Bacteriology Practice Questions

Q: Which of the following is the causative agent of Yaws?

Treponema pertenue. This question tests your knowledge of the **Non-venereal Treponematoses**, a group of chronic diseases caused by bacteria morphologically and serologically identical to *Treponema pallidum* (the cause of syphilis), but transmitted via non-sexual contact. ### **Explanation of Options** * **A. Treponema pertenue (Correct):** This is the causative agent of **Yaws**. It is the most common non-venereal treponematosis, primarily affecting children in humid, tropical regions. It affects the skin, bones, and joints. The characteristic primary lesion is the "Mother Yaw." * **B. Treponema carateum:** This agent causes **Pinta**. It is found exclusively in Central and South America and affects only the skin (causing pigmentary changes), without involving bones or internal organs. * **C. Treponema pallidum (subspecies pallidum):** This is the causative agent of **Venereal Syphilis**, a sexually transmitted infection with primary, secondary, and tertiary stages, including congenital transmission. * **D. Treponema endemicum:** This agent causes **Endemic Syphilis (Bejel)**. It is typically found in arid regions (Middle East/Africa) and is transmitted through contaminated drinking vessels or mouth-to-mouth contact. ### **High-Yield Clinical Pearls for NEET-PG** * **Morphology:** All Treponemes are thin, spiral-shaped organisms (spirochetes) that cannot be grown on culture media. They are visualized using **Dark-ground microscopy**. * **Serology:** Patients with Yaws, Pinta, or Bejel will test **positive** for both non-specific (VDRL/RPR) and specific (TPHA/FTA-ABS) syphilis tests, making history and clinical presentation vital for diagnosis. * **Treatment:** The drug of choice for all Treponematoses (including Yaws) is a single dose of **Benzathine Penicillin G** (or Azithromycin as an alternative). * **Eradication:** Yaws is targeted by the WHO for global eradication (the "Morges Strategy"). India was declared Yaws-free in 2016.

Q: Fournier gangrene is caused by which type of infection?

Mixed infection. **Explanation:** **Fournier gangrene** is a life-threatening, rapidly progressing **necrotizing fasciitis** of the perineal, perianal, and genital regions. **1. Why "Mixed Infection" is correct:** The underlying pathophysiology involves a **polymicrobial (Type I necrotizing fasciitis)** synergistic infection. It typically involves a mixture of **aerobes** (such as *E. coli*, *Klebsiella*, and *Proteus*) and **anaerobes** (such as *Bacteroides* and *Peptostreptococcus*). These organisms work in synergy: aerobes consume oxygen, creating a low-redox environment that favors the rapid multiplication of anaerobes. This leads to endarteritis obliterans, resulting in skin and subcutaneous tissue necrosis. **2. Why other options are incorrect:** * **Clostridium welchii (C. perfringens):** While it can cause gas gangrene (clostridial myonecrosis), it is rarely the sole isolate in Fournier gangrene. * **Proteus:** This is a common aerobic isolate in the mix, but it does not cause the condition in isolation. * **Streptococcus:** Group A Streptococcus is the primary cause of Type II necrotizing fasciitis (usually affecting limbs), but Fournier gangrene is classically Type I (polymicrobial). **3. NEET-PG High-Yield Pearls:** * **Risk Factors:** Diabetes Mellitus (most common), chronic alcoholism, and local trauma/surgery. * **Clinical Feature:** "Crepitus" on palpation due to gas-forming organisms. * **Management:** This is a **surgical emergency**. Treatment requires aggressive surgical debridement, broad-spectrum antibiotics, and sometimes hyperbaric oxygen therapy. * **Anatomy:** The infection spreads along the **Colles’ fascia**, Scarpa’s fascia, and Dartos fascia.

Q: Lactobacilli are numerous in carious lesions primarily because they:

Are secondary invaders in the carious process. **Explanation:** The development of dental caries is a multi-stage process involving different microbial populations. The correct answer is **D** because Lactobacilli are considered **secondary invaders** rather than the primary initiators of the lesion. 1. **Why Option D is Correct:** Dental caries begins with the colonization of acidogenic bacteria like *Streptococcus mutans*. These primary colonizers lower the local pH. Lactobacilli are acidophilic (acid-loving) and aciduric (acid-tolerant) organisms. They do not easily colonize clean tooth surfaces; instead, they thrive in the acidic environment created by the primary invaders. Once a cavity (carious lesion) has formed, Lactobacilli proliferate rapidly, contributing to the **progression** of the lesion deeper into the dentin. 2. **Why Other Options are Incorrect:** * **Option A:** *Streptococcus mutans* is the main causative agent/initiator of dental caries, not Lactobacilli. * **Option B:** The production of insoluble extracellular polysaccharides (glucans) is a hallmark of *S. mutans*, which allows it to form a sticky biofilm (dental plaque). Lactobacilli generally lack this mechanism. * **Option C:** Lactobacilli have a low affinity for smooth enamel surfaces. They require a pre-existing niche, such as a pit, fissure, or an established carious cavity, to colonize. **High-Yield Clinical Pearls for NEET-PG:** * **Initiator of Caries:** *Streptococcus mutans* (Gram-positive cocci). * **Progression of Caries:** *Lactobacillus acidophilus* (Gram-positive bacilli). * **Stephan Curve:** A graph representing the drop and recovery of plaque pH after sugar consumption; the critical pH for enamel demineralization is **5.5**. * **Dextran:** The specific extracellular polysaccharide produced by *S. mutans* from sucrose that facilitates adherence.

Q: Bacilli Calmette Guerin (BCG) vaccine contains avirulent strains of which type of mycobacteria?

Bovine tubercle bacilli. **Explanation:** The **Bacilli Calmette-Guerin (BCG)** vaccine is a live attenuated vaccine used for protection against tuberculosis. It is derived from an attenuated (avirulent) strain of **Mycobacterium bovis** (Bovine tubercle bacilli). The vaccine was developed by Albert Calmette and Camille Guerin, who subcultured the virulent bovine strain 230 times over 13 years on a medium containing glycerinated bile and potato. This process resulted in the loss of virulence while maintaining the immunogenicity required to provide cross-protection against *Mycobacterium tuberculosis*. **Analysis of Options:** * **Option A (Human tubercle bacilli):** While *M. tuberculosis* is the primary cause of TB in humans, it is not the source of the BCG vaccine. * **Option B (Avian tubercle bacilli):** *M. avium* complex primarily affects birds and immunocompromised humans (HIV/AIDS) but is not used in vaccine production. * **Option D (Atypical mycobacteria):** These are Non-Tuberculous Mycobacteria (NTM) like *M. kansasii* or *M. marinum*. They are environmental organisms and are not used for the BCG vaccine. **High-Yield Clinical Pearls for NEET-PG:** * **Method of Administration:** Intradermal injection using a **26G (Omega) needle** and a Tuberculin syringe. * **Site:** Left deltoid (standardized for uniform survey recording). * **Dose:** 0.1 mL (0.05 mL for neonates below 4 weeks of age). * **Diluent:** Normal Saline (Distilled water is avoided as it causes irritation). * **Phenomenon:** A wheal is formed immediately; a papule develops after 2–3 weeks, which may ulcerate and eventually leaves a permanent **depressed scar** (the hallmark of BCG vaccination). * **Protective Effect:** Highly effective against **miliary TB and TB meningitis** in children, though its efficacy against adult pulmonary TB varies geographically.

Q: Which of the following bacteria is a common cause of watery diarrhea in children?

Enteropathogenic E. coli (EPEC). **Explanation:** **Enteropathogenic *E. coli* (EPEC)** is the correct answer because it is a classic cause of endemic and epidemic **watery diarrhea**, specifically in **infants and children** in developing countries. The underlying mechanism involves the **"Attaching and Effacing" (A/E) lesion**. EPEC uses **Bundle-Forming Pili (BFP)** for initial attachment and the **Tir-Intimin interaction** to cause effacement of microvilli and actin pedestal formation. This disruption of the intestinal absorptive surface leads to malabsorption and watery stools. **Analysis of Incorrect Options:** * **Enterohemorrhagic *E. coli* (EHEC):** Causes **bloody diarrhea** (hemorrhagic colitis) and is associated with **Hemolytic Uremic Syndrome (HUS)** due to Shiga-like toxins (Stx1, Stx2). * **Enteroinvasive *E. coli* (EIEC):** Clinically resembles *Shigella* infection. It invades the colonic epithelium, leading to **dysentery** (blood and mucus in stools) and fever. * **Enteroaggregative *E. coli* (EAEC):** Associated with **persistent diarrhea** (lasting >14 days) in both children and HIV patients. It shows a characteristic "stacked brick" adhesion pattern. **High-Yield Clinical Pearls for NEET-PG:** * **ETEC (Enterotoxigenic):** Most common cause of **Traveler’s diarrhea**; produces Heat-labile (LT) and Heat-stable (ST) toxins. * **EPEC Mnemonic:** **P** is for **P**ediatric (Infantile diarrhea). * **EHEC Key Fact:** Does **not** ferment Sorbitol (Sorbitol MacConkey Agar is used for screening *E. coli* O157:H7). * **Site of Action:** EPEC, ETEC, and EAEC primarily affect the **small intestine**, whereas EHEC and EIEC affect the **colon**.

Q: In cystic fibrosis, which is the second most common organism to cause infection?

Staphylococcus aureus. **Explanation:** In **Cystic Fibrosis (CF)**, the thick, inspissated mucus in the airways provides a nidus for recurrent bacterial infections. The prevalence of specific pathogens is highly **age-dependent**, which is a frequent focus of NEET-PG questions. * **Staphylococcus aureus (Correct):** It is the **second most common** organism overall and the **most common** organism isolated from the respiratory tract of children and infants with CF. As patients age, its prevalence is eventually surpassed by *Pseudomonas*. * **Pseudomonas aeruginosa:** This is the **most common** organism overall across the entire lifespan of a CF patient and the leading cause of chronic colonization and lung function decline in adults. **Why the other options are incorrect:** * **Streptococci & Pneumococci (A & C):** While *Streptococcus pneumoniae* can cause community-acquired pneumonia in CF patients, it is not a characteristic or predominant pathogen associated with the pathophysiology of CF bronchiectasis. * **Klebsiella (B):** This is a common cause of gram-negative pneumonia in the general population (especially in diabetics or alcoholics) but is not a primary pathogen specific to the CF airway. **High-Yield Clinical Pearls for NEET-PG:** 1. **Age-wise dominance:** * Infancy/Childhood: *Staphylococcus aureus* (Most common). * Adulthood: *Pseudomonas aeruginosa* (Most common). 2. **Burkholderia cepacia:** Its presence is a poor prognostic sign ("Cepacia syndrome") and is often a contraindication for lung transplantation. 3. **Haemophilus influenzae:** Also a common early colonizer in young children with CF. 4. **Treatment:** For *S. aureus*, use Methicillin-sensitive (MSSA) or MRSA-targeted antibiotics; for *Pseudomonas*, inhaled Tobramycin or Colistin is often used.

Q: Which of the following organisms normally inhabits the female genital tract and is strongly associated with salpingitis and ovarian abscesses?

Mycoplasma hominis. **Explanation:** The correct answer is **Mycoplasma hominis**. **1. Why Mycoplasma hominis is correct:** *Mycoplasma hominis* is a common commensal of the lower genitourinary tract in sexually active individuals. Unlike most other Mycoplasma species, it can be easily cultured on artificial media (Fried-egg colonies). It is a significant opportunistic pathogen associated with **Pelvic Inflammatory Disease (PID)**, specifically causing **salpingitis** (inflammation of the fallopian tubes) and **tubo-ovarian abscesses**. It is also implicated in post-abortal and postpartum fever. **2. Why other options are incorrect:** * **Mycoplasma fermentans (Option A):** While it has been isolated from the urogenital tract and linked to respiratory illnesses in immunocompromised patients (like those with HIV), it is not the primary agent associated with acute salpingitis or ovarian abscesses. * **Mycoplasma pneumoniae (Option C):** This is a strictly respiratory pathogen. It causes "Walking Pneumonia" (Primary Atypical Pneumonia) and is associated with Bullous Myringitis and Cold Agglutinin disease. It does not inhabit the genital tract. * **Mycoplasma orale (Option D):** As the name suggests, this species is part of the normal flora of the oral cavity and oropharynx. It is generally considered non-pathogenic. **3. NEET-PG High-Yield Pearls:** * **Ureaplasma urealyticum:** Another genital Mycoplasma; it is distinguished by its ability to produce **urease** and is a major cause of Non-Gonococcal Urethritis (NGU) in men. * **Cell Wall:** All Mycoplasma species lack a cell wall (contain sterols instead), making them inherently resistant to Beta-lactams (Penicillins/Cephalosporins). * **Treatment:** Macrolides (Azithromycin) or Tetracyclines (Doxycycline) are the drugs of choice. Note that *M. hominis* is often resistant to Erythromycin but sensitive to Clindamycin.

Q: Which organism commonly causes intravenous biofilm formation?

Staphylococcus epidermidis. **Explanation:** **Staphylococcus epidermidis** is the correct answer because it is the most common cause of infections associated with indwelling medical devices (e.g., intravenous catheters, prosthetic valves, and shunts). As a commensal of the skin, it gains entry during device insertion. Its primary virulence factor is the ability to produce an **extracellular polysaccharide matrix (slime/glycocalyx)**, which facilitates the formation of a **biofilm**. This biofilm protects the bacteria from the host immune system and systemic antibiotics, often necessitating the removal of the device for successful treatment. **Analysis of Incorrect Options:** * **Acinetobacter baumannii:** While it can form biofilms and is a major cause of ventilator-associated pneumonia and ICU-acquired infections, it is not the *most common* organism associated specifically with IV catheter biofilms compared to Coagulase-Negative Staphylococci (CoNS). * **Neisseria meningitidis:** This is an encapsulated gram-negative diplococcus primarily responsible for meningitis and meningococcemia. It does not typically form biofilms on prosthetic hardware. * **Mycobacterium species:** While some rapidly growing mycobacteria can cause post-surgical wound infections, they are not standard causes of intravenous catheter-related biofilm infections. **Clinical Pearls for NEET-PG:** * **Coagulase-Negative Staphylococci (CoNS):** *S. epidermidis* is the most frequent CoNS isolate. * **Novobiocin Sensitivity:** *S. epidermidis* is Novobiocin **sensitive**, whereas *S. saprophyticus* (causes UTI) is Novobiocin **resistant**. * **Biofilm Marker:** The *ica* (intercellular adhesion) locus is often involved in the production of the polysaccharide intercellular adhesin (PIA) in *S. epidermidis* biofilms. * **Treatment:** Vancomycin is usually the drug of choice due to widespread methicillin resistance (MRSE).

Q: Actinomycetoma is caused by all of the following EXCEPT:

Madura mycosis. **Explanation:** Mycetoma is a chronic, granulomatous, subcutaneous infection characterized by a triad of **localized swelling, underlying bone destruction, and the presence of discharging sinuses containing grains.** It is etiologically classified into two types: 1. **Actinomycetoma:** Caused by filamentous aerobic bacteria. 2. **Eumycetoma:** Caused by true fungi. **Why "Madura mycosis" is the correct answer:** Madura mycosis (or Eumycetoma) is caused by true fungi, most commonly *Madurella mycetomatis*. It is distinct from Actinomycetoma because it requires antifungal therapy (e.g., Itraconazole) and often surgical intervention, whereas Actinomycetoma is treated with long-term antibiotics (e.g., Welsh regimen). **Analysis of Incorrect Options:** * **Actinomyces:** While *Actinomyces israelii* typically causes endogenous actinomycosis, certain species can cause actinomycetoma. * **Nocardia:** Species like *Nocardia brasiliensis* are the most common cause of actinomycetoma worldwide. * **Streptomyces:** *Streptomyces somaliensis* is a well-recognized causative agent of actinomycetoma, often producing yellow/white grains. **High-Yield Clinical Pearls for NEET-PG:** * **Grains:** The color of the grain provides a diagnostic clue. **Black grains** are pathognomonic for Eumycetoma (*Madurella*). **Yellow/White grains** can be seen in both, but are common in *Nocardia* and *Streptomyces*. * **Treatment:** Actinomycetoma responds well to the **Welsh Regimen** (Amikacin + Cotrimoxazole). * **Diagnosis:** The most definitive diagnosis is made by histopathological examination of the grains and culture on Sabouraud Dextrose Agar (SDA) or Lowenstein-Jensen (LJ) medium.

Question 1

Which of the following is the causative agent of Yaws?

Accepted Answer

Treponema pertenue

Answer

Treponema carateum

Answer

Treponema pallidum

Answer

Treponema endemicum

Question 2

Fournier gangrene is caused by which type of infection?

Accepted Answer

Mixed infection

Answer

Clostridium welchii

Answer

Proteus

Answer

Streptococcus

Question 3

Lactobacilli are numerous in carious lesions primarily because they:

Accepted Answer

Are secondary invaders in the carious process

Answer

Are the main causative agent of caries

Answer

Can produce insoluble extracellular polysaccharides

Answer

Can attach to smooth enamel surfaces

Question 4

Bacilli Calmette Guerin (BCG) vaccine contains avirulent strains of which type of mycobacteria?

Accepted Answer

Bovine tubercle bacilli

Answer

Human tubercle bacilli

Answer

Avian tubercle bacilli

Answer

Atypical mycobacteria

Question 5

Which of the following bacteria is a common cause of watery diarrhea in children?

Accepted Answer

Enteropathogenic E. coli (EPEC)

Answer

Enterohemorrhagic E. coli (EHEC)

Answer

Enteroinvasive E. coli (EIEC)

Answer

Enteroaggregative E. coli (EAEC)

Question 6

In cystic fibrosis, which is the second most common organism to cause infection?

Accepted Answer

Staphylococcus aureus

Answer

Streptococci

Answer

Klebsiella

Answer

Pneumococci

Question 7

A 65-year-old man, post-cardiac bypass surgery, is on postoperative broad-spectrum antibiotic prophylaxis. Several days later, he develops fever, abdominal pain, and bloody diarrhea. Colonoscopic biopsy shows a thick mucopurulent exudate. Which of the following is the most likely etiology of this patient's gastrointestinal disorder?

Accepted Answer

Clostridium difficile

Answer

Clostridium botulinum

Answer

Clostridium perfringens

Answer

Clostridium tetani

Question 8

Which of the following organisms normally inhabits the female genital tract and is strongly associated with salpingitis and ovarian abscesses?

Accepted Answer

Mycoplasma hominis

Answer

Mycoplasma fermentans

Answer

Mycoplasma pneumoniae

Answer

Mycoplasma orale

Question 9

Which organism commonly causes intravenous biofilm formation?

Accepted Answer

Staphylococcus epidermidis

Answer

Acinetobacter baumannii

Answer

Neisseria meningitidis

Answer

Mycobacterium species

Question 10

Actinomycetoma is caused by all of the following EXCEPT:

Accepted Answer

Madura mycosis

Answer

Actinomyces

Answer

Nocardia

Answer

Streptomyces

Bacteriology — MCQs

Bacteriology — MCQs

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