Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1411: A seminal smell on culture is produced by which bacterium?

A. Pneumococci
B. V. cholerae
C. Proteus (Correct Answer)
D. Yersinia

Explanation: **Explanation:** The correct answer is **Proteus**. The characteristic "seminal" or "fishy/ammoniacal" odor associated with *Proteus* species (particularly *P. mirabilis* and *P. vulgaris*) is due to the production of volatile amines and the breakdown of urea. *Proteus* is a highly motile, Gram-negative rod known for its "swarming" growth on agar. **Analysis of Options:** * **Proteus (Correct):** Produces a distinct seminal or putrid odor. It is also characterized by its ability to produce urease, which increases urinary pH, leading to the formation of struvite (triple phosphate) stones. * **Pneumococci (*S. pneumoniae*):** These are Gram-positive cocci that do not produce a seminal smell. They are typically identified by their alpha-hemolytic "draughtsman" appearance on blood agar and bile solubility. * **Vibrio cholerae:** This bacterium is famously associated with a **"fishy" or "sweetish"** odor, but it is more classically described as having a "rice-water" stool association. * **Yersinia:** *Yersinia pestis* or *Y. enterocolitica* do not have a seminal odor. *Y. enterocolitica* is known for its "bull’s eye" appearance on CIN agar and psychrophilic nature. **High-Yield Clinical Pearls for NEET-PG:** * **Proteus:** Swarming growth (inhibited by 6% salt or boric acid), Dienes phenomenon, and Phenylalanine Deaminase (TDA/PPA) positive. * **Other Characteristic Odors:** * *Pseudomonas aeruginosa:* Fruity/Grape-like/Corn-tortilla odor. * *Staphylococcus aureus:* Old musty/Sock-like odor. * *Pasteurella multocida:* Musty/Mousy odor. * *Eikenella corrodens:* Bleach-like odor. * *Actinomyces:* Earthy/Musty odor.

Question 1412: Which of the following causes both cutaneous and respiratory diphtheria?

A. Corynebacterium diphtheria (Correct Answer)
B. Corynebacterium ulcerans
C. Corynebacterium xerosis
D. Corynebacterium hoffmanii

Explanation: **Explanation:** **Corynebacterium diphtheriae** is the primary pathogen responsible for diphtheria. It can manifest in two main clinical forms: **Respiratory diphtheria**, characterized by the classic greyish-white "pseudomembrane" on the tonsils and pharynx, and **Cutaneous diphtheria**, which presents as non-healing chronic ulcers often covered by a greyish membrane. Both forms are caused by the same organism, and the systemic complications (myocarditis, polyneuritis) result from the absorption of the **DTX (Diphtheria Toxin)** produced by lysogenic strains carrying the *tox* gene. **Analysis of Options:** * **Corynebacterium ulcerans (Option B):** While this species can produce the diphtheria toxin and cause a diphtheria-like illness (often zoonotic, via raw milk), it is primarily associated with skin infections or mild pharyngitis and is not the classic cause of the dual clinical presentation described. * **Corynebacterium xerosis (Option C):** This is a commensal of the conjunctiva and skin. It is rarely pathogenic, occasionally causing endocarditis or opportunistic infections in immunocompromised hosts, but never diphtheria. * **Corynebacterium hoffmanii (C. pseudodiphtheriticum) (Option D):** A normal inhabitant of the nasopharynx. It lacks the *tox* gene and does not cause diphtheria; it is generally considered non-pathogenic unless causing rare lower respiratory infections in the elderly. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, club-shaped bacilli arranged in **Chinese letter patterns** (cuneiform). * **Granules:** Presence of **Volutin/Metachromatic granules** (Babes-Ernst granules) seen with Albert’s stain. * **Culture:** Grows on **Löffler's serum slope** (rapid growth) and **Potassium Tellurite Agar** (black colonies). * **Virulence Test:** **Elek’s Gel Precipitation Test** is the gold standard for detecting toxin production. * **Schick Test:** Used to determine the immune status of an individual against diphtheria.

Question 1413: An organism grown on agar shows green colored colonies. What is the likely organism?

A. Staphylococcus
B. Streptococcus
C. Pseudomonas (Correct Answer)
D. E. coli

Explanation: **Explanation:** The correct answer is **Pseudomonas aeruginosa**. This organism is a non-fermenting Gram-negative bacillus known for its ability to produce various water-soluble pigments, which impart a characteristic color to the colonies on nutrient agar or Mueller-Hinton agar. **Why Pseudomonas is correct:** The characteristic green color is primarily due to the production of two pigments: 1. **Pyocyanin:** A blue-green pigment (unique to *P. aeruginosa*). 2. **Pyoverdin (Fluorescein):** A yellow-green fluorescent pigment. When these pigments combine, they produce the classic "grassy green" appearance. Some strains may also produce Pyorubin (red) or Pyomelanin (brown/black). **Why other options are incorrect:** * **Staphylococcus:** Typically produces golden-yellow (*S. aureus*) or white (*S. epidermidis*) colonies. * **Streptococcus:** Colonies are usually small, translucent, or greyish. While they produce "alpha-hemolysis" (greenish discoloration) *around* the colony on Blood Agar, the colonies themselves are not green. * **E. coli:** On routine media like Nutrient Agar, it produces moist, greyish-white colonies. On MacConkey agar, it produces bright pink (lactose-fermenting) colonies. **Clinical Pearls for NEET-PG:** * **Odor:** *Pseudomonas* produces a characteristic **fruity, grape-like, or corn-tortilla odor**. * **Oxidase Test:** It is **Oxidase positive** (distinguishes it from Enterobacteriaceae). * **Clinical Association:** It is a leading cause of nosocomial infections, particularly in burn patients (blue-green pus) and cystic fibrosis patients. * **Culture Media:** Cetrimide agar is the selective medium used for its isolation.

Question 1414: Granulomatous infantisepticum is caused by which of the following?

A. HSV-1
B. HSV-2
C. Listeria (Correct Answer)
D. Group B streptococcus

Explanation: **Explanation:** **Listeria monocytogenes** is the causative agent of **Granulomatosis infantiseptica**, a severe, life-threatening form of early-onset neonatal listeriosis. This condition occurs due to *in utero* (transplacental) infection. It is characterized by the formation of disseminated pyogenic granulomas and microabscesses in multiple organs, including the liver, spleen, lungs, and brain. If not treated promptly, it often leads to abortion, stillbirth, or premature delivery. **Analysis of Options:** * **HSV-1 & HSV-2:** While Herpes Simplex Virus can cause neonatal herpes (typically acquired during passage through the birth canal), it presents with skin-eye-mouth (SEM) lesions, encephalitis, or disseminated disease, but it does not cause the specific pathological entity of granulomatosis infantiseptica. * **Group B Streptococcus (GBS):** *S. agalactiae* is the most common cause of neonatal sepsis and meningitis. While it causes early-onset disease, it typically presents as respiratory distress or pneumonia rather than disseminated granulomatous lesions. **High-Yield Clinical Pearls for NEET-PG:** * **Microbiology:** *Listeria* is a Gram-positive, non-spore-forming motile bacillus. It exhibits characteristic **"Tumbling motility"** at 25°C and is catalase-positive. * **Cold Enrichment:** It can grow at low temperatures (4°C), a property used for its isolation. * **CAMP Test:** *Listeria* shows a positive CAMP test (similar to GBS), producing a rectangular zone of hemolysis. * **Transmission:** Usually via contaminated food (unpasteurized milk, soft cheese, deli meats). * **Treatment:** **Ampicillin** is the drug of choice (Listeria is inherently resistant to all cephalosporins).

Question 1415: A patient presented with bloody diarrhea. Laboratory examination revealed a lactose-positive, glucose-fermenting, Gram-negative rod. The patient's infection is most likely due to which organism?

A. Salmonellae enterica
B. Escherichia coli (Correct Answer)
C. Shigella sonnei
D. Helicobacter pylori

Explanation: ### Explanation The correct answer is **Escherichia coli**. The diagnosis is based on the interpretation of biochemical characteristics provided in the clinical vignette. **1. Why Escherichia coli is correct:** The laboratory findings describe a **Gram-negative rod** that is **Lactose-positive** (pink colonies on MacConkey agar) and **Glucose-fermenting**. Among the common causes of bloody diarrhea (dysentery), *E. coli*—specifically **Enterohemorrhagic E. coli (EHEC)** and **Enteroinvasive E. coli (EIEC)**—fits this biochemical profile perfectly. EHEC (notably O157:H7) causes bloody diarrhea via Shiga-like toxins, while EIEC invades the colonic mucosa. **2. Why the other options are incorrect:** * **Salmonella enterica:** While it is a Gram-negative rod that ferments glucose, it is **Lactose-non-fermenting** (appears pale on MacConkey agar). It typically causes enteric fever or inflammatory diarrhea, but the lactose status rules it out. * **Shigella sonnei:** Although a major cause of bloody diarrhea (bacillary dysentery), *Shigella* species are **Lactose-non-fermenters** (though *S. sonnei* is a late/slow lactose fermenter, it is clinically categorized as a non-fermenter) and are non-motile. * **Helicobacter pylori:** This is a Gram-negative **curved/spiral** rod associated with peptic ulcers and gastritis, not acute bloody diarrhea. **High-Yield Clinical Pearls for NEET-PG:** * **Lactose Fermenters (CEEK):** **C**itrobacter, **E**. coli, **E**nterobacter, **K**lebsiella. * **EHEC (O157:H7):** Does **not** ferment Sorbitol (tested on SMAC agar). It is associated with **Hemolytic Uremic Syndrome (HUS)** due to Shiga toxin. * **EIEC:** Clinically indistinguishable from Shigellosis; it uses actin tails for cell-to-cell spread. * **Golden Rule:** If a question mentions "Lactose-positive" and "Diarrhea," always prioritize *E. coli*.

Question 1416: Which of the following statements is FALSE regarding Streptococcus pyogenes?

A. Causes necrotizing fasciitis
B. Is beta-hemolytic
C. M protein is a virulence factor
D. Is resistant to bacitracin (Correct Answer)

Explanation: ### Explanation The correct answer is **D (Is resistant to bacitracin)** because *Streptococcus pyogenes* (Group A Streptococcus) is characteristically **sensitive** to bacitracin. This sensitivity is a classic laboratory diagnostic feature used to differentiate it from other beta-hemolytic streptococci, such as *S. agalactiae* (Group B), which is bacitracin-resistant. #### Analysis of Options: * **Option A (Causes necrotizing fasciitis):** This is true. *S. pyogenes* is the primary cause of "flesh-eating disease" (Type II necrotizing fasciitis), a rapidly progressing infection of the deep fascia. * **Option B (Is beta-hemolytic):** This is true. When grown on blood agar, *S. pyogenes* produces a wide, clear zone of complete hemolysis due to the production of hemolysins (Streptolysin O and S). * **Option C (M protein is a virulence factor):** This is true. The M protein is the most important virulence factor; it is anti-phagocytic, inhibits complement activation, and is responsible for molecular mimicry leading to Acute Rheumatic Fever. #### NEET-PG High-Yield Pearls: * **PYR Test:** *S. pyogenes* is **PYR positive** (L-pyrrolidonyl arylamidase), which is a more specific test than bacitracin sensitivity. * **ASO Titer:** Elevated Anti-Streptolysin O (ASO) titers indicate recent infection, useful for diagnosing post-streptococcal sequelae (Rheumatic Fever/PSGN). * **Drug of Choice:** Penicillin remains the treatment of choice as *S. pyogenes* has not yet developed clinical resistance to it. * **Scarlet Fever:** Caused by Erythrogenic toxins (Pyrogenic exotoxins A, B, and C).

Question 1417: All of the following are true of exotoxins, except:

A. Protein in nature
B. Antigenic
C. Heat stable (Correct Answer)
D. Highly toxic

Explanation: **Explanation:** The correct answer is **C (Heat stable)** because exotoxins are primarily **heat-labile** (heat-sensitive). ### 1. Why "Heat stable" is the correct (false) statement: Exotoxins are proteins secreted by both Gram-positive and Gram-negative bacteria. Because they are proteins, they are typically denatured by heat (usually at temperatures above 60°C). * **Exception to remember:** The *Staphylococcal* enterotoxin and *E. coli* heat-stable toxin (ST) are notable exceptions that remain stable at high temperatures. In contrast, **Endotoxins** (Lipopolysaccharides) are characteristically heat-stable. ### 2. Analysis of Incorrect Options: * **A. Protein in nature:** Exotoxins are polypeptide chains secreted by living bacterial cells. This allows them to be neutralized by specific antibodies. * **B. Antigenic:** Due to their high molecular weight and protein structure, exotoxins are highly immunogenic. They induce the formation of high-titer antibodies called **antitoxins**. This property is exploited to create **toxoids** (detoxified toxins that retain antigenicity) for vaccines like Tetanus and Diphtheria. * **C. Highly toxic:** Exotoxins are among the most poisonous substances known. They have specific mechanisms of action (e.g., inhibiting protein synthesis or blocking neurotransmitters) and possess a very low lethal dose (LD50) compared to endotoxins. ### High-Yield Clinical Pearls for NEET-PG: * **Source:** Exotoxins are secreted by living cells; Endotoxins are released only upon cell lysis. * **Genetics:** Exotoxin genes are often carried on **plasmids** or **bacteriophages** (e.g., Diphtheria toxin is encoded by the *tox* gene of the Beta-phage). * **Mechanism:** Many exotoxins are **A-B toxins**, where the 'B' subunit binds to the cell surface and the 'A' subunit possesses the enzymatic activity. * **Key Comparison:** Endotoxins are poorly antigenic, cannot be toxoided, and are heat-stable even at 100°C.

Question 1418: From which source is the antibiotic gramicidin S derived?

A. Bacillus brevis (Correct Answer)
B. Clostridium difficile
C. Staphylococcus epidermidis
D. Streptococcus bovis

Explanation: **Explanation:** **Gramicidin S** (Gramicidin Soviet) is a cyclopeptide antibiotic effective against Gram-positive bacteria and some fungi. It was first isolated in 1942 by Gause and Brazhnikova from a specific strain of **Bacillus brevis**. Unlike the linear gramicidins (found in Tyrothricin), Gramicidin S is a cyclic decapeptide. Its mechanism of action involves disrupting the bacterial cytoplasmic membrane, leading to increased permeability and cell death. **Analysis of Options:** * **Bacillus brevis (Correct):** This soil-dwelling, aerobic, spore-forming bacterium is the natural source of Gramicidin S. It is a classic example of a "Bacillus" species producing polypeptide antibiotics (similar to *B. polymyxa* producing Polymyxin). * **Clostridium difficile (Incorrect):** This is an anaerobic, spore-forming Gram-positive rod known for causing pseudomembranous colitis. It produces toxins (Toxin A and B) rather than therapeutic antibiotics. * **Staphylococcus epidermidis (Incorrect):** A common skin commensal and coagulase-negative staphylococcus (CoNS). While it can produce bacteriocins (like epidermin), it is not the source of Gramicidin. * **Streptococcus bovis (Incorrect):** A Group D Streptococcus associated with endocarditis and colorectal cancer. It does not produce Gramicidin. **High-Yield Clinical Pearls for NEET-PG:** * **Toxicity:** Gramicidin S is highly hemolytic; therefore, it is **never administered systemically**. It is strictly used for **topical** applications (e.g., infected wounds, throat lozenges). * **Mechanism:** It acts as a surfactant/detergent that disrupts the lipid bilayer of the bacterial cell membrane. * **Bacillus Sources:** Remember that many polypeptide antibiotics come from the genus *Bacillus*: * *B. brevis* → Gramicidin * *B. licheniformis* → Bacitracin * *B. polymyxa* → Polymyxin B

Question 1419: What is the causative agent of syphilis?

A. Treponema penana
B. Treponema endemicum
C. Treponema carateum
D. Treponema pallidum (Correct Answer)

Explanation: **Explanation:** **Treponema pallidum subsp. pallidum** is the causative agent of **Syphilis**, a chronic systemic sexually transmitted infection (STI). It is a thin, motile spirochete that cannot be grown on artificial culture media and is typically visualized using **dark-field microscopy** or silver impregnation stains (e.g., Levaditi stain). **Analysis of Options:** * **Treponema pallidum (Option D):** This is the correct answer. It causes Venereal Syphilis, characterized by stages: Primary (painless chancre), Secondary (condyloma lata, maculopapular rash), Latent, and Tertiary (gummas, neurosyphilis, aortitis). * **Treponema endemicum (Option B):** This subspecies causes **Endemic Syphilis (Bejel)**, primarily found in arid regions of Africa and the Middle East. It is transmitted via non-sexual contact (sharing utensils). * **Treponema carateum (Option C):** This agent causes **Pinta**, a skin disease found in Central and South America. It only affects the skin and does not involve internal organs. * **Treponema pertenue (Option A - misspelled as 'penana'):** *T. pallidum subsp. pertenue* causes **Yaws**, characterized by skin, bone, and joint lesions in tropical climates. **High-Yield Clinical Pearls for NEET-PG:** * **Screening Test:** VDRL and RPR (Non-specific/Reaginic tests). Note: VDRL is the test of choice for **Neurosyphilis** (using CSF). * **Confirmatory Test:** FTA-ABS and TPHA (Specific treponemal tests). * **Drug of Choice:** **Benzathine Penicillin G** is the gold standard for all stages. * **Jarisch-Herxheimer Reaction:** An acute febrile reaction occurring shortly after starting treatment due to the release of endotoxins from dying spirochetes.

Question 1420: A false-positive VDRL test is seen in which of the following conditions?

A. Lepromatous Leprosy
B. Infectious mononucleosis (Correct Answer)
C. HIV
D. Pregnancy

Explanation: **Explanation:** The **VDRL (Venereal Disease Research Laboratory)** test is a non-treponemal screening test for Syphilis. It detects **reagin antibodies** (IgM and IgG) directed against a cardiolipin-cholesterol-lecithin antigen. Because cardiolipin is a normal component of human mitochondrial membranes, any condition causing significant tissue damage or immune stimulation can lead to the production of these antibodies, resulting in a **Biological False Positive (BFP)** result. **Why Infectious Mononucleosis is the correct answer:** Infectious Mononucleosis (caused by EBV) is a classic cause of an **acute BFP** (lasting <6 months). The intense B-cell proliferation and hepatic involvement associated with the virus lead to the release of cardiolipin and the subsequent production of anti-cardiolipin antibodies, triggering a positive VDRL despite the absence of *Treponema pallidum*. **Analysis of other options:** * **Lepromatous Leprosy:** While Leprosy is a well-known cause of BFP in VDRL, it is typically associated with the **Tuberculoid** form or chronic cases. In many standardized NEET-PG contexts, viral infections like Infectious Mononucleosis or Malaria are prioritized as "classic" acute causes. * **HIV & Pregnancy:** Both are recognized causes of BFP VDRL. However, in the context of this specific question (often derived from standard textbooks like Ananthanarayan), **Infectious Mononucleosis** is frequently highlighted as the high-yield prototype for viral-induced false positives. **High-Yield Clinical Pearls for NEET-PG:** * **BFP Categorization:** * **Acute (<6 months):** Recent immunizations (e.g., smallpox), acute viral infections (Hepatitis, IM, Chickenpox), Malaria. * **Chronic (>6 months):** SLE (most common), Leprosy, Rheumatoid Arthritis, IV drug use. * **Confirmatory Test:** Any positive VDRL must be confirmed with a treponemal-specific test like **FTA-ABS** or **TPHA** to rule out a BFP. * **Prozone Phenomenon:** Can cause a false *negative* VDRL in secondary syphilis due to excessive antibody titers.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

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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