. **Why other options are incorrect:** * **Bordetella pertussis:** Produces colonies described as **'Mercury drops'** or **'Bisected pearls'** on Bordet-Gengou medium. * **Vibrio cholerae:** On TCBS agar, it produces large, smooth, **yellow colonies** (due to sucrose fermentation). On MacConkey agar, colonies are initially colorless but may become pinkish (late lactose fermenter). * **Mycobacterium leprae:** This organism is **obligate intracellular** and **cannot be grown on artificial culture media**. It is typically studied via inoculation into the footpads of mice or nine-banded armadillos. **Clinical Pearls for NEET-PG:** 1. **Sulfur Granules:** *Actinomyces* infections (Actinomycosis) are characterized by "sulfur granules" in pus—yellowish specks that are actually masses of filamentous bacteria. 2. **Lumpy Jaw:** The most common clinical presentation is cervicofacial actinomycosis, often following dental trauma. 3. **Ray Fungus:** *Actinomyces* was historically mistaken for a fungus because of its branching filaments (hence the name "Actino-" meaning ray). 4. **Treatment:** Penicillin G is the drug of choice (long-term).
Practice by Chapter
Staphylococci
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Streptococci and Enterococci
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Neisseria and Moraxella
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Corynebacterium and Listeria
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Bacillus and Clostridium
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Enterobacteriaceae
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Vibrio, Aeromonas, and Plesiomonas
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Pseudomonas and Related Bacteria
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Haemophilus and HACEK Group
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Bordetella and Brucella
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Mycobacteria
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Spirochetes
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