Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 821: Which of the following mycoplasmas has been implicated as a cause of nongonococcal urethritis (NGU)?

A. Mycoplasma hominis
B. Mycoplasma pneumoniae
C. Mycoplasma fermentans
D. Ureaplasma urealyticum (Correct Answer)

Explanation: **Explanation:** **Ureaplasma urealyticum** is the correct answer because it is a well-established cause of **nongonococcal urethritis (NGU)** in men and has been associated with pregnancy complications and neonatal infections. It is unique among mycoplasmas due to its ability to produce **urease**, which hydrolyzes urea into ammonia—a key biochemical marker used for its identification. **Analysis of Incorrect Options:** * **Mycoplasma hominis:** While it is a common inhabitant of the lower genitourinary tract, it is primarily associated with **pelvic inflammatory disease (PID)**, post-abortal fever, and postpartum fever, rather than NGU. It is also distinguished by its ability to metabolize arginine. * **Mycoplasma pneumoniae:** This is a strictly respiratory pathogen. It is the leading cause of **primary atypical pneumonia** ("walking pneumonia") and is transmitted via respiratory droplets, not sexual contact. * **Mycoplasma fermentans:** This species is considered an opportunistic pathogen and has been isolated from the blood and tissues of patients with HIV/AIDS, but it is not a primary cause of NGU. **High-Yield Clinical Pearls for NEET-PG:** * **NGU Etiology:** *Chlamydia trachomatis* (D-K) is the most common cause, followed by *Ureaplasma urealyticum* and *Mycoplasma genitalium*. * **Cell Wall:** Mycoplasmas lack a peptidoglycan cell wall; therefore, they are **intrinsically resistant to Beta-lactams** (Penicillins/Cephalosporins). * **Culture:** They require sterols (cholesterol) for growth and produce characteristic **"fried-egg" colonies** on PPLO agar (except *M. pneumoniae*, which has a granular appearance). * **Treatment:** Macrolides (Azithromycin) or Tetracyclines (Doxycycline) are the drugs of choice.

Question 822: Regarding Proteus, which statement is true?

A. Musty odor
B. Most common infection caused is respiratory tract infection
C. PPA test positive (Correct Answer)
D. Esthiomene is a complication in females

Explanation: ### Explanation **Correct Answer: C. PPA test positive** **Why it is correct:** *Proteus* species belong to the tribe **Proteeae** (which includes *Proteus*, *Morganella*, and *Providencia*). A key biochemical hallmark of this tribe is the ability to produce the enzyme **phenylalanine deaminase**. * **The PPA Test:** This test detects the oxidative deamination of phenylalanine into **phenylpyruvic acid (PPA)**. When 10% ferric chloride is added to the culture, it reacts with PPA to produce a characteristic **dark green color**, signifying a positive result. **Analysis of Incorrect Options:** * **A. Musty odor:** *Proteus* is famous for its **fishy or "putrid" odor**. A "musty" or "mousy" odor is characteristic of *Haemophilus influenzae*. * **B. Most common infection:** While *Proteus* can cause various infections, its most common clinical presentation is **Urinary Tract Infection (UTI)**, not respiratory tract infection. It is notorious for causing "struvite" or staghorn calculi due to its potent urease activity. * **D. Esthiomene:** This is a chronic hypertrophic ulceration of the vulva, which is a late complication of **Lymphogranuloma Venereum (LGV)** caused by *Chlamydia trachomatis* (serotypes L1-L3), not *Proteus*. **High-Yield Clinical Pearls for NEET-PG:** * **Swarming Growth:** *Proteus mirabilis* and *P. vulgaris* exhibit "swarming" on blood agar due to their peritrichous flagella. This can be inhibited by increasing agar concentration (6%) or adding boric acid/chloral hydrate. * **Dienes Phenomenon:** Used to differentiate two strains of *Proteus*; a line of inhibited growth occurs where two different strains meet. * **Urease Positive:** *Proteus* produces urease, which splits urea into ammonia, raising urinary pH and leading to the formation of **Staghorn Calculi** (Magnesium ammonium phosphate). * **Weil-Felix Reaction:** An agglutination test using *Proteus* antigens (OX19, OX2, OXK) to diagnose Rickettsial infections (Heterophile agglutination).

Question 823: Which one of the following enteric organisms is anaerogenic and non-motile?

A. Shigella sonnei (Correct Answer)
B. Salmonella typhi
C. Proteus mirabilis
D. Klebsiella pneumonia

Explanation: ### Explanation The correct answer is **Shigella sonnei**. In microbiology, **anaerogenic** refers to an organism that does not produce gas (CO₂ and H₂) during the fermentation of carbohydrates. **Non-motile** refers to the absence of flagella. **Why Shigella sonnei is correct:** All species of the genus *Shigella* are characteristically **non-motile** and **anaerogenic** (with the rare exception of certain *S. flexneri* serotypes). They are Gram-negative, non-lactose fermenting (NLF) bacilli (though *S. sonnei* is a late-lactose fermenter). Their lack of motility is a key diagnostic feature used to differentiate them from other Enterobacteriaceae. **Analysis of Incorrect Options:** * **Salmonella typhi:** While it is anaerogenic (it produces acid but no gas from glucose), it is **motile** via peritrichous flagella. * **Proteus mirabilis:** This organism is highly **motile** (exhibiting characteristic "swarming" on agar) and is **aerogenic** (produces gas during fermentation). * **Klebsiella pneumoniae:** While it is **non-motile** (due to a thick polysaccharide capsule), it is characteristically **aerogenic**, producing abundant gas during the fermentation of lactose and other sugars. **NEET-PG High-Yield Pearls:** * **The "Non-Motile" Rule:** Remember the mnemonic **"K-S-Y"** for non-motile Enterobacteriaceae: *Klebsiella*, *Shigella*, and *Yersinia* (at 37°C; *Yersinia* is motile at 25°C). * **Gas Production:** *Shigella* and *S. typhi* are the two major enteric pathogens that do **not** produce gas from glucose. * **S. sonnei specific:** It is the only *Shigella* species that is a **late-lactose fermenter** (forming pale pink colonies on MacConkey agar after 48 hours) and is Indole negative.

Question 824: Which bacterium is associated with serotype O117:K1:H7?

A. Mycobacterium TB
B. Mycobacterium leprae
C. E. coli (Correct Answer)
D. Klebsiella

Explanation: **Explanation:** The correct answer is **C. E. coli**. The classification of *Escherichia coli* is based on its antigenic structure, specifically the **Kauffman-White classification**. This system identifies bacteria based on three primary antigens: 1. **O Antigen (Somatic):** Located on the lipopolysaccharide of the cell wall. 2. **K Antigen (Capsular):** An acidic polysaccharide antigen that covers the O antigen. 3. **H Antigen (Flagellar):** Proteinaceous antigen found on the flagella. The specific serotype **O117:K1:H7** is a classic example of this nomenclature. Notably, the **K1 capsular antigen** is a high-yield fact for NEET-PG, as it is chemically identical to the Group B polysaccharide of *Neisseria meningitidis* and is the primary virulence factor responsible for **neonatal meningitis** caused by *E. coli*. **Why other options are incorrect:** * **Mycobacterium tuberculosis & M. leprae:** These are acid-fast bacilli (AFB) characterized by high lipid content (mycolic acids) in their cell walls. They do not possess the O:K:H antigenic framework used for Gram-negative enteric bacteria. * **Klebsiella:** While *Klebsiella* possesses O and K antigens (it is non-motile, so it lacks H antigens), its serotyping does not follow the O117:K1:H7 pattern. *Klebsiella* is primarily typed based on its prominent polysaccharide capsule (K antigens). **High-Yield Clinical Pearls for NEET-PG:** * **K1 Antigen:** Associated with 80% of *E. coli* neonatal meningitis cases. * **Vi Antigen:** A specific type of K (capsular) antigen found in *Salmonella Typhi*. * **H Antigen:** Useful for identifying motile strains; absent in *Shigella* and *Klebsiella*.

Question 825: A patient with AIDS returned from Haiti with acute diarrhea. The stool revealed an oval organism (8 to 9 um in diameter) that was acid-fast and fluoresced blue under ultraviolet light. What is the most likely identification of this organism?

A. Cyclospora (Correct Answer)
B. Giardia
C. Cryptosporidium
D. Enterocytozoon

Explanation: ### Explanation The clinical presentation and laboratory findings point specifically to **Cyclospora cayetanensis**, a coccidian parasite that is a common cause of traveler’s diarrhea and opportunistic infections in immunocompromised patients (like those with AIDS). **Why Cyclospora is correct:** 1. **Size:** The organism measures **8–10 µm**, which is the classic "double the size" rule compared to *Cryptosporidium*. 2. **Acid-fast staining:** It is **variably acid-fast**, meaning some oocysts stain red while others appear as "ghosts." 3. **Autofluorescence:** This is the pathognomonic feature for NEET-PG. Under UV light (330–365 nm), *Cyclospora* oocysts exhibit **intense blue fluorescence** due to the presence of phenolic compounds in the oocyst wall. 4. **Epidemiology:** It is frequently associated with travel to tropical regions like Haiti or the consumption of contaminated imported produce (e.g., raspberries, basil). **Why other options are incorrect:** * **Cryptosporidium:** While also acid-fast and common in AIDS, it is significantly smaller (**4–6 µm**) and **does not fluoresce** under UV light. * **Giardia:** This is a flagellated protozoan, not a coccidian. It is **not acid-fast** and has a characteristic pear shape with "falling leaf" motility. * **Enterocytozoon (Microsporidia):** These are much smaller (**1–2 µm**) and require special stains like Modified Trichrome or Calcofluor White; they are not typically identified by standard acid-fast staining. **High-Yield Pearls for NEET-PG:** * **Size Comparison:** *Cryptosporidium* (4–6 µm) < *Cyclospora* (8–10 µm) < *Isospora/Cystoisospora* (25–30 µm). * **Fluorescence:** Only *Cyclospora* (Blue) and *Isospora* (Green/Blue) show autofluorescence. * **Treatment:** Unlike *Cryptosporidium* (Nitazoxanide), *Cyclospora* is treated with **Trimethoprim-Sulfamethoxazole (TMP-SMX)**.

Question 826: Bacillary angiomatosis is caused by which organism?

A. Bartonella henselae (Correct Answer)
B. Bartonella quintana
C. Bartonella bacilliformis
D. Coxiella burnetii

Explanation: **Explanation:** **Bacillary Angiomatosis (BA)** is a vascular proliferative disease primarily seen in immunocompromised individuals (especially HIV/AIDS patients with low CD4 counts). It is characterized by red, papular, "cherry-angioma" like skin lesions that can involve internal organs. 1. **Why Option A is Correct:** **Bartonella henselae** is the most common cause of Bacillary Angiomatosis. It is also the causative agent of **Cat Scratch Disease (CSD)**. In CSD, it typically causes regional lymphadenopathy in immunocompetent hosts, whereas, in immunocompromised hosts, it manifests as the angioproliferative lesions of BA. 2. **Why Other Options are Incorrect:** * **Bartonella quintana:** While it can also cause Bacillary Angiomatosis (transmitted by the human body louse), it is more classically associated with **Trench Fever** and endocarditis. In the context of standard NEET-PG questions, if both are present, *B. henselae* is the more frequent clinical association for BA. * **Bartonella bacilliformis:** This organism causes **Carrion’s disease**, which has two phases: Oroya fever (acute hemolytic anemia) and Verruga peruana (skin nodules). It is restricted to the Andes mountains (Peru/Ecuador). * **Coxiella burnetii:** This is the causative agent of **Q fever**. It does not cause vascular skin lesions; it typically presents as an interstitial pneumonia or culture-negative endocarditis. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** *B. henselae* is transmitted via **cat fleas/scratches**, while *B. quintana* is transmitted by **lice**. * **Histology:** BA lesions show lobular proliferation of small blood vessels with **Warthin-Starry silver stain** showing clusters of bacilli. * **Treatment:** The drug of choice for Bacillary Angiomatosis is **Erythromycin** or Doxycycline. * **Differential:** BA can clinically mimic **Kaposi Sarcoma** (caused by HHV-8), but they are distinguished histologically (BA has neutrophils; KS has spindle cells).

Question 827: Which of the following statements is true about Bacteroides?

A. It is gram-positive bacilli.
B. It is strictly aerobic.
C. It may cause peritonitis. (Correct Answer)
D. Presence in stool culture indicates a need for treatment.

Explanation: **Explanation:** **Bacteroides** (specifically *Bacteroides fragilis*) are the most common anaerobic bacteria found in the human colon. 1. **Why Option C is Correct:** *Bacteroides* species are commensals of the gastrointestinal tract. However, when the mucosal barrier is breached—due to trauma, surgery, or a ruptured appendix—they escape into the sterile peritoneal cavity. Their **capsular polysaccharide** is a key virulence factor that induces abscess formation, leading to **secondary peritonitis**. 2. **Why Other Options are Incorrect:** * **Option A:** *Bacteroides* are **Gram-negative bacilli**. They are often pleomorphic and pale-staining. * **Option B:** They are **obligate (strict) anaerobes**. They lack catalase and superoxide dismutase, making them unable to survive in high-oxygen environments. * **Option D:** Since *Bacteroides* are part of the **normal resident flora** of the colon (outnumbering *E. coli* 1000:1), their presence in a stool culture is a normal finding and does not indicate infection or a need for treatment. **High-Yield Clinical Pearls for NEET-PG:** * **Bile Resistance:** *B. fragilis* is unique among anaerobes because it can grow in **20% bile**, a feature used for its laboratory identification (Bile Esculin Agar). * **Antibiotic Profile:** They are characteristically resistant to penicillin due to **beta-lactamase production**. The drug of choice is typically **Metronidazole**. * **LPS Activity:** Unlike most Gram-negative bacteria, *Bacteroides* LPS has low toxicity and does not typically cause classic endotoxic shock.

Question 828: A malignant pustule is a characteristic feature of which of the following conditions?

A. Anthrax (Correct Answer)
B. Actinomycosis
C. Tetanus
D. Gas gangrene

Explanation: **Explanation:** **Anthrax (Option A)** is the correct answer. The term **"Malignant Pustule"** refers to the characteristic skin lesion of **Cutaneous Anthrax**, caused by *Bacillus anthracis*. Despite the name, it is neither malignant (cancerous) nor a true pustule (as it lacks pus). It typically begins as a painless papule that progresses to a vesicle containing bluish-black fluid, eventually rupturing to form a **painless, depressed black eschar** surrounded by marked non-pitting edema. The lesion is "malignant" in its historical sense, referring to its potential to spread systemically if untreated. **Why the other options are incorrect:** * **Actinomycosis (Option B):** Characterized by chronic granulomatous lesions, typically in the cervicofacial region, presenting with multiple abscesses and draining sinuses that discharge **"sulfur granules."** * **Tetanus (Option C):** Caused by *Clostridium tetani* neurotoxins. It presents with muscular rigidity (lockjaw, opisthotonus) rather than a specific diagnostic skin lesion. * **Gas Gangrene (Option D):** Caused by *Clostridium perfringens*. It presents with rapidly spreading myonecrosis, crepitus (gas in tissues), and a foul-smelling discharge, but not a black eschar. **High-Yield Clinical Pearls for NEET-PG:** * **Causative Agent:** *Bacillus anthracis* (Gram-positive, spore-forming, non-motile "box-car" shaped bacilli). * **McFadyean’s Reaction:** Used for presumptive identification (capsule staining with polychrome methylene blue). * **Medusa Head Appearance:** Characteristic morphology of colonies on blood agar. * **Occupational Hazard:** Also known as **"Hide Porter’s Disease"** or "Woolsorter’s Disease" (pulmonary form). * **Virulence Factors:** Encapsulated (poly-D-glutamic acid) and produces tripartite toxin (Edema factor, Lethal factor, and Protective antigen).

Question 829: Which of the following pigments is produced exclusively by Pseudomonas aeruginosa?

A. Pyoverdin
B. Pyorubin
C. Pyocyanin (Correct Answer)
D. Pyomelanin

Explanation: **Explanation:** *Pseudomonas aeruginosa* is a non-fermenting Gram-negative bacillus known for its ability to produce various water-soluble pigments. **Why Pyocyanin is the Correct Answer:** **Pyocyanin** is a blue-green pigment and is the only pigment produced **exclusively** by *Pseudomonas aeruginosa*. It is a virulence factor that generates reactive oxygen species (ROS) to cause tissue damage and interfere with human ciliary function. In clinical practice, this pigment is responsible for the characteristic "blue-pus" seen in infected wounds or burns and the greenish tint on culture media like Nutrient Agar. **Analysis of Incorrect Options:** * **Pyoverdin (A):** A yellow-green fluorescent pigment (siderophore). While produced by *P. aeruginosa*, it is also produced by other *Pseudomonas* species (e.g., *P. fluorescens*, *P. putida*). * **Pyorubin (B):** A red-brown pigment produced by some strains of *P. aeruginosa*, but it can also be seen in other bacteria and is not unique to this species. * **Pyomelanin (D):** A brown-black pigment produced by certain hyper-oxidative strains of *P. aeruginosa*, but it is also produced by other organisms like *Legionella pneumophila*. **NEET-PG High-Yield Pearls:** 1. **Culture Characteristics:** *P. aeruginosa* produces a characteristic **fruity/grape-like odor** (due to aminoacetophenone). 2. **Growth:** It can grow at **42°C**, a feature used to differentiate it from other *Pseudomonas* species. 3. **Biochemical Tests:** It is **Oxidase positive** and Catalase positive. 4. **Clinical Association:** It is a leading cause of nosocomial pneumonia, Ecthyma gangrenosum in neutropenic patients, and chronic lung infections in **Cystic Fibrosis** patients.

Question 830: Gram stain is a:

A. Simple stain
B. Differential stain (Correct Answer)
C. Negative stain
D. None of the above

Explanation: **Explanation:** **1. Why "Differential Stain" is correct:** A differential stain uses more than one dye to distinguish between different groups of bacteria or different parts of a single organism based on their chemical or structural properties. The Gram stain, developed by **Christian Gram**, differentiates bacteria into two broad groups—**Gram-positive** (purple) and **Gram-negative** (pink)—based on the composition of their cell walls. Gram-positive bacteria have a thick peptidoglycan layer that retains the primary stain (Crystal Violet), while Gram-negative bacteria have a thin layer and an outer membrane that loses the primary stain and takes up the counterstain (Safranin). **2. Why other options are incorrect:** * **Simple Stain:** Uses only a single dye (e.g., Methylene blue) to color the bacteria. It helps visualize the size, shape, and arrangement but cannot differentiate between species. * **Negative Stain:** Colors the background rather than the organism itself (e.g., India Ink for *Cryptococcus neoformans*). It is used to demonstrate capsules, which do not take up ordinary stains. **3. High-Yield Clinical Pearls for NEET-PG:** * **Steps of Gram Stain:** Primary Stain (Crystal Violet) → Mordant (Gram’s Iodine) → Decolorizer (Acetone/Alcohol) → Counterstain (Safranin). * **The most crucial step** is decolorization; over-decolorizing can make Gram-positives appear Gram-negative. * **Gram-variable organisms:** Some bacteria (like *Gardnerella vaginalis* or aging cultures) may show inconsistent staining. * **Non-Gram staining organisms:** Remember the mnemonic **"These Microbes May Lack Real Color"** (Treponema, Mycobacteria, Mycoplasma, Legionella, Rickettsia, Chlamydia). These require alternative methods like Acid-fast or Giemsa stains.

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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Mycobacteria

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Spirochetes

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