Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 371: An unimmunized, 2-year-old boy presents with drooling from the mouth, elevated temperature, and enlarged tonsils. During attempts at intubation, no gray-white membrane is observed but the epiglottis appears "beefy" red and edematous. Which of the following is the most likely organism?

A. Haemophilus haemolyticus
B. Haemophilus influenzae (Correct Answer)
C. Klebsiella pneumoniae
D. Mycoplasma pneumoniae

Explanation: ### Explanation The clinical presentation of drooling, high fever, and a "beefy red," edematous epiglottis in an unimmunized child is a classic description of **Acute Epiglottitis**. **1. Why Haemophilus influenzae is correct:** Historically, **Haemophilus influenzae type b (Hib)** was the most common cause of acute epiglottitis in children. It is a Gram-negative coccobacillus that requires Factors X (hemin) and V (NAD) for growth. The "beefy red" appearance is due to intense inflammation of the supraglottic structures, which can lead to life-threatening airway obstruction. The lack of a gray-white membrane helps differentiate this from Diphtheria. Since the child is unimmunized, he lacked the Hib vaccine, making him highly susceptible. **2. Why the other options are incorrect:** * **A. Haemophilus haemolyticus:** This is generally considered commensal flora of the upper respiratory tract and is rarely pathogenic; it does not cause epiglottitis. * **C. Klebsiella pneumoniae:** While it causes severe pneumonia (characterized by "currant jelly" sputum), it is not a typical cause of acute epiglottitis in children. * **D. Mycoplasma pneumoniae:** This is the leading cause of "Atypical Pneumonia" (Walking Pneumonia) in older children and young adults. It typically presents with a persistent dry cough and interstitial infiltrates, not acute airway obstruction. **3. High-Yield NEET-PG Pearls:** * **X-ray Sign:** The characteristic finding on a lateral neck X-ray is the **"Thumb sign"** (swollen epiglottis). * **Clinical Triad:** The "3 Ds"—Drooling, Dysphagia, and Distress (Stridor). * **Management:** Do NOT examine the throat with a tongue depressor as it may trigger fatal laryngospasm. Secure the airway first. * **Culture:** Grows on **Chocolate Agar** (contains released X and V factors). It shows the **"Satellitism"** phenomenon around *S. aureus* on blood agar.

Question 372: In the treatment of patients who have cholera, the use of a drug that inhibits adenyl cyclase would be expected to have which of the following characteristics?

A. Block the action of cholera toxin (Correct Answer)
B. Eradicate the organism
C. Increase fluid secretion
D. Kill the patient immediately

Explanation: ### Explanation **Mechanism of Action (Why A is correct):** The pathogenesis of *Vibrio cholerae* is primarily mediated by the **Cholera Toxin (Choleragen)**. This A-B type enterotoxin works by ADP-ribosylating the **Gs alpha subunit** of the G-protein complex. This leads to the permanent activation of the enzyme **adenyl cyclase**, resulting in intracellular accumulation of **cyclic AMP (cAMP)**. High cAMP levels stimulate the CFTR channels, leading to the massive efflux of chloride ions and water into the intestinal lumen, causing "rice-water stools." Therefore, a drug that inhibits adenyl cyclase would directly counteract the toxin's biochemical pathway, effectively blocking its physiological action and reducing fluid loss. **Analysis of Incorrect Options:** * **B. Eradicate the organism:** Adenyl cyclase is a host cell enzyme involved in signaling; inhibiting it does not have bactericidal or bacteriostatic effects on the bacteria itself. Antibiotics (like Azithromycin or Doxycycline) are required for eradication. * **C. Increase fluid secretion:** Inhibiting adenyl cyclase would *decrease* cAMP levels, thereby **decreasing** fluid secretion. Increasing secretion would worsen the dehydration. * **D. Kill the patient immediately:** Adenyl cyclase is vital for many cellular processes, but its temporary inhibition in the gut to treat cholera would be therapeutic, not acutely lethal. **NEET-PG High-Yield Pearls:** * **Toxin Type:** Cholera toxin is an **AB5 toxin** (1 A subunit, 5 B subunits). * **Receptor:** The B subunit binds to the **GM1 ganglioside** receptor on enterocytes. * **Stool Characteristic:** Non-inflammatory, "rice-water" appearance (contains mucus, epithelial cells, and vibrios). * **Gold Standard Treatment:** Aggressive fluid and electrolyte replacement (ORS/IV fluids). Antibiotics are secondary but help reduce shedding. * **Culture Media:** TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar—colonies appear **yellow** due to sucrose fermentation.

Question 373: All of the following statements about staphylococci are TRUE, EXCEPT:

A. Toxic shock syndrome may affect both males and females.
B. Many individuals are nasal carriers of S. aureus.
C. Impetigo is caused only by S. aureus. (Correct Answer)
D. S. epidermidis is often responsible for infective endocarditis in intravenous drug abusers.

Explanation: ### Explanation **1. Why Option C is the Correct Answer (The Exception):** Impetigo is a highly contagious superficial skin infection. While **Staphylococcus aureus** is the most common cause globally, it is **not the only** causative agent. **Streptococcus pyogenes (Group A Streptococcus)** is also a significant cause of impetigo, particularly the "non-bullous" variety. In many cases, both organisms may be isolated from the same lesion. Therefore, the statement that it is caused *only* by *S. aureus* is incorrect. **2. Analysis of Other Options:** * **Option A:** Toxic Shock Syndrome (TSS) was historically associated with high-absorbency tampon use in women. However, "non-menstrual TSS" occurs in both males and females due to surgical wound infections, burns, or colonized sites producing the TSST-1 toxin. * **Option B:** Approximately 20–30% of the healthy population are persistent **nasal carriers** of *S. aureus*, which serves as a major reservoir for community and hospital-acquired infections. * **Option C:** *S. epidermidis* (a Coagulase-negative Staphylococcus) is a common skin commensal. It is a leading cause of infective endocarditis in patients with prosthetic valves and is frequently isolated from endocarditis cases in **intravenous drug abusers (IVDAs)**, though *S. aureus* remains the most common overall in IVDAs. **3. NEET-PG High-Yield Pearls:** * **Impetigo Types:** Bullous impetigo is almost exclusively caused by *S. aureus* (due to exfoliative toxins), whereas non-bullous impetigo can be caused by both *S. aureus* and *S. pyogenes*. * **Catalase vs. Coagulase:** All Staphylococci are Catalase positive (distinguishes them from Streptococci). *S. aureus* is Coagulase positive, while *S. epidermidis* and *S. saprophyticus* are Coagulase-negative (CoNS). * **Novobiocin Sensitivity:** *S. epidermidis* is sensitive, while *S. saprophyticus* (common cause of UTI in young females) is resistant.

Question 374: All of the following Proteus species form indole EXCEPT?

A. P. mirabilis (Correct Answer)
B. P. vulgaris
C. P. rettgeri
D. P. morganii

Explanation: **Explanation:** The genus *Proteus* belongs to the family Enterobacteriaceae. A key biochemical test used to differentiate between the species of this genus is the **Indole test**, which determines the ability of an organism to produce the enzyme tryptophanase to break down the amino acid tryptophan into indole. 1. **Why P. mirabilis is correct:** *Proteus mirabilis* is characteristically **indole-negative**. This is a high-yield distinguishing feature because *P. mirabilis* is the most common cause of human infections (especially UTIs) within this genus. 2. **Why the other options are incorrect:** * **P. vulgaris:** This species is **indole-positive**. It is often differentiated from *P. mirabilis* in the lab using this specific test. * **P. rettgeri & P. morganii:** While these have been reclassified into their own genera (*Providencia rettgeri* and *Morganella morganii*), they were historically grouped with *Proteus* and are both **indole-positive**. **NEET-PG High-Yield Pearls:** * **Swarming Growth:** *Proteus* species exhibit "swarming" (concentric waves of growth) on solid media like blood agar due to their peritrichous flagella. This can be inhibited by increasing agar concentration (6%) or adding chloral hydrate/PABA. * **Urease Production:** All *Proteus* species are strong urease producers. This raises urinary pH, leading to the formation of **Struvite stones** (Triple phosphate/Staghorn calculi). * **Dienes Phenomenon:** A method used to differentiate two strains of *Proteus*; if two different strains meet on an agar plate, a line of inhibited growth forms between them. * **Weil-Felix Reaction:** Antigens of certain *Proteus* strains (OX-19, OX-2, OX-K) cross-react with antibodies against Rickettsiae, serving as a diagnostic tool.

Question 375: What is the primary transmission route for pneumonic plague?

A. Mice
B. Tick
C. Inhalation (Correct Answer)
D. Flea

Explanation: **Explanation:** **Pneumonic plague** is the most severe and fatal form of plague caused by the Gram-negative coccobacillus *Yersinia pestis*. The correct answer is **Inhalation** because pneumonic plague is primarily transmitted via **respiratory droplets** from person to person or through the inhalation of contaminated aerosols. This leads to a rapid, overwhelming primary pneumonia with a high mortality rate if not treated within 24 hours. **Analysis of Incorrect Options:** * **Mice (A):** While rodents (like rats and mice) serve as the primary **natural reservoir** for *Y. pestis*, they do not directly transmit the pneumonic form to humans. * **Tick (B):** Ticks are not vectors for plague. They are associated with diseases like Rickettsial infections or Lyme disease. * **Flea (D):** The bite of an infected rat flea (*Xenopsylla cheopis*) is the classic transmission route for **Bubonic plague**, not pneumonic plague. However, if bubonic plague spreads hematogenously to the lungs, it causes *secondary* pneumonic plague, which can then spread via inhalation to others. **High-Yield NEET-PG Pearls:** * **Safety Warning:** *Yersinia pestis* is a potential **Bioterrorism agent** (Category A) due to its ability to spread via aerosol (Inhalation). * **Microscopy:** Shows characteristic **"Safety-pin appearance"** (bipolar staining) with Wayson or Giemsa stain. * **Virulence Factor:** The **F1 antigen** (capsular polysaccharide) is highly diagnostic. * **Drug of Choice:** **Streptomycin** or Gentamicin; Doxycycline is used for prophylaxis.

Question 376: Where do gonococci initially infect?

A. Vagina
B. Cervix (Correct Answer)
C. Uterus
D. Fallopian tubes

Explanation: **Explanation:** The primary site of infection for *Neisseria gonorrhoeae* in the female genital tract is the **Cervix** (specifically the endocervix). **Why the Cervix is the correct answer:** Gonococci have a high affinity for **columnar epithelium**. The endocervix is lined with simple columnar epithelium, making it the most common site of initial infection in adult females. The bacteria use pili to attach to these non-ciliated columnar cells, leading to local inflammation (cervicitis). **Why other options are incorrect:** * **Vagina:** In adult females, the vagina is lined by **stratified squamous epithelium**, which is resistant to gonococcal invasion. However, in prepubertal girls (where the epithelium is thin), gonococci can cause vulvovaginitis. * **Uterus & Fallopian tubes:** These are sites of **ascending infection**. While gonococci can spread to the endometrium and fallopian tubes, causing Pelvic Inflammatory Disease (PID) and salpingitis, these are secondary complications rather than the initial site of infection. **High-Yield NEET-PG Pearls:** * **Gold Standard Diagnosis:** Culture on **Thayer-Martin medium** (a selective VCN medium: Vancomycin, Colistin, Nystatin). * **Microscopy:** Gram-negative kidney-shaped diplococci, typically found **intracellularly** within polymorphonuclear leukocytes (neutrophils). * **Clinical Presentation:** Often asymptomatic in females (up to 50-70%), which facilitates the silent spread of the disease. * **Treatment:** Due to increasing resistance, the current CDC/WHO recommendation is usually a single dose of **Ceftriaxone** (IM). Always screen for co-infection with *Chlamydia trachomatis*.

Question 377: Neisseria is morphologically:

A. Gram negative bacilli
B. Gram positive bacilli
C. Gram negative cocci (Correct Answer)
D. Gram positive cocci

Explanation: **Explanation:** The genus *Neisseria* consists of aerobic, non-motile, and non-spore-forming bacteria. Morphologically, they are characterized as **Gram-negative cocci** that typically occur in pairs (**diplococci**) with adjacent sides flattened, often described as **kidney-shaped** or coffee-bean shaped. **Why Option C is correct:** *Neisseria* species (specifically *N. meningitidis* and *N. gonorrhoeae*) possess a typical Gram-negative cell wall structure containing lipooligosaccharide (LOS). On Gram staining, they do not retain the crystal violet dye but take up the counterstain (safranin/neutral red), appearing pink or red under the microscope. **Why other options are incorrect:** * **Options A & B (Bacilli):** *Neisseria* are spherical (cocci), not rod-shaped (bacilli). Common Gram-negative bacilli include *E. coli* and *Pseudomonas*, while Gram-positive bacilli include *Bacillus* and *Clostridium* species. * **Option D (Gram-positive cocci):** This category includes genera like *Staphylococcus* and *Streptococcus*, which appear purple on Gram stain due to their thick peptidoglycan layer. **High-Yield Clinical Pearls for NEET-PG:** * **Oxidase & Catalase:** All *Neisseria* species are **Oxidase positive** and **Catalase positive**. * **Sugar Utilization:** *N. meningitidis* ferments both **M**altose and **G**lucose (M=M), whereas *N. gonorrhoeae* ferments only **G**lucose (G=G). * **Culture:** They are fastidious and grow best on **Thayer-Martin Medium** (a selective Chocolate agar). * **Microscopic Appearance:** In clinical samples (like urethral discharge or CSF), they are often found **intracellularly** within polymorphonuclear leukocytes (neutrophils).

Question 378: In a case of neonatal meningitis, the etiologic bacteria is found to have properties of beta hemolysis, bacitracin resistance, and CAMP positive. Which of the following is the most likely causative agent?

A. S. pyogenes
B. S. agalactiae (Correct Answer)
C. S. pneumoniae
D. E. faecalis

Explanation: **Explanation:** The clinical presentation of neonatal meningitis combined with the laboratory findings points directly to **Streptococcus agalactiae (Group B Streptococcus - GBS)**. **Why S. agalactiae is correct:** * **CAMP Test Positive:** This is the hallmark test for GBS. The organism produces the "CAMP factor," which acts synergistically with the beta-lysin of *Staphylococcus aureus* to produce an enhanced zone of hemolysis (arrowhead shape) on blood agar. * **Bacitracin Resistance:** Unlike Group A Strep, GBS is resistant to bacitracin. * **Beta-hemolysis:** GBS typically shows a narrow zone of beta-hemolysis. * **Clinical Context:** S. agalactiae is the leading cause of neonatal sepsis and meningitis, usually acquired from the maternal vaginal flora during birth. **Why other options are incorrect:** * **S. pyogenes (Group A Strep):** While it is beta-hemolytic, it is **Bacitracin sensitive** and CAMP negative. It primarily causes pharyngitis and skin infections, not neonatal meningitis. * **S. pneumoniae:** Shows **alpha-hemolysis** (greenish discoloration) and is bile soluble and optochin sensitive. It is a common cause of meningitis in adults/children, but not the primary agent in the immediate neonatal period. * **E. faecalis:** Usually shows **gamma-hemolysis** (non-hemolytic) and is characterized by growth in 6.5% NaCl and bile esculin hydrolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Hippurate Hydrolysis:** S. agalactiae is also Hippurate hydrolysis positive. * **Prevention:** Screening pregnant women at 35–37 weeks and administering intrapartum penicillin is the standard preventive measure. * **Top 3 causes of Neonatal Meningitis:** 1. *S. agalactiae*, 2. *E. coli* (K1 strain), 3. *Listeria monocytogenes*.

Question 379: All the statements about nontuberculous mycobacteria (NTM) are TRUE, EXCEPT:

A. NTM are ubiquitous in soil and water
B. No human-to-human transmission of NTM
C. Disseminated disease denotes significant immune dysfunction
D. Pulmonary disease is usually associated with systemic immunodeficiency. (Correct Answer)

Explanation: ### Explanation **1. Why Option D is the Correct Answer (The False Statement):** While disseminated NTM infection is a hallmark of systemic immunodeficiency (like advanced HIV/AIDS), **pulmonary NTM disease** typically occurs in patients with **local structural lung defects** rather than systemic immune dysfunction. Common predisposing factors include bronchiectasis, COPD, previous tuberculosis (cavitary lesions), or cystic fibrosis. In some cases, such as "Lady Windermere Syndrome" (caused by *M. avium* complex), it occurs in elderly women who suppress the cough reflex, despite having a normal systemic immune system. **2. Analysis of Other Options:** * **Option A (True):** NTM are environmental saprophytes. They are **ubiquitous**, found commonly in soil, natural water bodies, and domestic plumbing (showers/taps). * **Option B (True):** Unlike *M. tuberculosis*, NTM infections are acquired directly from the environment. There is **no documented human-to-human** or animal-to-human transmission (with the rare, controversial exception of *M. abscessus* in cystic fibrosis clinics). * **Option C (True):** Disseminated NTM (especially *MAC*) is an AIDS-defining illness. It signifies severe immunosuppression (typically CD4 counts **<50 cells/µL**) or specific genetic defects in the IFN-γ/IL-12 pathway. **3. NEET-PG Clinical Pearls:** * **Runyon Classification:** NTM are classified based on growth rate and pigment production (Photochromogens, Scotochromogens, Non-photochromogens, and Rapid growers). * **Most Common NTM:** *Mycobacterium avium* complex (MAC) is the most frequent cause of both pulmonary and disseminated NTM disease worldwide. * **Buruli Ulcer:** Caused by *M. ulcerans*; it is the third most common mycobacterial disease in immunocompetent hosts after TB and Leprosy. * **Swimming Pool Granuloma:** Caused by *M. marinum*.

Question 380: What is true about Bordetella pertussis?

A. Adheres to normal mucosa
B. Destroys cilia
C. Causes local tissue destruction
D. All of the above (Correct Answer)

Explanation: **Explanation:** *Bordetella pertussis*, the causative agent of Whooping Cough, is a highly specialized respiratory pathogen. Its pathogenesis is primarily mediated by its ability to colonize the respiratory epithelium and release potent toxins. 1. **Adherence to Mucosa (Option A):** The first step in infection is attachment. *B. pertussis* uses several adhesins, most notably **Filamentous Hemagglutinin (FHA)** and **Pertactin**, to bind specifically to the ciliated epithelial cells of the respiratory tract. 2. **Destruction of Cilia (Option B):** Once attached, the bacteria release **Tracheal Cytotoxin (TCT)**. This toxin is a peptidoglycan fragment that specifically targets and inhibits the movement of cilia, eventually leading to the death and extrusion of ciliated cells (ciliostasis). 3. **Local Tissue Destruction (Option C):** The combined action of **Pertussis Toxin (PT)**, **Adenylate Cyclase Toxin (ACT)**, and **Dermonecrotic Toxin** leads to local mucosal damage, inflammation, and impaired host immune response. This destruction triggers the characteristic violent coughing fits as the body attempts to clear mucus without the aid of functional cilia. Since all three mechanisms are integral to its pathogenesis, **Option D** is correct. **High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** Regan-Lowe medium (preferred) or Bordet-Gengou (potato-blood-glycerol) agar. * **Mercury Drop Appearance:** Characteristic colony morphology on agar. * **Lymphocytosis:** Unlike most bacterial infections, Pertussis causes a marked absolute lymphocytosis (due to Pertussis Toxin blocking lymphocyte reentry into lymph nodes). * **Stages:** Catarrhal (most infectious), Paroxysmal (whooping cough), and Convalescent. * **Drug of Choice:** Erythromycin (or other Macrolides).

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

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