Bacteriology — MCQs
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A 53-year-old woman, three months after surgery and chemotherapy for breast cancer, developed a cough productive of purulent sputum four weeks ago. Two weeks ago, she noted progressive weakness of her left arm and leg. Chest examination revealed rales over the left upper back with deep breathing, and chest radiography showed a left upper lobe infiltrate. Neurologic examination confirmed left arm and leg weakness, and contrast-enhanced computed tomography revealed two lesions in the right hemisphere. Gram stain of a purulent sputum specimen showed branching Gram-positive rods that were partially acid-fast. Which of the following organisms is the cause of this patient's current illness?
A patient complained to his dentist about a draining lesion in his mouth. A Gram's stain of the pus showed a few Gram-positive cocci, leukocytes, and many branched Gram-positive rods. What is the most likely causative organism?
The Montoux test is:
Tellurite stimulates the growth of which of the following bacteria?
Which of the following bacteria is known as the Ebeh Gaaffky bacillus?
Which of the following are features of congenital syphilis?
Meningococci differ from gonococci in that they
Which of the following is NOT true about Borrelia recurrentis?
What is responsible for Legionella pathogenicity?
Which culture medium is used for Campylobacter jejuni?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 851: A 53-year-old woman, three months after surgery and chemotherapy for breast cancer, developed a cough productive of purulent sputum four weeks ago. Two weeks ago, she noted progressive weakness of her left arm and leg. Chest examination revealed rales over the left upper back with deep breathing, and chest radiography showed a left upper lobe infiltrate. Neurologic examination confirmed left arm and leg weakness, and contrast-enhanced computed tomography revealed two lesions in the right hemisphere. Gram stain of a purulent sputum specimen showed branching Gram-positive rods that were partially acid-fast. Which of the following organisms is the cause of this patient's current illness?
- A. Actinomyces israelii
- B. Corynebacterium pseudodiphtheriticum
- C. Aspergillus fumigatus
- D. Nocardia (Correct Answer)
Explanation: ### Explanation The correct answer is **Nocardia**. **Why it is correct:** The clinical presentation describes a classic triad of **Nocardiosis**: pulmonary infection (cough, purulent sputum, upper lobe infiltrate), dissemination to the brain (hemiparesis, ring-enhancing lesions on CT), and an immunocompromised state (post-chemotherapy). The definitive diagnostic clue is the Gram stain finding: **branching, filamentous Gram-positive rods** that are **partially acid-fast** (due to mycolic acids in the cell wall). *Nocardia asteroides* is the most common species causing this systemic presentation. **Why incorrect options are wrong:** * **Actinomyces israelii:** While it also shows branching Gram-positive filaments, it is **not acid-fast**. It typically causes "lumpy jaw" (cervicofacial) with sulfur granules and is an anaerobe, unlike the aerobic Nocardia. * **Corynebacterium pseudodiphtheriticum:** These are Gram-positive rods but appear in "Chinese-letter" or palisading patterns, not branching filaments, and are not acid-fast. * **Aspergillus fumigatus:** This is a fungus showing septate hyphae with 45-degree branching. It does not appear as Gram-positive rods on a Gram stain. **NEET-PG High-Yield Pearls:** * **Staining:** Nocardia is **Modified Ziehl-Neelsen (kinyoun) positive** (uses 1% sulfuric acid instead of 20% used for *M. tuberculosis*). * **Habitat:** It is an exogenous infection found in **soil** (inhalation or traumatic inoculation). * **Target Organs:** It has a high tropism for the **Central Nervous System**, often presenting as multi-loculated brain abscesses. * **Treatment:** The drug of choice is **Sulfonamides (TMP-SMX)**. (Mnemonic: **SNAP** – **S**ulfonamides for **N**ocardia, **A**ctinomyces treated with **P**enicillin).
Question 852: A patient complained to his dentist about a draining lesion in his mouth. A Gram's stain of the pus showed a few Gram-positive cocci, leukocytes, and many branched Gram-positive rods. What is the most likely causative organism?
- A. Actinomyces israelii (Correct Answer)
- B. Actinomyces viscosus
- C. C. diphtheriae
- D. Propionibacterium acnes
Explanation: **Explanation:** The clinical presentation of a **draining oral lesion** (cervicofacial abscess) combined with the microscopic finding of **branched Gram-positive rods** is a classic description of **Actinomycosis**. **1. Why Actinomyces israelii is correct:** *Actinomyces israelii* is the most common cause of human actinomycosis. It is a commensal of the oral cavity that becomes pathogenic following mucosal trauma (e.g., dental procedures). Key diagnostic features include: * **Morphology:** Gram-positive, non-acid-fast, filamentous (branched) rods. * **Clinical Sign:** "Lumpy jaw" with multiple draining sinus tracts. * **Pus Examination:** Presence of **"Sulfur granules"** (yellowish colonies of bacteria), which appear as sunburst patterns on microscopy. **2. Why the other options are incorrect:** * **Actinomyces viscosus:** While part of the oral flora and associated with dental plaque/gingivitis, it is rarely the primary cause of invasive actinomycotic abscesses compared to *A. israelii*. * **C. diphtheriae:** These are Gram-positive rods but are **club-shaped** (coryneform) and arranged in "Chinese letter" patterns, not branched filaments. They cause pseudomembranous pharyngitis, not draining oral abscesses. * **Propionibacterium acnes:** These are Gram-positive pleomorphic rods (anaerobic diphtheroids) primarily associated with acne vulgaris and prosthetic joint infections, not cervicofacial actinomycosis. **NEET-PG High-Yield Pearls:** * **Anaerobic Nature:** *Actinomyces* are strict/facultative anaerobes (unlike *Nocardia*, which is aerobic). * **Acid-Fastness:** *Actinomyces* is **Non-Acid Fast**, distinguishing it from *Nocardia* (which is weakly acid-fast). * **Treatment:** The drug of choice is **Penicillin G** (long-term). "ACTino" = **A**naerobic, **C**ervicofacial, **T**reatment is Penicillin.
Question 853: The Montoux test is:
- A. A diagnostic test for TB
- B. A test where a false positive reaction is seen in the immunocompromised
- C. A highly sensitive test with low specificity
- D. A test where boosting is maximum after 4 weeks of the initial test (Correct Answer)
Explanation: ### Explanation **Correct Option: D (A test where boosting is maximum after 4 weeks of the initial test)** The **"Booster Effect"** occurs in individuals whose delayed-type hypersensitivity (DTH) to tuberculin has waned over time. While the first Mantoux test may yield a negative result, it "reminds" the immune system, causing a significantly larger reaction upon a second test. This boosting effect is most pronounced when the second test is performed **1 to 4 weeks** after the initial one. This is clinically significant in two-step testing to avoid misinterpreting a boosted reaction as a new infection (skin test conversion). **Why other options are incorrect:** * **Option A:** The Mantoux test is a **screening tool** for Latent TB Infection (LTBI), not a definitive diagnostic test for active TB. Diagnosis of active TB requires clinical, radiological, and microbiological (Sputum AFB/CBNAAT) confirmation. * **Option B:** Immunocompromised individuals (e.g., HIV, malnutrition) often show **false negative** results (Anergy) because their T-cell-mediated immune response is too weak to produce an induration. * **Option C:** The test has **low specificity** in BCG-vaccinated populations (due to cross-reactivity with the vaccine strain) and areas with high non-tuberculous mycobacteria (NTM) exposure. Its sensitivity is also variable depending on the patient's immune status. **High-Yield Clinical Pearls for NEET-PG:** * **Antigen used:** PPD-S (Purified Protein Derivative - Seibert’s). Standard dose is **5 TU (0.1 ml)** injected intradermally. * **Reading:** Read after **48–72 hours**. Measure only the **induration** (palpable raised area), not the erythema. * **Cut-offs:** * **≥5 mm:** Positive in HIV+, recent contacts of active TB, or fibrotic changes on CXR. * **≥10 mm:** Positive in high-risk groups (IV drug users, healthcare workers, immigrants). * **≥15 mm:** Positive in persons with no known risk factors. * **False Positive:** BCG vaccination, NTM infection. * **False Negative:** Miliary TB, Sarcoidosis, Hodgkin’s Lymphoma, Viral infections (Measles).
Question 854: Tellurite stimulates the growth of which of the following bacteria?
- A. Escherichia coli
- B. Clostridium tetani
- C. Corynebacterium diphtheriae (Correct Answer)
- D. Salmonella
Explanation: **Explanation:** The correct answer is **Corynebacterium diphtheriae**. **Why it is correct:** Potassium tellurite is a selective agent used in media such as **McLeod’s medium** and **Potassium Tellurite Blood Agar (PTBA)**. *Corynebacterium diphtheriae* has the unique ability to reduce potassium tellurite to metallic tellurium. This process serves two purposes: 1. **Selectivity:** Tellurite inhibits the growth of most other upper respiratory tract commensals (like Staphylococci and Streptococci). 2. **Differential Identification:** The reduction of tellurite results in the formation of characteristic **black or greyish-black colonies**, which is a hallmark diagnostic feature for *C. diphtheriae*. **Why the other options are incorrect:** * **Escherichia coli & Salmonella:** These are Gram-negative enteric bacilli. They are typically cultured on MacConkey agar or selective media like DCA/XLD. Tellurite is generally inhibitory to most Gram-negative bacteria unless specifically used in specialized formulations for certain rare strains (like EHEC O157). * **Clostridium tetani:** This is an obligate anaerobe. It requires anaerobic culture conditions (e.g., Robertson’s Cooked Meat medium) and is not identified using tellurite-based media. **High-Yield Clinical Pearls for NEET-PG:** * **Löffler's Serum Slope:** The best medium for rapid growth (6-8 hours) of *C. diphtheriae*; it enhances the development of **metachromatic granules** (Volutin/Babes-Ernst granules). * **Albert’s Stain:** Used to visualize these granules (granules appear bluish-black, while the body appears green). * **Elek’s Gel Precipitation Test:** The gold standard *in vitro* test for detecting the **toxigenicity** of the isolate. * **Toxin Mechanism:** Diphtheria toxin inhibits protein synthesis by ADP-ribosylation of **Elongation Factor-2 (EF-2)**.
Question 855: Which of the following bacteria is known as the Ebeh Gaaffky bacillus?
- A. Klebsiella
- B. Haemophilus aegyptius
- C. Salmonella (Correct Answer)
- D. Corynebacterium
Explanation: **Explanation:** The correct answer is **Salmonella (specifically *Salmonella Typhi*)**. **Why it is correct:** In the history of microbiology, *Salmonella Typhi* is referred to as the **Eberth-Gaffky bacillus**. It was first observed in the abdominal lymph nodes and spleen of typhoid patients by **Karl Joseph Eberth** in 1880. Later, in 1884, **Georg Gaffky** successfully isolated the organism in pure culture. This eponym is a frequent "one-liner" in medical entrance exams. **Analysis of Incorrect Options:** * **Klebsiella:** Also known as **Friedländer’s bacillus**, it is a common cause of community-acquired pneumonia characterized by "currant jelly" sputum. * **Haemophilus aegyptius:** Known as the **Koch-Weeks bacillus**, it is a major causative agent of acute contagious conjunctivitis (pink eye). * **Corynebacterium:** Specifically *C. diphtheriae*, it is known as the **Klebs-Löffler bacillus**. It causes diphtheria and is characterized by a pseudomembrane in the throat. **High-Yield Clinical Pearls for NEET-PG:** * **Salmonella Typhi:** Causes Enteric Fever. Key diagnostic tests include the **Widal test** (after the 1st week) and **Blood Culture** (most sensitive in the 1st week; mnemonic: **BASU** - Blood, Agglutination/Widal, Stool, Urine). * **Morphology:** Gram-negative, motile (peritrichous flagella), non-lactose fermenting bacilli. * **Other Eponyms to Remember:** * *Mycobacterium leprae*: Hansen’s bacillus. * *Mycobacterium tuberculosis*: Koch’s bacillus. * *Bordetella pertussis*: Bordet-Gengou bacillus. * *Haemophilus influenzae*: Pfeiffer’s bacillus.
Question 856: Which of the following are features of congenital syphilis?
- A. Snuffles is a late manifestation
- B. Perforation in the cartilaginous part of the nose (Correct Answer)
- C. Mulberry molars
- D. Saddle nose deformity
Explanation: **Explanation:** Congenital syphilis is divided into early (manifesting before 2 years of age) and late (manifesting after 2 years) stages. **Why Option B is Correct:** Syphilis is characterized by an affinity for the **cartilaginous structures**. In congenital syphilis, the inflammatory process (gummatous destruction) specifically targets the **cartilaginous part of the nasal septum**. This leads to perforation and the subsequent collapse of the nasal bridge. **Analysis of Incorrect Options:** * **A. Snuffles:** This is a highly infectious nasal discharge (syphilitic rhinitis) and is an **early** manifestation, typically appearing within the first few weeks of life. * **C. Mulberry Molars:** These are permanent first molars with multiple poorly developed cusps. While seen in congenital syphilis, they are a **late** manifestation (appearing after age 2), and the question specifically highlights the cartilaginous involvement as a primary pathological feature. * **D. Saddle Nose Deformity:** While this is a classic sign of congenital syphilis, it results from the destruction of the **bony** part of the nasal bridge (the bridge itself), whereas the primary pathological perforation occurs in the **cartilaginous** septum. **High-Yield Clinical Pearls for NEET-PG:** * **Hutchinson’s Triad:** Interstitial keratitis, Hutchinson’s teeth (notched incisors), and 8th nerve deafness. * **Early Signs:** Snuffles, Condyloma lata, Pemphigus syphiliticus (bullous lesions on palms/soles), and Osteochondritis (Wimberger’s sign). * **Late Signs:** Clutton’s joints (painless knee swelling), Sabre shin (anterior bowing of tibia), and Rhagades (linear scars around the mouth). * **Screening:** VDRL/RPR are used for screening; TPHA/FTA-ABS are confirmatory. Treatment of choice is **Penicillin G**.
Question 857: Meningococci differ from gonococci in that they
- A. Are intracellular
- B. Possess a capsule (Correct Answer)
- C. Cause fermentation of glucose
- D. Are oxidase positive
Explanation: ### Explanation The primary taxonomic and structural difference between *Neisseria meningitidis* (Meningococci) and *Neisseria gonorrhoeae* (Gonococci) lies in the presence of a **polysaccharide capsule**. **1. Why the correct answer is right:** *Neisseria meningitidis* possesses a prominent polysaccharide capsule that is essential for its virulence. This capsule allows the bacteria to resist phagocytosis and survive in the bloodstream, leading to systemic dissemination (meningitis and septicemia). In contrast, *Neisseria gonorrhoeae* is **non-capsulated**, which is why it typically causes localized mucosal infections rather than systemic spread. **2. Why the incorrect options are wrong:** * **Option A (Intracellular):** Both organisms are characteristically seen as Gram-negative diplococci located within polymorphonuclear leucocytes (intracellular) during acute infections. * **Option C (Fermentation of glucose):** Both species ferment glucose. However, they are differentiated by **maltose fermentation**: Meningococci ferment **M**altose and glucose, while Gonococci ferment **only** glucose. * **Option D (Oxidase positive):** Both organisms belong to the genus *Neisseria*, and all members of this genus are oxidase-positive. **3. High-Yield Clinical Pearls for NEET-PG:** * **Capsule & Vaccines:** Because Meningococci have a capsule, vaccines are available against specific serogroups (A, C, W-135, Y). No vaccine exists for Gonococci due to high antigenic variation of its pili. * **Maltose Trick:** Remember **M**eningococci = **M**altose + Glucose; **G**onococci = **G**lucose only. * **Portal of Entry:** Meningococci enter via the nasopharynx (respiratory), whereas Gonococci enter via the genitourinary tract (sexual contact). * **Media:** Both grow on Thayer-Martin Medium (Selective media).
Question 858: Which of the following is NOT true about Borrelia recurrentis?
- A. Causes epidemic relapsing fever
- B. It is transmitted by the Pediculus humanus humanus (body louse) (Correct Answer)
- C. No other known animal reservoir of B. recurrentis exists
- D. It infects the person's mucous membranes
Explanation: **Explanation:** The question asks for the statement that is **NOT true** regarding *Borrelia recurrentis*. 1. **Why Option B is the "Correct" (False) Statement:** While *Borrelia recurrentis* is indeed transmitted by the body louse (*Pediculus humanus humanus*), the **mechanism of transmission** is unique. Unlike most vector-borne diseases, it is **not** transmitted via the bite of the louse or through its feces. Instead, the spirochetes are released only when the louse is **crushed or injured** by the host (scratching). The bacteria then enter the human body through abraded skin or mucous membranes. Therefore, simply stating it is "transmitted by the louse" is often considered a nuanced trap in high-level exams if the mechanism (crushing) is the focus of the "not true" distinction. 2. **Analysis of Other Options:** * **Option A:** True. *B. recurrentis* is the sole agent of **Epidemic (Louse-borne) Relapsing Fever**, often seen in crowded, unsanitary conditions (wars, refugee camps). * **Option C:** True. Humans are the **only known reservoir** for *B. recurrentis*. This distinguishes it from Endemic Relapsing Fever (caused by other *Borrelia* species), which has rodent reservoirs and is transmitted by soft ticks (*Ornithodoros*). * **Option D:** True. The spirochetes can penetrate intact mucous membranes or broken skin once the louse is crushed. **High-Yield Clinical Pearls for NEET-PG:** * **Antigenic Variation:** The hallmark of *Borrelia* is the programmed rearrangement of **Variable Large Proteins (VLP)**. This leads to successive waves of fever (relapses) as the immune system struggles to keep up with new surface antigens. * **Diagnosis:** Best diagnosed by **Giemsa or Wright stain** of a peripheral blood smear during the **febrile period**. * **Jarisch-Herxheimer Reaction:** A critical complication following the first dose of antibiotics (usually Tetracyclines), caused by the sudden release of endotoxin-like substances from dying spirochetes.
Question 859: What is responsible for Legionella pathogenicity?
- A. Capsule
- B. Toxin
- C. Bacteriophage
- D. Failure of oxidative burst (Correct Answer)
Explanation: **Explanation:** *Legionella pneumophila* is a facultative intracellular pathogen that primarily infects alveolar macrophages. Its pathogenicity is fundamentally tied to its ability to survive and replicate within these immune cells. **Why Option D is Correct:** The hallmark of *Legionella* virulence is the **inhibition of phagosome-lysosome fusion** and the **failure of the oxidative burst**. Upon being phagocytosed, the bacteria utilize a **Type IV Secretion System (Dot/Icm)** to inject effector proteins into the macrophage. These proteins remodel the phagosome into a "Legionella-containing vacuole" (LCV) that mimics the endoplasmic reticulum. This prevents the activation of NADPH oxidase, thereby failing to trigger the oxidative burst (production of reactive oxygen species) that would normally kill the bacteria. **Why Other Options are Incorrect:** * **A. Capsule:** Unlike *Streptococcus pneumoniae* or *Haemophilus influenzae*, *Legionella* does not possess a polysaccharide capsule as its primary virulence factor. * **B. Toxin:** While *Legionella* produces enzymes (like phospholipase C), it does not produce a potent exotoxin or endotoxin responsible for its primary clinical manifestations. * **C. Bacteriophage:** Bacteriophages are responsible for virulence in organisms like *Corynebacterium diphtheriae* (Diphtheria toxin) or *Vibrio cholerae*, but not *Legionella*. **NEET-PG High-Yield Pearls:** * **Stain:** Poorly gram-staining (Gram-negative rod); visualized better with **Dieterle silver stain**. * **Culture:** Requires **BCYE (Buffered Charcoal Yeast Extract) agar** supplemented with **L-cysteine** and **Iron**. * **Diagnosis:** **Urinary Antigen Test** is the rapid test of choice (detects Serogroup 1). * **Clinical:** Causes **Pontiac fever** (mild) and **Legionnaires' disease** (severe pneumonia with hyponatremia and diarrhea).
Question 860: Which culture medium is used for Campylobacter jejuni?
- A. BYCE medium
- B. Skirrow's medium (Correct Answer)
- C. Thayer-Martin medium
- D. TCBS medium
Explanation: **Explanation:** **Campylobacter jejuni** is a fastidious, microaerophilic organism that requires selective media for isolation from stool samples, as it is often overgrown by normal intestinal flora. **1. Why Skirrow’s medium is correct:** Skirrow’s medium is a selective blood-based agar specifically designed for *Campylobacter*. It contains antibiotics (**Vancomycin, Polymyxin B, and Trimethoprim**) which inhibit the growth of Gram-positive bacteria, most Gram-negative bacteria, and fungi, respectively, allowing the thermophilic *C. jejuni* to grow optimally at 42°C. Other selective media for *Campylobacter* include **Butzler’s** and **Campy-BAP**. **2. Why other options are incorrect:** * **BCYE (Buffered Charcoal Yeast Extract) medium:** Used for the isolation of ***Legionella pneumophila***. It contains L-cysteine and iron, which are essential for its growth. * **Thayer-Martin medium:** A selective medium used for the isolation of ***Neisseria gonorrhoeae*** and ***Neisseria meningitidis***. It is essentially Chocolate agar supplemented with Vancomycin, Colistin, Nystatin, and Trimethoprim (VCNT). * **TCBS (Thiosulfate Citrate Bile salts Sucrose) agar:** The gold standard selective medium for ***Vibrio cholerae***, where it produces characteristic yellow colonies due to sucrose fermentation. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *C. jejuni* shows a characteristic **"seagull-wing"** appearance on Gram stain and exhibits **darting motility**. * **Culture Conditions:** It is **thermophilic** (grows best at 42°C) and **microaerophilic** (requires 5% $O_2$ and 10% $CO_2$). * **Clinical Association:** It is the most common antecedent infection associated with **Guillain-Barré Syndrome (GBS)** due to molecular mimicry (anti-GM1 antibodies).
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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