Bacteriology — MCQs
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What is MYPA agar used for?
Which of the following is NOT true about Neisseria gonorrhoeae?
The 'string of pearls' appearance on microscopy is characteristic of which bacterium?
Which streptodornase is most antigenic in human beings?
A 30-year-old woman presents with vaginitis. She complains of a slightly increased, malodorous discharge that is gray-white in color, thin, and homogenous. Clue cells are discovered upon microscopic examination of the discharge. Which of the following organisms is the most likely cause of her infection?
All of the following organisms cause interstitial pneumonia except?
Ritter's disease is caused by?
The Weil-Felix test is useful in the diagnosis of which of the following conditions, except?
A farmer with a history of handling cows presents with an ulcer over the hand with a central black eschar. Which statement regarding the organism responsible is false?
Which of the following is NOT seen in Mycoplasma pneumoniae infections?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 491: What is MYPA agar used for?
- A. Bacillus anthracis
- B. Bacillus cereus (Correct Answer)
- C. Campylobacter
- D. Staphylococcus aureus
Explanation: **Explanation:** **MYPA (Mannitol Egg Yolk Polymyxin Agar)** is the standard selective and differential medium used for the isolation and enumeration of **Bacillus cereus**. The medium works based on three key components: 1. **Mannitol:** *B. cereus* is mannitol-negative; therefore, it does not ferment mannitol, and the colonies remain **pink/red** (due to the phenol red indicator). 2. **Egg Yolk:** *B. cereus* produces the enzyme **Lecithinase**, which breaks down the lecithin in egg yolk, creating a characteristic **opaque halo** around the colonies. 3. **Polymyxin B:** This acts as a selective agent that inhibits the growth of most Gram-negative bacteria and other competing flora. **Analysis of Incorrect Options:** * **Bacillus anthracis:** While related, *B. anthracis* is typically isolated on **PLET medium** (Polymyxin, Lysozyme, EDTA, Thallous acetate). Unlike *B. cereus*, *B. anthracis* is non-hemolytic and non-motile. * **Campylobacter:** This requires specialized media like **Skirrow’s medium** or **Butzler’s medium**, and microaerophilic conditions for growth. * **Staphylococcus aureus:** The classic selective medium for *S. aureus* is **Mannitol Salt Agar (MSA)**, where it fermentatively produces yellow colonies. **High-Yield Clinical Pearls for NEET-PG:** * **B. cereus Food Poisoning:** Occurs in two forms: **Emetic type** (associated with reheated fried rice, caused by the heat-stable toxin *Cereulide*) and **Diarrheal type** (associated with meat/vegetables, caused by heat-labile enterotoxin). * **Other Media for B. cereus:** PEMBA (Polymyxin Pyruvate Egg Yolk Mannitol Bromothymol Blue Agar). * **Nagler Reaction:** *B. cereus* is Nagler positive (lecithinase activity), a property it shares with *Clostridium perfringens*.
Question 492: Which of the following is NOT true about Neisseria gonorrhoeae?
- A. Gram positive cocci (Correct Answer)
- B. Causes stricture of the urethra
- C. Involves seminal vesicles and spreads to the epididymis
- D. Drug of choice is ceftriaxone
Explanation: ### Explanation **1. Why Option A is the Correct Answer (The "False" Statement):** *Neisseria gonorrhoeae* is a **Gram-negative diplococcus**, not Gram-positive. Under a microscope, they typically appear as kidney-bean or coffee-bean shaped pairs with adjacent sides flattened. They are characteristically found within polymorphonuclear leukocytes (intracellular). **2. Analysis of Other Options:** * **Option B (Stricture of the urethra):** This is a classic chronic complication in males. Repeated or untreated gonococcal urethritis leads to inflammation and subsequent fibrosis of the urethral canal, resulting in narrowing (strictures). * **Option C (Spread to epididymis):** In males, the infection can ascend from the urethra to involve the prostate, seminal vesicles, and epididymis. Acute epididymitis is a common complication that can lead to infertility if bilateral. * **Option D (Drug of choice):** Due to widespread resistance to penicillin and fluoroquinolones, **Ceftriaxone** (a third-generation cephalosporin) is the current drug of choice. It is often given as a single intramuscular dose. **3. High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** Thayer-Martin medium (selective) or Chocolate agar (non-selective). * **Biochemical Test:** Oxidase positive and ferments **only glucose** (unlike *N. meningitidis*, which ferments both glucose and maltose). * **Virulence Factor:** Pili are the most important for attachment to mucosal surfaces and inhibiting phagocytosis. * **Co-infection:** Often co-exists with *Chlamydia trachomatis*; hence, dual therapy (Ceftriaxone + Azithromycin/Doxycycline) was traditionally recommended. * **Neonatal manifestation:** Causes *Ophthalmia neonatorum*, occurring within 2–5 days of birth.
Question 493: The 'string of pearls' appearance on microscopy is characteristic of which bacterium?
- A. Clostridium species
- B. Bacillus anthracis (Correct Answer)
- C. Streptococcus pneumoniae
- D. Staphylococcus aureus
Explanation: **Explanation:** The **'string of pearls' appearance** is a classic diagnostic feature of **Bacillus anthracis**. This phenomenon occurs when the bacteria are cultured on agar containing low concentrations of **penicillin** (0.05–0.5 units/mL). Penicillin induces the formation of large, spherical protoplasts that remain attached in chains, resembling a string of pearls under the microscope. This test is specifically used to differentiate *B. anthracis* from other non-pathogenic *Bacillus* species (like *B. cereus*), which are typically resistant to penicillin and do not show this reaction. **Analysis of Incorrect Options:** * **Clostridium species:** These are anaerobic, gram-positive bacilli. While some species (like *C. perfringens*) show a "target hemolysis" on blood agar, they do not exhibit the string of pearls phenomenon. * **Streptococcus pneumoniae:** These appear as gram-positive, lancet-shaped diplococci. They are characterized by the "Quellung reaction" and "draughtsman" or "checkerboard" colony morphology. * **Staphylococcus aureus:** These are gram-positive cocci in clusters (grape-like). They are identified by gold-yellow colonies and positive catalase/coagulase tests. **High-Yield Clinical Pearls for NEET-PG:** * **Colony Morphology:** *B. anthracis* produces **"Medusa head"** colonies (frosted glass appearance with wavy outgrowths). * **McFadyean’s Reaction:** Uses polychrome methylene blue to demonstrate the **pink-colored capsule** (composed of poly-D-glutamic acid). * **Selective Medium:** **PLET medium** (Polymyxin, Lysozyme, Ethylene diamine tetra-acetic acid, and Thallous acetate). * **Virulence Factors:** Encoded on plasmids **pXO1** (toxins: PA, LF, EF) and **pXO2** (capsule).
Question 494: Which streptodornase is most antigenic in human beings?
- A. Streptodornase A
- B. Streptodornase B (Correct Answer)
- C. Streptodornase C
- D. Streptodornase D
Explanation: **Explanation:** Streptodornase is a deoxyribonuclease (DNase) produced by *Streptococcus pyogenes* (Group A Streptococcus). It functions by liquefying thick, purulent exudates by degrading DNA, which facilitates the spread of the infection. There are four immunologically distinct types of streptodornase: A, B, C, and D. **Why Streptodornase B is the correct answer:** Among the four types, **Streptodornase B** is the most antigenic in humans. Following a streptococcal infection, the body produces specific antibodies against this enzyme, known as **Anti-DNase B**. This is clinically significant because while the Anti-Streptolysin O (ASO) titer may remain low or negative in patients with streptococcal skin infections (pyoderma/impetigo), the Anti-DNase B titer consistently rises. Therefore, it is the most reliable serological marker for documenting previous streptococcal skin infections and associated post-streptococcal glomerulonephritis (PSGN). **Analysis of Incorrect Options:** * **Streptodornase A, C, and D:** While these enzymes are produced by various strains of Streptococci and perform similar biochemical functions (DNA degradation), they are significantly less immunogenic than Type B. They do not elicit a diagnostic antibody response useful for clinical serology. **Clinical Pearls for NEET-PG:** * **ASO vs. Anti-DNase B:** ASO is the gold standard for diagnosing Rheumatic Fever (following pharyngitis). Anti-DNase B is the gold standard for diagnosing PSGN (following skin infections). * **Therapeutic Use:** A mixture of Streptokinase and Streptodornase (e.g., Varidase) is used for the enzymatic debridement of wounds to dissolve clots and thick exudates. * **Biochemical property:** Streptodornase requires Magnesium (Mg²⁺) ions for its activation.
Question 495: A 30-year-old woman presents with vaginitis. She complains of a slightly increased, malodorous discharge that is gray-white in color, thin, and homogenous. Clue cells are discovered upon microscopic examination of the discharge. Which of the following organisms is the most likely cause of her infection?
- A. Candida albicans
- B. Trichomonas vaginalis
- C. Escherichia coli
- D. Gardnerella vaginalis (Correct Answer)
Explanation: ### Explanation The clinical presentation described is a classic case of **Bacterial Vaginosis (BV)**, primarily caused by **Gardnerella vaginalis**. **Why Gardnerella vaginalis is correct:** Bacterial Vaginosis is characterized by a shift in vaginal flora from protective Lactobacilli to anaerobic bacteria, including *Gardnerella vaginalis*. The diagnosis is based on **Amsel’s Criteria** (3 out of 4 required): 1. **Thin, homogenous, gray-white discharge** that coats the vaginal walls. 2. **Clue cells** on wet mount (vaginal epithelial cells studded with coccobacilli, giving a "stippled" or "shaggy" appearance). 3. **Vaginal pH > 4.5**. 4. **Positive Whiff test** (fishy odor upon adding 10% KOH due to the release of amines). **Why the other options are incorrect:** * **Candida albicans:** Causes Vulvovaginal Candidiasis. It presents with a thick, "curdy" or **cottage-cheese-like** discharge, intense pruritus, and a normal pH (<4.5). Microscopy shows pseudohyphae and budding yeast. * **Trichomonas vaginalis:** A protozoan infection causing Trichomoniasis. It presents with a **profuse, frothy, green-yellow** discharge and a "strawberry cervix" (punctate hemorrhages). Motile pear-shaped trophozoites are seen on wet mount. * **Escherichia coli:** While a common cause of UTIs, it is not a primary cause of the specific clinical syndrome of malodorous vaginitis with clue cells. **High-Yield Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Nugent Scoring (Gram stain of vaginal discharge). * **Drug of Choice:** Metronidazole (Oral or Gel). * **Clue Cells:** These are the most pathognomonic finding for BV. * **Note:** BV is a "vaginosis," not "vaginitis," because there is a lack of significant inflammation (no pus cells/leukocytes) despite the discharge.
Question 496: All of the following organisms cause interstitial pneumonia except?
- A. Pneumocystis carinii
- B. Mycoplasma pneumoniae
- C. Legionella pneumophila
- D. Haemophilus influenzae (Correct Answer)
Explanation: ### Explanation The key to answering this question lies in distinguishing between **Typical** and **Atypical** pneumonia patterns. **1. Why Haemophilus influenzae is the correct answer:** *Haemophilus influenzae* is a classic cause of **Typical Pneumonia**. It typically results in **lobar pneumonia** or bronchopneumonia characterized by intra-alveolar purulent exudate. On a chest X-ray, this manifests as dense consolidation rather than interstitial infiltrates. It is a pyogenic bacterium that triggers a robust neutrophil response within the air spaces. **2. Why the other options are incorrect:** The other three organisms are classic causes of **Interstitial (Atypical) Pneumonia**, where the inflammation is primarily confined to the alveolar septa (interstitium) rather than the alveoli themselves. * **Pneumocystis carinii (jirovecii):** An opportunistic fungus in immunocompromised patients (especially HIV) that causes bilateral "ground-glass" interstitial opacities. * **Mycoplasma pneumoniae:** The most common cause of "Walking Pneumonia." It lacks a cell wall and characteristically causes patchy interstitial infiltrates that look "worse than the patient feels." * **Legionella pneumophila:** While it can cause severe consolidation, it is classified as an "atypical" pathogen because it does not grow on standard media and often presents with interstitial patterns or multi-lobar involvement alongside systemic symptoms. **3. NEET-PG High-Yield Pearls:** * **Typical Pneumonia (Consolidation):** *S. pneumoniae* (most common), *H. influenzae*, *Staph. aureus*, *Klebsiella*. * **Atypical Pneumonia (Interstitial):** *Mycoplasma*, *Chlamydia*, *Legionella*, Viruses (Influenza, RSV, CMV), and *Pneumocystis*. * **Mycoplasma** is associated with **Cold Agglutinins** (anti-I antibodies). * **Legionella** is associated with **hyponatremia** and exposure to contaminated water/air conditioning systems.
Question 497: Ritter's disease is caused by?
- A. Bacillus anthracis
- B. Streptococcus pneumoniae
- C. Staphylococcus aureus (Correct Answer)
- D. Pseudomonas aeruginosa
Explanation: **Explanation:** **Ritter’s disease**, also known as **Staphylococcal Scalded Skin Syndrome (SSSS)**, is caused by specific strains of **Staphylococcus aureus** (primarily Phage Group II, types 71 and 55). The pathogenesis involves the production of **Exfoliative toxins (Epidermolytic toxins) A and B**. These toxins act as serine proteases that specifically target and cleave **Desmoglein-1**, a protein responsible for cell-to-cell adhesion in the *stratum granulosum* of the epidermis. This leads to diffuse erythema and widespread sloughing of the skin, typically in neonates and young children. **Analysis of Options:** * **Staphylococcus aureus (Correct):** It is the definitive causative agent. The clinical hallmark is a positive **Nikolsky sign** (dislodgement of intact superficial epidermis by slight rubbing). * **Bacillus anthracis:** Causes Anthrax. Cutaneous anthrax presents as a painless "malignant pustule" with a central black eschar, not diffuse skin peeling. * **Streptococcus pneumoniae:** Primarily causes respiratory infections (pneumonia), meningitis, and otitis media. It does not produce exfoliative toxins. * **Pseudomonas aeruginosa:** Associated with "Ecthyma gangrenosum" in immunocompromised patients and "Hot tub folliculitis," but not Ritter's disease. **High-Yield NEET-PG Pearls:** * **Site of Cleavage:** Stratum granulosum (superficial). This distinguishes it from Toxic Epidermal Necrolysis (TEN), where the cleavage is at the dermo-epidermal junction (deep). * **Culture:** In Ritter’s disease, the skin blisters are usually **sterile** because the damage is toxin-mediated from a distant site (e.g., nasopharynx or conjunctiva). * **Bullous Impetigo:** A localized form of SSSS where the toxin remains restricted to the site of infection; here, blisters *will* yield *S. aureus* on culture.
Question 498: The Weil-Felix test is useful in the diagnosis of which of the following conditions, except?
- A. Epidemic typhus
- B. Scrub typhus
- C. Rocky Mountain Spotted Fever (RMSF)
- D. Trench fever (Correct Answer)
Explanation: **Explanation:** The **Weil-Felix test** is a heterophile agglutination test used for the diagnosis of Rickettsial diseases. It relies on a cross-reaction between antibodies produced during certain rickettsial infections and the somatic (O) antigens of specific strains of **Proteus vulgaris (OX-19, OX-2)** and **Proteus mirabilis (OX-K)**. **Why Trench Fever is the correct answer:** Trench fever is caused by ***Bartonella quintana***. Unlike Rickettsia, Bartonella species do not share cross-reactive antigens with Proteus strains. Therefore, the Weil-Felix test is **negative** in Trench fever. Other Rickettsial diseases that are Weil-Felix negative include Q fever (*Coxiella burnetii*) and Rickettsialpox (*R. akari*). **Analysis of Incorrect Options:** * **Epidemic typhus (*R. prowazekii*):** Characteristically shows a strongly positive reaction with **OX-19**. * **Scrub typhus (*Orientia tsutsugamushi*):** This is the only major rickettsial disease that reacts exclusively with **OX-K**. * **Rocky Mountain Spotted Fever (*R. rickettsii*):** Being part of the Spotted Fever Group, it typically shows positive reactions with both **OX-19 and OX-2**. **High-Yield Clinical Pearls for NEET-PG:** * **Summary Table:** * **Typhus Group** (Epidemic/Endemic): OX-19 (+++), OX-2 (+) * **Spotted Fever Group**: OX-19 (+), OX-2 (+++) * **Scrub Typhus**: OX-K (+++); OX-19 and OX-2 are negative. * **Limitations:** The test lacks high sensitivity and specificity; definitive diagnosis is now preferred via Immunofluorescence Assay (IFA) or PCR. * **Mnemonic:** "K" for **K**atayama/Scrub (OX-K).
Question 499: A farmer with a history of handling cows presents with an ulcer over the hand with a central black eschar. Which statement regarding the organism responsible is false?
- A. Gram positive bacillus
- B. Non-motile
- C. Spore bearing
- D. Capsule is made up of polyglutamic acid. (Correct Answer)
Explanation: ### Explanation The clinical presentation of a **central black eschar** in a farmer (occupational exposure to cattle) is a classic description of **Cutaneous Anthrax**, caused by *Bacillus anthracis*. **1. Why Option D is the "False" statement (in the context of this specific question):** Actually, there appears to be a technical error in the question's marking. In standard microbiology, *Bacillus anthracis* **is** characterized by a capsule made of **poly-D-glutamic acid**. However, in many competitive exams, if this is marked as the "false" statement, it is often a trick regarding the **biochemical nature** (e.g., polypeptide vs. polysaccharide). Most bacterial capsules are polysaccharides; *B. anthracis* is the unique exception with a **polypeptide** capsule. If the option intended to imply it is a polysaccharide, it would be false. *Note: If this is a "Select the False statement" question and D is marked correct, it implies the statement as written is considered incorrect by the examiner, likely due to a distinction between "polyglutamic acid" and "poly-D-glutamic acid" or a typo in the provided key.* **2. Analysis of Other Options:** * **Option A (Gram-positive bacillus):** True. *B. anthracis* is a large, square-ended, Gram-positive rod often described as having a "bamboo stick" appearance. * **Option B (Non-motile):** True. This is a key differentiating feature. While most *Bacillus* species are motile, *B. anthracis* is **non-motile** (Non-motile, Non-hemolytic, and sensitive to Penicillin). * **Option C (Spore-bearing):** True. It forms central, elliptical, non-bulging spores. Spores are formed in the soil or culture but **never** in the living host tissue. **3. Clinical Pearls for NEET-PG:** * **McFadyean’s Reaction:** Used to visualize the capsule using polychrome methylene blue (appears as amorphous purple material around blue bacilli). * **Medusa Head Appearance:** Characteristic morphology of colonies on agar. * **String of Pearls Reaction:** Occurs when grown on agar containing low concentrations of penicillin. * **Virulence Factors:** Encoded on plasmids **pXO1** (Toxins: Lethal factor, Edema factor, Protective antigen) and **pXO2** (Capsule).
Question 500: Which of the following is NOT seen in Mycoplasma pneumoniae infections?
- A. Diffuse infiltration of lungs
- B. Cannot be cultured routinely
- C. Cefotaxime is the drug of choice (Correct Answer)
- D. Serology is useful in diagnosis
Explanation: **Explanation:** The correct answer is **C**, as **Cefotaxime is NOT the drug of choice** for *Mycoplasma pneumoniae*. **1. Why Cefotaxime is incorrect (The Medical Concept):** *Mycoplasma pneumoniae* is a unique bacterium that **lacks a cell wall**. Cefotaxime is a third-generation Cephalosporin (Beta-lactam) that acts by inhibiting cell wall synthesis. Since *Mycoplasma* has no cell wall target, all Beta-lactams, Penicillins, and Vancomycin are inherently ineffective. The drugs of choice are protein synthesis inhibitors, specifically **Macrolides (Azithromycin)**, Tetracyclines (Doxycycline), or Fluoroquinolones. **2. Analysis of other options:** * **Option A (Diffuse infiltration):** *M. pneumoniae* typically causes "Atypical Pneumonia." Radiologically, this presents as diffuse interstitial or patchy infiltrates that often look much worse than the patient’s clinical symptoms (termed "walking pneumonia"). * **Option B (Cannot be cultured routinely):** While it can be grown on specialized media (e.g., PPLO agar/Eaton’s agar), it is extremely fastidious and slow-growing (taking 1–3 weeks). Therefore, culture is not used for routine clinical diagnosis. * **Option D (Serology is useful):** Because culture is slow, diagnosis relies on serology. **Cold agglutinins** (IgM antibodies against RBC I-antigen) are a classic bedside test, though specific ELISA for IgM/IgG or PCR is now preferred. **High-Yield Clinical Pearls for NEET-PG:** * **Appearance:** "Fried egg" colonies on Eaton's agar. * **Cell Membrane:** Contains **sterols** (unique among bacteria), which provide structural integrity in the absence of a cell wall. * **Complications:** Bullous myringitis, Stevens-Johnson Syndrome, and autoimmune hemolytic anemia (due to cold agglutinins). * **Gram Stain:** Does not Gram stain because it lacks a cell wall.
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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