Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1101: Which of the following microorganisms constitutes the normal flora of the oral cavity?

A. Escherichia coli
B. Staphylococcus epidermidis
C. Branhamella catarrhalis (Correct Answer)
D. Picornavirus

Explanation: **Explanation:** The oral cavity and upper respiratory tract harbor a diverse ecosystem of commensal microorganisms. **Branhamella catarrhalis** (now officially reclassified as **Moraxella catarrhalis**) is a Gram-negative aerobic diplococcus that is a well-recognized member of the normal flora of the nasopharynx and oral cavity. While usually commensal, it can become an opportunistic pathogen causing otitis media in children and exacerbations of COPD in adults. **Analysis of Options:** * **A. Escherichia coli:** This is a Gram-negative rod primarily found as normal flora in the **lower gastrointestinal tract** (large intestine). Its presence in the oral cavity is transient and not considered normal flora. * **B. Staphylococcus epidermidis:** This is the predominant commensal of the **skin**. While *Staphylococcus aureus* can sometimes colonize the anterior nares, *S. epidermidis* is not a primary resident of the oral cavity. * **D. Picornavirus:** Viruses are generally not considered "normal flora" as they are obligate intracellular entities. Picornaviruses (like Rhinovirus or Poliovirus) are associated with infections rather than commensal colonization. **High-Yield NEET-PG Pearls:** * **Predominant Oral Flora:** The most common organisms in the mouth are **Viridans group streptococci** (e.g., *S. mutans, S. sanguis*). * **Moraxella (Branhamella) catarrhalis:** It is **oxidase-positive** and **catalase-positive**. It is a common cause of "culture-negative" looking sputum in COPD patients because it resembles Neisseria species. * **Gingival Crevices:** These are often colonized by **anaerobes** like *Bacteroides* and *Fusobacterium*, which are implicated in aspiration pneumonia.

Question 1102: A 45-year-old man presents with two painless beefy red ulcers in the inguinal region. A biopsy and Giemsa stain reveal Donovan bodies. What is the most likely diagnosis?

A. Behcet's syndrome
B. Glomus tumor
C. Renal agenesis
D. Granuloma inguinale (Correct Answer)

Explanation: **Explanation:** The clinical presentation of **painless, beefy red ulcers** in the inguinal region, combined with the histological finding of **Donovan bodies**, is pathognomonic for **Granuloma inguinale** (also known as Donovanosis). **1. Why Granuloma Inguinale is Correct:** Granuloma inguinale is caused by the Gram-negative intracellular bacterium *Klebsiella granulomatis* (formerly *Calymmatobacterium granulomatis*). It typically presents as chronic, progressive, painless, highly vascular (beefy red) ulcers that bleed easily on touch. Diagnosis is confirmed by identifying **Donovan bodies**—safety-pin-shaped organisms seen within the cytoplasm of large mononuclear cells (macrophages) on Giemsa or Wright stain. **2. Why the Other Options are Incorrect:** * **Behcet’s Syndrome:** A multisystem inflammatory disorder characterized by the triad of recurrent oral ulcers, genital ulcers, and uveitis. These ulcers are typically painful and do not show Donovan bodies. * **Glomus Tumor:** A benign, exquisitely painful vascular tumor usually found under the nail bed (subungual). It does not present as inguinal ulceration. * **Renal Agenesis:** A congenital condition where one or both kidneys fail to develop. It has no association with genital ulcers or microbiological findings. **3. High-Yield Clinical Pearls for NEET-PG:** * **Causative Agent:** *Klebsiella granulomatis*. * **Pathognomonic Sign:** Donovan bodies (intracellular "safety-pin" appearance). * **Clinical Feature:** "Pseudobubo" (inguinal swelling due to granulation tissue, not true lymphadenopathy). * **Drug of Choice:** Azithromycin (1g orally once a week or 500mg daily for at least 3 weeks). * **Differential:** Syphilis (painless chancre but not beefy red) and Chancroid (painful ulcers).

Question 1103: Watercan perineum is caused by which bacterium?

A. Escherichia coli
B. Enterococcus fecalis
C. Neisseria gonorrhoeae (Correct Answer)
D. Treponema pallidium

Explanation: **Explanation:** **Watercan perineum** is a classic clinical manifestation of chronic, untreated **Neisseria gonorrhoeae** infection. The underlying mechanism involves gonococcal urethritis leading to inflammation and subsequent **urethral stricture**. When the stricture obstructs urine flow, proximal pressure increases, causing the formation of periurethral abscesses. These abscesses eventually rupture through the skin of the perineum and scrotum, creating multiple chronic discharging sinuses. When the patient voids, urine leaks through these multiple openings, resembling the spray of a watering can. **Analysis of Options:** * **Neisseria gonorrhoeae (Correct):** It is the primary cause of inflammatory urethral strictures in males, which is the prerequisite for developing a watercan perineum. * **Escherichia coli:** While a common cause of urinary tract infections (UTIs), it typically causes acute cystitis or pyelonephritis rather than the chronic fibrotic strictures required for sinus formation. * **Enterococcus fecalis:** A common commensal and cause of nosocomial UTIs, but it does not possess the specific virulence factors to cause extensive urethral scarring and multiple fistulae. * **Treponema pallidum:** The causative agent of Syphilis. It presents with chancres (primary), rashes (secondary), or gummas (tertiary), but does not typically cause urethral strictures or perineal sinuses. **High-Yield Clinical Pearls for NEET-PG:** * **Thayer-Martin Medium:** The selective medium used for isolating *N. gonorrhoeae*. * **Pili:** The most important virulence factor for attachment to mucosal surfaces. * **Co-infection:** Always screen for *Chlamydia trachomatis* in patients with Gonorrhea. * **Treatment:** Ceftriaxone (IM) is the drug of choice for uncomplicated gonococcal infections.

Question 1104: Which is the most common microorganism known to cause tropical pyomyositis?

A. Streptococcus viridans
B. Staphylococcus albus
C. Klebsiella pneumoniae
D. Staphylococcus aureus (Correct Answer)

Explanation: **Explanation:** **Tropical Pyomyositis** is a primary intramuscular abscess involving large skeletal muscles (most commonly the quadriceps or gluteal muscles). It is typically seen in tropical climates and often follows minor local trauma. **Why Staphylococcus aureus is correct:** *Staphylococcus aureus* is the causative agent in **70% to 90%** of all cases. The pathogenesis involves a transient bacteremia (often from an occult site) that seeds into a muscle previously damaged by trauma, vigorous exercise, or nutritional deficiencies. Its potent arsenal of toxins and enzymes (like coagulase and leukocidins) facilitates tissue necrosis and abscess formation within the muscle fascia. **Why other options are incorrect:** * **Streptococcus viridans:** These are commensals of the oral cavity and are primarily associated with subacute bacterial endocarditis, not primary muscle abscesses. * **Staphylococcus albus (S. epidermidis):** This is a coagulase-negative staphylococcus (CoNS). While it is a common cause of prosthetic valve endocarditis and catheter-associated infections, it lacks the virulence factors to cause pyomyositis in healthy muscle. * **Klebsiella pneumoniae:** While Gram-negative bacilli can cause pyomyositis in immunocompromised or diabetic patients, they are rare in the classic "tropical" presentation compared to *S. aureus*. **High-Yield Clinical Pearls for NEET-PG:** * **Most common site:** Quadriceps muscle. * **Clinical Stages:** 1. Invasive stage (cramping/pain), 2. Suppurative stage (abscess formation - most common presentation), 3. Late stage (sepsis). * **Diagnosis:** Ultrasound or MRI is the imaging modality of choice to localize the deep-seated abscess. * **Treatment:** Incision and drainage (I&D) combined with antibiotics (cloxacillin or vancomycin if MRSA is suspected).

Question 1105: Which of the following statements regarding Salmonella infection is true?

A. Flagellar antigen forms chalky granular clumps.
B. Somatic antigen is a protein.
C. Absence of Vi antibody indicates a poor prognosis. (Correct Answer)
D. Serological classification is by Craige's scheme.

Explanation: ### Explanation **1. Why the Correct Answer is Right:** In Enteric fever (Typhoid), the **Vi (Virulence) antigen** is a capsular polysaccharide that covers the O antigen, protecting the bacteria from phagocytosis. During the course of the infection, the development of Vi antibodies is a sign of a robust immune response. Clinically, the **absence of Vi antibodies** in a confirmed case of typhoid fever is associated with a **poor prognosis**, as it suggests a failure of the host's immune system to recognize and respond to the virulent capsular component of *Salmonella Typhi*. Conversely, the persistence of Vi antibodies after clinical recovery is a high-yield marker for identifying **chronic carriers**. **2. Why the Other Options are Incorrect:** * **Option A:** This describes the **Somatic (O) antigen**. O-agglutination results in compact, **chalky granular clumps**. In contrast, **Flagellar (H) antigens** form large, loose, **fluffy cotton-wool-like clumps**. * **Option B:** The **Somatic (O) antigen** is a **lipopolysaccharide (LPS)** complex (specifically the phospholipid-polysaccharide part). The **Flagellar (H) antigen** is the one made of protein (flagellin). * **Option D:** Serological classification (serotyping) of *Salmonella* is based on the **Kauffman-White scheme**, which identifies O and H antigens. **Craige’s tube method** is used specifically for demonstrating **phase variation** in flagellar antigens, not for general serological classification. **3. NEET-PG High-Yield Pearls:** * **Widal Test:** Measures antibodies against O and H antigens. O antibodies appear early and disappear early (indicate recent infection); H antibodies appear late and persist. * **Carrier Detection:** The most reliable screening tool for the typhoid carrier state is the **Vi antibody titer** (followed by stool/bile culture). * **Enrichment Media:** Selenite F broth and Tetrathionate broth are used for *Salmonella* isolation from stool. * **Selective Media:** Wilson and Blair’s Bismuth Sulfite Medium (jet black colonies with metallic sheen).

Question 1106: Trachoma is caused by which of the following serotypes of Chlamydia?

A. D - K
B. L1 - L3
C. L5 - L7
D. A, B, Ba, C (Correct Answer)

Explanation: **Explanation:** *Chlamydia trachomatis* is an obligate intracellular bacterium classified into several serovars (serotypes) based on differences in its Major Outer Membrane Protein (MOMP). These serotypes have distinct tissue tropisms and clinical manifestations. **Correct Answer: D (A, B, Ba, C)** Serotypes **A, B, Ba, and C** are the causative agents of **Trachoma**, a chronic keratoconjunctivitis. It is the leading infectious cause of preventable blindness worldwide. These serotypes primarily infect the ocular epithelium and are transmitted via hand-to-eye contact, flies, and fomites. * *Mnemonic:* **A**-**C** causes **B**lindness (**A**frican **B**lindness **C**hlamydia). **Incorrect Options:** * **Option A (D - K):** These serotypes cause **Genital Infections** (Non-gonococcal urethritis, cervicitis, PID) and **Inclusion Conjunctivitis** in adults and neonates (Ophthalmia neonatorum). Unlike Trachoma, these do not typically lead to blindness. * **Option B (L1 - L3):** These serotypes cause **Lymphogranuloma Venereum (LGV)**, a systemic sexually transmitted infection characterized by painful inguinal lymphadenopathy (buboes) and proctitis. * **Option C (L5 - L7):** These serotypes do not exist in the standard classification of *Chlamydia trachomatis*. **High-Yield Clinical Pearls for NEET-PG:** * **SAFE Strategy (WHO):** **S**urgery (for trichiasis), **A**ntibiotics (Azithromycin), **F**acial cleanliness, and **E**nvironmental improvement. * **Diagnosis:** Inclusion bodies called **Halberstaedter-Prowazek bodies** (intracytoplasmic) can be seen on Giemsa stain. * **Drug of Choice:** A single oral dose of **Azithromycin** is the preferred treatment for Trachoma control programs. * **Pathogenesis:** Blindness in Trachoma results from chronic inflammation leading to conjunctival scarring, entropion (inward turning of eyelids), and trichiasis (eyelashes rubbing the cornea).

Question 1107: Corynebacterium diphtheriae is:

A. Gram positive (Correct Answer)
B. Resistant to penicillin
C. Gram negative
D. Resistant to chloramphenicol

Explanation: **Explanation:** *Corynebacterium diphtheriae*, the causative agent of diphtheria, is a classic **Gram-positive**, non-motile, non-spore-forming bacillus. Under the microscope, these organisms exhibit a characteristic "Chinese letter" or cuneiform arrangement due to incomplete separation during binary fission (snapping division). **Why the other options are incorrect:** * **Option B & D:** *C. diphtheriae* is generally **sensitive** to a wide range of antibiotics, including **Penicillin** (the drug of choice) and Erythromycin. It is also susceptible to Chloramphenicol. Antibiotics are used to stop toxin production and eradicate the carrier state, though they do not neutralize toxin already bound to tissues. * **Option C:** As a Gram-positive organism, it retains the crystal violet stain due to its thick peptidoglycan layer, unlike Gram-negative bacteria which possess an outer membrane and stain pink/red. **High-Yield Clinical Pearls for NEET-PG:** * **Metachromatic Granules:** Also known as Volutin or Babes-Ernst granules; these are best visualized with **Albert’s stain** (appear bluish-black against a green cytoplasm). * **Culture Media:** Highly selective media include **Loeffler’s Serum Slope** (rapid growth) and **Potassium Tellurite Agar** (colonies appear grey-black). * **Virulence Factor:** The Diphtheria toxin (an AB toxin) is produced only by strains lysogenized by the **beta-phage**. It acts by inhibiting protein synthesis via ADP-ribosylation of **EF-2**. * **Toxin Detection:** The **Elek’s gel precipitation test** is the gold standard for detecting toxigenicity.

Question 1108: Transient colonization is caused by which of the following?

A. Herpes Simplex Virus (HSV)
B. Trichomonas vaginalis
C. Staphylococcus aureus
D. All of the above (Correct Answer)

Explanation: ### Explanation **Concept of Colonization** Colonization refers to the presence and multiplication of microorganisms on a host surface (skin or mucous membranes) without causing immediate tissue invasion or immune response. * **Resident Flora:** Permanent inhabitants of a specific site. * **Transient Flora:** Microorganisms that inhabit the skin or mucous membranes temporarily (hours to weeks). They are derived from the environment, do not establish themselves permanently, and can be potential pathogens if the host's immunity is compromised. **Why "All of the Above" is Correct:** 1. **Herpes Simplex Virus (HSV):** While HSV is known for latency in nerve ganglia, it undergoes **transient colonization** of the mucosal surfaces (oropharynx or genitals) during periods of asymptomatic shedding. 2. **Trichomonas vaginalis:** This parasite colonizes the vaginal or urethral mucosa. It does not become part of the permanent resident flora and is typically acquired through contact, representing a transient state until treated or cleared. 3. **Staphylococcus aureus:** This is a classic example of transient flora. About 20-30% of the general population are "persistent carriers" in the anterior nares, but a large majority are **transient colonizers** who harbor the bacteria for short periods. **Clinical Pearls for NEET-PG:** * **Hand Hygiene:** The primary goal of handwashing in hospitals is to remove **transient flora** (like *S. aureus* or Gram-negative bacilli) acquired from patients, as these are the most common causes of nosocomial infections. * **Resident Flora:** Examples include *Staphylococcus epidermidis* (skin), *Viridans streptococci* (mouth), and *Lactobacillus* (vagina). * **Carrier State:** A person colonized by a potential pathogen (like *S. typhi* or *N. meningitidis*) without clinical disease is a "carrier," a key concept in epidemiology.

Question 1109: What is the most common organism causing hemolytic uremic syndrome?

A. Campylobacter
B. Salmonella
C. Enterohemorrhagic E coli (Correct Answer)
D. Shigella

Explanation: **Explanation:** **Hemolytic Uremic Syndrome (HUS)** is a clinical triad characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. **Why Enterohemorrhagic E. coli (EHEC) is correct:** EHEC, specifically the **O157:H7 serotype**, is the most common cause of HUS worldwide. The pathogenesis involves the production of **Shiga-like toxins (Verotoxins)**. These toxins enter the bloodstream and bind to **Gb3 receptors**, which are highly expressed on glomerular endothelial cells. This leads to endothelial damage, microthrombi formation, and subsequent mechanical destruction of red blood cells (schistocytes) and platelet consumption. **Analysis of Incorrect Options:** * **Campylobacter:** While *C. jejuni* is a common cause of bloody diarrhea, it is more classically associated with **Guillain-Barré Syndrome** rather than HUS. * **Salmonella:** *Salmonella* species cause gastroenteritis and enteric fever but are not typically associated with the Shiga-toxin-mediated pathway required for HUS. * **Shigella:** Specifically *Shigella dysenteriae* type 1, produces the potent **Shiga toxin** and can cause HUS. However, it is much less common globally than EHEC as an etiology. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Presentation:** A child with a history of bloody diarrhea (hemorrhagic colitis) followed by oliguria and pallor. * **Diagnosis:** Peripheral smear shows **Schistocytes** (helmet cells) and decreased platelets. * **Management Tip:** Avoid antibiotics and anti-motility agents in suspected EHEC diarrhea, as they may increase toxin release and the risk of HUS. * **Atypical HUS:** Caused by genetic mutations in the alternative complement pathway (e.g., Factor H deficiency), not by infection.

Question 1110: All of the following are true about Chlamydia except?

A. Gram positive (Correct Answer)
B. Causes trachoma
C. Causative organism of psittacosis
D. Are also called basophilic viruses

Explanation: **Explanation:** **Chlamydia** species are unique, obligate intracellular bacteria. The correct answer is **Option A** because Chlamydia are **Gram-negative** organisms, not Gram-positive. Although they possess an inner and outer membrane similar to Gram-negative bacteria, they lack a traditional peptidoglycan layer (the "peptidoglycan paradox"), making them difficult to visualize with a standard Gram stain. **Analysis of Options:** * **Option B (Causes trachoma):** *Chlamydia trachomatis* (Serotypes A, B, Ba, and C) is the leading infectious cause of blindness worldwide, characterized by chronic follicular conjunctivitis. * **Option C (Causative organism of psittacosis):** *Chlamydia psittaci* causes psittacosis (parrot fever), a zoonotic pneumonia transmitted through the inhalation of dried bird excreta. * **Option D (Basophilic viruses):** Historically, Chlamydiae were misclassified as "basophilic viruses" because they are filterable, obligate intracellular parasites and form large inclusion bodies that stain blue (basophilic) with Giemsa stain. **High-Yield NEET-PG Pearls:** 1. **Life Cycle:** Exists in two forms—the **Elementary Body (EB)**, which is infectious and extracellular, and the **Reticulate Body (RB)**, which is the metabolically active, replicative intracellular form. 2. **Staining:** Best visualized using **Giemsa, Castaneda, or Gimenez stains**. 3. **Diagnosis:** **NAAT (Nucleic Acid Amplification Test)** is the gold standard for diagnosis. 4. **Treatment:** Macrolides (Azithromycin) or Tetracyclines (Doxycycline) are the drugs of choice.

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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