Bacteriology Practice Questions

Q: A 60-year-old female presents with diarrhea and abdominal pain. She recently completed a course of Clindamycin, has a history of diabetes mellitus, and is on oral hypoglycemic agents. Stool culture revealed gram-positive motile rods. Colonoscopy revealed characteristic findings. Which of the following microorganisms is most likely responsible for her condition?

Clostridium difficile. **Explanation:** The clinical presentation is a classic case of **Pseudomembranous Colitis** caused by **_Clostridioides difficile_** (formerly *Clostridium*). **Why Option A is correct:** The most significant risk factor for *C. difficile* infection (CDI) is recent **broad-spectrum antibiotic use**, particularly **Clindamycin**, fluoroquinolones, or cephalosporins. These drugs disrupt the normal colonic flora, allowing *C. difficile* to overgrow and release toxins (Toxin A/Enterotoxin and Toxin B/Cytotoxin). The "characteristic findings" on colonoscopy refer to **pseudomembranes** (yellow-white plaques of fibrin and inflammatory debris). Microbiologically, *C. difficile* is a **Gram-positive, spore-forming, motile anaerobic rod**. **Why incorrect options are wrong:** * **Options B, C, & D (E. coli variants):** These are all **Gram-negative bacilli**. While they cause diarrhea, they are not typically associated with post-antibiotic disruption of gut flora. * **EIEC** causes dysentery similar to Shigella. * **EAEC** causes persistent watery diarrhea. * **STEC (EHEC)** is associated with Hemolytic Uremic Syndrome (HUS) and bloody diarrhea, but not pseudomembrane formation. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Oral **Fidaxomicin** or **Vancomycin** (Note: Metronidazole is no longer the first-line agent for non-severe cases in updated guidelines). * **Diagnosis:** The most sensitive test is NAAT (PCR) for toxin genes; the most specific is the Cell Cytotoxicity Assay. * **Morphology:** Described as having a **"Drumstick appearance"** (sub-terminal spores) and colonies on CCFA agar have a **"Ground glass appearance"** with a characteristic **horse-manure odor**.

Q: Which of the following organisms is most likely to be implicated as a cause of urethritis that persists after antibiotic therapy for gonorrhea?

Chlamydia. This question addresses the clinical phenomenon of **Post-Gonococcal Urethritis (PGU)**, a high-yield topic in NEET-PG Microbiology and Venereology. ### **Explanation** **Chlamydia trachomatis (Serotypes D-K)** is the most common cause of non-gonococcal urethritis (NGU). In many cases of acute urethritis, there is a **co-infection** with both *Neisseria gonorrhoeae* and *Chlamydia trachomatis*. When a patient is treated with beta-lactams (like Ceftriaxone) for gonorrhea, the *N. gonorrhoeae* is eradicated. However, *Chlamydia* is an obligate intracellular organism that lacks a traditional peptidoglycan cell wall; therefore, it is inherently resistant to beta-lactams. If the initial treatment does not include a macrolide (Azithromycin) or a tetracycline (Doxycycline), the *Chlamydia* persists, leading to symptoms that reappear or continue after the gonorrhea is cured. ### **Why Incorrect Options are Wrong** * **Actinomyces:** These are Gram-positive anaerobic bacilli known for causing "lumpy jaw" and sulfur granules. They are not associated with sexually transmitted urethritis. * **Mycobacteria:** While *M. tuberculosis* can cause genitourinary TB, it typically presents with "sterile pyuria" and chronic systemic symptoms, not acute post-gonococcal urethritis. * **Nocardia:** These are aerobic, acid-fast branching filaments that primarily cause pulmonary or cutaneous infections in immunocompromised hosts. ### **Clinical Pearls for NEET-PG** * **Dual Therapy:** Due to high rates of co-infection, the standard of care for urethritis often involves treating both organisms simultaneously (e.g., Ceftriaxone + Doxycycline). * **Incubation Period:** *N. gonorrhoeae* has a short incubation (2–5 days), while *C. trachomatis* has a longer incubation (7–21 days), contributing to the "delayed" presentation of PGU. * **Diagnosis:** Nucleic Acid Amplification Test (NAAT) is the gold standard for diagnosing *Chlamydia*.

Q: Why is Streptococcus referred to as a flesh-eating bacteria?

Pyrogenic exotoxin. **Explanation:** *Streptococcus pyogenes* (Group A Streptococcus) is termed "flesh-eating bacteria" due to its ability to cause **Necrotizing Fasciitis**, a rapidly progressive destruction of subcutaneous tissue and fascia. **Why Pyrogenic Exotoxin is correct:** The primary virulence factors responsible for this severe manifestation are **Streptococcal Pyrogenic Exotoxins (SPE types A, B, and C)**. These act as **superantigens**, non-specifically activating a massive number of T-cells. This triggers a "cytokine storm" (release of IL-1, IL-6, and TNF-α), leading to systemic shock and extensive tissue necrosis. SPE-B, specifically, is a cysteine protease that directly degrades host proteins, facilitating the rapid spread of the infection through tissue planes. **Analysis of Incorrect Options:** * **Streptolysin A:** This is a distractor; the two main hemolysins are Streptolysin O and S. * **Streptolysin O:** An oxygen-labile hemolysin that creates pores in cell membranes. While it aids in immune evasion, it is primarily used clinically for diagnosis (ASO titer) rather than being the primary driver of necrotizing fasciitis. * **Hyaluronidase:** Known as the "spreading factor," it breaks down hyaluronic acid in connective tissue. While it helps the bacteria invade, it does not cause the massive tissue death and systemic toxicity characteristic of "flesh-eating" disease. **NEET-PG High-Yield Pearls:** * **M-Protein:** The most important virulence factor for *S. pyogenes*; it is anti-phagocytic and mediates adhesion. * **Toxic Shock-Like Syndrome (TSLS):** Also mediated by Pyrogenic Exotoxins (Superantigens). * **Drug of Choice:** Penicillin G is the mainstay, but **Clindamycin** is added in necrotizing fasciitis because it inhibits protein synthesis, thereby shutting down exotoxin production (the "Eagle Effect").

Q: What is the causative organism of Bacillary Angiomatosis?

Bartonella. **Explanation:** **Bacillary Angiomatosis (BA)** is a vascular proliferative disease caused by Gram-negative, fastidious, pleomorphic bacilli of the genus **Bartonella**. It primarily affects immunocompromised individuals, particularly those with advanced HIV/AIDS (CD4 count <100 cells/mm³). 1. **Why Bartonella is Correct:** The two primary species involved are ***Bartonella henselae*** (transmitted via cat scratches/fleas) and ***Bartonella quintana*** (transmitted via human body lice). These bacteria induce endothelial cell proliferation, leading to the formation of characteristic "mulberry-like" red-to-purple skin nodules that can mimic Kaposi Sarcoma. Diagnosis is confirmed via biopsy showing lobular vascular proliferation and **Warthin-Starry silver stain**, which highlights the bacteria. 2. **Why Other Options are Incorrect:** * **Brucella:** Causes Brucellosis (Undulant fever), characterized by drenching sweats and hepatosplenomegaly, but does not cause vascular skin lesions. * **Listeria:** A Gram-positive rod causing meningitis or sepsis, especially in neonates and the elderly; it is not associated with angiomatous lesions. * **Borrelia:** Causes Lyme disease (*B. burgdorferi*) or Relapsing fever (*B. recurrentis*). While Lyme presents with a rash (*Erythema migrans*), it is not a vascular proliferative lesion. **High-Yield Clinical Pearls for NEET-PG:** * **Differential Diagnosis:** Must distinguish BA from **Kaposi Sarcoma** (caused by HHV-8). BA shows neutrophilic infiltrate on histology, whereas Kaposi shows spindle cells and slit-like spaces. * **Treatment:** The drug of choice for Bacillary Angiomatosis is **Erythromycin** or Doxycycline. * **Other Bartonella Diseases:** *B. henselae* also causes Cat Scratch Disease (in immunocompetent hosts), and *B. quintana* causes Trench Fever.

Q: About yaws, all are true except:

Later stages can involve heart and nerves. **Explanation:** Yaws is a chronic, non-venereal treponematosis caused by **Treponema pallidum subspecies pertenue**. It primarily affects children in tropical regions and is characterized by skin, bone, and joint involvement. **Why Option D is the correct answer (The "Except"):** Unlike venereal syphilis, yaws **does not** involve the cardiovascular system (heart) or the central nervous system (nerves). It is strictly a disease of the skin, bone, and cartilage. The late stage of yaws is characterized by destructive lesions like **gangosa** (rhinopharyngitis mutilans) and **goundou** (exostosis of the maxilla), but visceral involvement is absent. **Analysis of other options:** * **Option A:** Correct. The causative agent is *T. pallidum pertenue*. It is morphologically indistinguishable from the agent of syphilis. * **Option B:** Correct. Yaws is transmitted via **direct skin-to-skin contact** with the exudate of early lesions. It is not sexually transmitted. * **Option C:** Correct. Secondary yaws is characterized by generalized skin eruptions and **osteoperiostitis** (involving long bones and fingers, leading to "sabre tibia" or polydactylitis). **NEET-PG High-Yield Pearls:** * **Endemicity:** Found in humid, tropical "hotspots." India was declared **Yaws-free** by the WHO in 2016 (the first country to be so certified under the 2012 roadmap). * **Diagnosis:** Serological tests (VDRL, TPHA) are positive but cannot distinguish yaws from syphilis. * **Treatment:** A single dose of **Azithromycin** (30 mg/kg) is the current WHO-recommended treatment (Morgues strategy). Benzathine Penicillin is the alternative. * **Key Lesion:** The initial lesion is called the **"Mother Yaw"** (frambesioma).

Q: What is true about H. influenza?

Invasive strains are most common. **Explanation:** *Haemophilus influenzae* is a small, pleomorphic Gram-negative coccobacillus. Understanding its classification and growth requirements is high-yield for NEET-PG. **1. Why Option C is Correct:** *H. influenzae* is divided into **capsulated (typable)** and **non-capsulated (non-typable)** strains. While non-typable strains cause localized mucosal infections (like otitis media), the **capsulated strains** (specifically **Type b**) are highly invasive. The capsule (polyribosylribitol phosphate - PRP) allows the bacteria to resist phagocytosis and enter the bloodstream, leading to severe systemic conditions like meningitis, epiglottitis, and septic arthritis. In the context of clinically significant, severe disease, these invasive strains are the primary focus. **2. Why Other Options are Incorrect:** * **Option A:** *H. influenzae* **cannot** grow on plain sheep blood agar because it requires both **Factor X (Hemin)** and **Factor V (NAD)**. Sheep blood contains NADase which destroys Factor V. It grows on **Chocolate Agar**, where heat lyses the RBCs, releasing both factors and inactivating NADase. * **Option B:** While non-typable strains exist, the most clinically significant and virulent strains are **capsulated**. * **Option D:** *H. influenzae* is a **Gram-negative** organism (stains pink/red). **Clinical Pearls for NEET-PG:** * **Satellitism:** *H. influenzae* grows on blood agar only when streaked alongside *Staphylococcus aureus*, which provides the necessary Factor V. * **Vaccine:** The Hib vaccine targets the **Type b capsule (PRP)**. * **Culture:** Best grown on Chocolate agar in 5–10% $CO_2$. * **Drug of Choice:** Ceftriaxone for invasive disease; Amoxicillin-clavulanate for mucosal infections.

Q: Watercan perineum is caused by which organism?

Neisseria gonorrhoeae. **Explanation:** **Watercan perineum** is a classic clinical manifestation of chronic **urethral stricture**, most commonly resulting from untreated or recurrent **gonococcal urethritis**. 1. **Why Neisseria gonorrhoeae is correct:** Infection with *N. gonorrhoeae* leads to severe inflammation of the urethral mucosa and periurethral glands (Glands of Littre). If inadequately treated, this progresses to periurethral abscesses. These abscesses eventually rupture through the skin of the perineum or scrotum, forming multiple fistulous tracts. When the patient voids, urine leaks through these multiple openings, resembling the spray of a **watering can**. 2. **Why other options are incorrect:** * **E. coli & Enterococcus fecalis:** While these are common causes of urinary tract infections (UTIs) and acute prostatitis, they do not typically cause the specific chronic inflammatory fibrosis and fistulization required to produce a watercan perineum. * **Treponema pallidum:** This is the causative agent of Syphilis. Its primary manifestations are chancres (primary), rashes (secondary), or gummas (tertiary), but it does not cause urethral strictures or perineal fistulae. **Clinical Pearls for NEET-PG:** * **Most common site:** The bulbar urethra is the most common site for post-inflammatory (gonococcal) strictures. * **Modern Etiology:** While *N. gonorrhoeae* is the classic textbook cause, in modern clinical practice, **iatrogenic trauma** (catheterization, cystoscopy) is the most common cause of urethral strictures overall. * **Management:** Initial management involves urinary diversion (suprapubic cystostomy) followed by definitive surgery, such as urethroplasty. * **Fournier’s Gangrene:** Do not confuse watercan perineum with Fournier’s gangrene, which is a rapidly progressing polymicrobial necrotizing fasciitis of the perineum.

Question 1

A 60-year-old female presents with diarrhea and abdominal pain. She recently completed a course of Clindamycin, has a history of diabetes mellitus, and is on oral hypoglycemic agents. Stool culture revealed gram-positive motile rods. Colonoscopy revealed characteristic findings. Which of the following microorganisms is most likely responsible for her condition?

Accepted Answer

Clostridium difficile

Answer

Enteroinvasive E. coli

Answer

Enteroaggregative E. coli

Answer

Shiga toxin-producing E. coli

Question 2

Which of the following organisms is most likely to be implicated as a cause of urethritis that persists after antibiotic therapy for gonorrhea?

Accepted Answer

Chlamydia

Answer

Actinomyces

Answer

Mycobacteria

Answer

Nocardia

Question 3

A 40-year-old male, who was in good health earlier, begins experiencing a chronic cough. Over the following 6 weeks, the cough gradually worsens and becomes productive. He is also coughing up blood, and notes weight loss, fever, and night sweats. A sputum sample is positive for acid-fast bacilli. Which of the following pathogenic mechanisms can be primarily attributed to the etiologic agent involved in this disease?

Accepted Answer

Cell-mediated hypersensitivity

Answer

Clogging of alveoli by large numbers of acid-fast mycobacteria

Answer

Humoral immunity

Answer

Specific cell adhesion sites

Question 4

Why is Streptococcus referred to as a flesh-eating bacteria?

Accepted Answer

Pyrogenic exotoxin

Answer

Streptolysin A

Answer

Streptolysin O

Answer

Hyaluronidase

Question 5

What is the causative organism of Bacillary Angiomatosis?

Accepted Answer

Bartonella

Answer

Brucella

Answer

Listeria

Answer

Borrelia

Question 6

About yaws, all are true except:

Accepted Answer

Later stages can involve heart and nerves

Answer

Caused by T. pallidum subspecies pertenue

Answer

Transmitted non-venereally

Answer

Secondary yaws can involve bones

Question 7

What is true about H. influenza?

Accepted Answer

Invasive strains are most common

Answer

Grown on sheep blood agar and CO2

Answer

It is not capsulated

Answer

Gram positive

Question 8

In a neutropenic patient, which of the following would be most commonly encountered as a cause of bacterial meningitis and brain abscess?

Accepted Answer

Pseudomonas aeruginosa

Answer

Staphylococcus aureus

Answer

Haemophilus influenzae

Answer

Bacteroides fragilis

Question 9

Watercan perineum is caused by which organism?

Accepted Answer

Neisseria gonorrhoeae

Answer

E. coli

Answer

Enterococcus fecalis

Answer

Treponema pallidum

Question 10

A 4-year-old girl with a history of hydrocephalus presents with severe headache and fever. She underwent surgery for a ventricular-peritoneal shunt 2 months ago. The neurologist suspects an infection. Which organism would most likely be isolated from the shunt tubing?

Accepted Answer

Staphylococcus epidermidis

Answer

Staphylococcus aureus

Answer

Streptococcus

Answer

Meningococcus

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?