Bacteriology — MCQs

A patient presents with burning epigastric pain and undergoes gastric biopsy. The tissue is cultured on chocolate agar and incubated in a microaerophilic environment at 37°C for 5 to 7 days. On the fifth day of incubation, colonies appear. Microscopic examination reveals curved, gram-negative, oxidase-positive rods. What is the most likely identity of this organism?

Q445Easy

The Widal test is a type of:

Q446Easy

Which toxin acts on cGMP?

Q447Easy

Gastrointestinal enteritis necroticans is caused by?

Q448Easy

Brucella melitensis is commonly found in which animal?

Q449Medium

All of the following are true regarding Pseudomonas aeruginosa, except:

Q450Medium

A patient with H. pylori infection is treated with drugs. What is the best method to detect the presence of residual H. pylori infection in this person?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 441: Microscopic examination of a specimen shows organisms with characters of both corynebacteria and streptococci and resemble diphtheroids in stained smears. What is the organism?

A. Proteus vulgaris
B. Proteus mirabilis
C. Vibrio
D. Listeria (Correct Answer)

Explanation: **Explanation:** The correct answer is **Listeria monocytogenes**. **Why Listeria is correct:** *Listeria monocytogenes* is a small, Gram-positive, non-spore-forming bacillus. On microscopic examination, it often appears as short chains or pairs, mimicking **Streptococci**. However, it also exhibits a pleomorphic morphology, frequently appearing as coccobacilli arranged in "V" or "L" shapes, which closely resembles **Corynebacteria (Diphtheroids)**. This unique "morphological bridge" between cocci and bacilli is a classic diagnostic feature described in microbiology textbooks. **Why the other options are wrong:** * **Proteus vulgaris & Proteus mirabilis:** These are Gram-negative bacilli belonging to the Enterobacteriaceae family. They are known for "swarming motility" on agar and do not resemble Gram-positive diphtheroids. * **Vibrio:** These are Gram-negative, comma-shaped (curved) rods with a characteristic "darting motility." They do not share morphological features with Streptococci or Corynebacteria. **High-Yield Clinical Pearls for NEET-PG:** * **Motility:** Shows characteristic **"Tumbling motility"** at 25°C (due to peritrichous flagella) but is non-motile at 37°C. * **Culture:** Exhibits **"Cold Enrichment"** (can grow at 4°C), making it a common cause of foodborne illness via refrigerated dairy/meat. * **Hemolysis:** Shows narrow zones of **beta-hemolysis** on blood agar, similar to Group B Streptococcus. * **CAMP Test:** Listeria is **CAMP test positive** (it enhances the zone of hemolysis of *S. aureus*). * **Clinical Significance:** A leading cause of neonatal meningitis, granulomatosis infantiseptica, and meningitis in immunocompromised adults.

Question 442: All of the following are true about Lyme disease except:

A. Borrelia burgdorferi replicates locally and invades locally
B. Infection progresses in spite of good humoral immunity
C. Polymorphonuclear pleocytosis in CSF suggests meningeal involvement (Correct Answer)
D. IgA intrathecally confirms meningitis

Explanation: ### Explanation Lyme disease, caused by the spirochete *Borrelia burgdorferi*, is a multisystem illness characterized by unique immunological and clinical patterns. **1. Why Option C is the Correct Answer (The False Statement):** In Neuroborreliosis (meningeal involvement in Lyme disease), the characteristic finding in the Cerebrospinal Fluid (CSF) is **Lymphocytic pleocytosis**, not polymorphonuclear (neutrophilic) pleocytosis. While neutrophils dominate in acute bacterial meningitis, spirochetal infections like Lyme and Syphilis typically present with a mononuclear/lymphocytic response. **2. Analysis of Other Options:** * **Option A:** *B. burgdorferi* initially replicates locally at the site of the tick bite (causing *Erythema migrans*) and then spreads through the dermis and lymphatics before hematogenous dissemination. * **Option B:** Despite a robust humoral (antibody) response, the organism can persist in the host. This is due to **antigenic variation** (VlsE surface protein) and its ability to reside in immunologically privileged sites (like the extracellular matrix), allowing the infection to progress to late stages. * **Option D:** The detection of intrathecal antibody production (specifically IgM, IgG, or IgA) against *B. burgdorferi* antigens is a gold-standard diagnostic criterion for confirming Lyme meningitis. **Clinical Pearls for NEET-PG:** * **Vector:** *Ixodes* tick (deer tick). * **Stage 1:** Erythema migrans (Bull’s eye rash). * **Stage 2:** Early disseminated (Bilateral Bell’s palsy is a high-yield association). * **Stage 3:** Late persistent (Chronic arthritis, Acrodermatitis chronica atrophicans). * **Treatment:** Doxycycline is the drug of choice; Ceftriaxone is used for neurological or cardiac manifestations.

Question 443: What contributes to the virulence of mycobacteria?

A. Mycotoxin production
B. Cell envelope (Correct Answer)
C. Resistance to inactivation by heat
D. Proteolytic bacteria

Explanation: **Explanation:** The virulence of *Mycobacterium tuberculosis* is primarily attributed to its unique, complex **cell envelope**, which is exceptionally rich in lipids (about 60% of its dry weight). Unlike most bacteria, mycobacteria do not produce classic exotoxins or endotoxins. Instead, their pathogenicity stems from their ability to survive and multiply within host macrophages. **Why the Cell Envelope is the Correct Answer:** The cell envelope contains several key components that facilitate virulence: * **Mycolic Acids:** Long-chain fatty acids that provide a physical barrier against detergents and many antibiotics. * **Cord Factor (Trehalose dimycolate):** Inhibits neutrophil migration and damages mitochondria. It is responsible for the characteristic "serpentine cord" growth pattern. * **Sulfatides:** Prevent phagosome-lysosome fusion, allowing the bacteria to survive intracellularly. * **Wax D:** Acts as an adjuvant, stimulating the delayed-type hypersensitivity (DTH) response. **Analysis of Incorrect Options:** * **A. Mycotoxin production:** Mycotoxins are secondary metabolites produced by **fungi** (e.g., Aflatoxin from *Aspergillus*), not mycobacteria. * **C. Resistance to inactivation by heat:** While mycobacteria are resistant to chemical disinfectants and desiccation, they are **not** particularly heat-resistant. They are easily killed by pasteurization and standard autoclaving. * **D. Proteolytic bacteria:** Mycobacteria are not primarily characterized by the production of extracellular proteolytic enzymes to invade tissues; their damage is largely mediated by the host's immune response. **NEET-PG High-Yield Pearls:** * **Acid-fastness:** Due to mycolic acid in the cell wall; stained by the Ziehl-Neelsen technique. * **Lipoarabinomannan (LAM):** A major surface glycolipid that suppresses T-cell activation. * **Generation time:** Very slow (12–20 hours), contributing to the chronic nature of the disease.

Question 444: A patient presents with burning epigastric pain and undergoes gastric biopsy. The tissue is cultured on chocolate agar and incubated in a microaerophilic environment at 37°C for 5 to 7 days. On the fifth day of incubation, colonies appear. Microscopic examination reveals curved, gram-negative, oxidase-positive rods. What is the most likely identity of this organism?

A. Campylobacter fetus
B. Campylobacter jejuni
C. Haemophilus influenzae
D. Helicobacter pylori (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Helicobacter pylori** The clinical presentation of epigastric pain (suggestive of peptic ulcer disease) combined with specific laboratory findings points directly to *Helicobacter pylori*. * **Microbiology:** It is a **curved (spiral), Gram-negative rod** that is **oxidase-positive**, catalase-positive, and strongly **urease-positive**. * **Culture Requirements:** It requires a **microaerophilic** environment (5–10% $O_2$) and enriched media like chocolate agar or Skirrow’s medium. Unlike many other bacteria, it is slow-growing, typically requiring **5 to 7 days** for colonies to appear. **Incorrect Options:** * **A & B. Campylobacter species:** While also curved, oxidase-positive, Gram-negative rods, *C. jejuni* is primarily associated with inflammatory diarrhea (dysentery). It grows better at **42°C** (thermophilic) and much faster (48 hours) than *H. pylori*. *C. fetus* is an opportunistic pathogen causing systemic infections, not localized gastric pathology. * **C. Haemophilus influenzae:** Although it grows on chocolate agar, it is a **pleomorphic coccobacillus** (not curved) and is typically associated with respiratory infections or meningitis, not epigastric pain. **High-Yield Clinical Pearls for NEET-PG:** * **Urease Activity:** The most characteristic feature of *H. pylori* is its potent urease production, which neutralizes gastric acid by producing ammonia, allowing survival in the stomach. This is the basis for the **Urea Breath Test** and **Rapid Urease Test (RUT)**. * **Virulence Factors:** Look for **CagA** (associated with gastric cancer) and **VacA** (vacuolating cytotoxin) in question stems. * **Associations:** *H. pylori* is a Type 1 Carcinogen; it is strongly linked to **MALToma** and **Gastric Adenocarcinoma**. * **Staining:** In biopsy sections, it is best visualized using **Warthin-Starry silver stain** or Giemsa stain.

Question 445: The Widal test is a type of:

A. Slide flocculation test
B. Immunodiffusion
C. Tube precipitation
D. Tube agglutination (Correct Answer)

Explanation: **Explanation:** The **Widal test** is a serological test used for the diagnosis of enteric fever (Typhoid and Paratyphoid). It is a classic example of a **direct agglutination test**, specifically the **tube agglutination** method. **1. Why the Correct Answer is Right:** In the Widal test, the patient’s serum (containing antibodies) is mixed with standardized bacterial suspensions of *Salmonella Typhi* (O and H antigens) and *S. Paratyphi* (AH and BH antigens). When specific antibodies are present, they cross-link with the particulate bacterial antigens, resulting in visible clumping or **agglutination**. The tube method (Dreyer’s tube for H and Felix tube for O) is preferred over the slide method as it allows for the quantification of antibody titers through serial dilution. **2. Why the Other Options are Wrong:** * **Slide flocculation test:** This involves soluble antigens reacting with antibodies to form visible flakes (floccules). A classic example is the **VDRL test** for Syphilis. * **Immunodiffusion:** This is a precipitation reaction in a gel medium (e.g., Elek’s gel precipitation test for Diphtheria toxin). * **Tube precipitation:** This involves soluble antigens reacting with antibodies in a liquid medium to form a visible precipitate at the interface (e.g., Ascoli’s thermoprecipitin test for Anthrax). **High-Yield Clinical Pearls for NEET-PG:** * **Antigens used:** 'O' (Somatic) is a group-specific antigen; 'H' (Flagellar) is species-specific. * **Interpretation:** 'O' agglutinins appear early and disappear early (indicates recent infection). 'H' agglutinins appear later but persist longer. * **Diagnostic Titer:** In India, a significant titer is usually **>1:80 for O** and **>1:160 for H**. * **Anamnestic Response:** A transient rise in Widal titers can occur during unrelated fevers (e.g., Malaria) in individuals previously vaccinated or infected with *Salmonella*.

Question 446: Which toxin acts on cGMP?

A. Heat-stable E. coli toxin (Correct Answer)
B. Heat-labile E. coli toxin
C. Cholera toxin
D. Shiga toxin

Explanation: **Explanation:** The correct answer is **Heat-stable E. coli toxin (STa)**. **1. Mechanism of the Correct Answer:** Enterotoxigenic *E. coli* (ETEC) produces two types of toxins: Heat-stable (ST) and Heat-labile (LT). The **Heat-stable toxin (STa)** binds to the **Guanylate cyclase-C** receptor on the apical membrane of enterocytes. This activation leads to an increase in intracellular **cyclic Guanosine Monophosphate (cGMP)**. Elevated cGMP levels activate protein kinase G, which inhibits sodium absorption and stimulates chloride secretion via the CFTR channel, resulting in secretory "Traveler’s diarrhea." **2. Analysis of Incorrect Options:** * **Heat-labile E. coli toxin (LT):** This toxin acts by ADP-ribosylation of the Gs protein, which activates **Adenylate cyclase**, leading to an increase in **cAMP** (not cGMP). * **Cholera toxin:** Produced by *Vibrio cholerae*, its mechanism is identical to the E. coli Heat-labile toxin (LT). It increases **cAMP** levels, causing massive electrolyte and water loss. * **Shiga toxin:** Produced by *Shigella dysenteriae* (and Shiga-like toxin by EHEC), it acts by inactivating the **28S ribosomal subunit**, thereby inhibiting protein synthesis. It does not involve cyclic nucleotides. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for ETEC:** "**S**table **G**uanylate, **L**abile **A**denylate" (**S**t. **G**eorge and **L**os **A**ngeles). * **cGMP-mediated toxins:** Heat-stable E. coli toxin and the toxin of *Yersinia enterocolitica*. * **cAMP-mediated toxins:** Cholera toxin, E. coli LT, Anthrax Edema Factor (calmodulin-dependent adenylate cyclase), and Pertussis toxin (via Gi inhibition). * **Clinical:** ETEC is the most common cause of Traveler’s diarrhea. STa is a small peptide that is not immunogenic, unlike the larger LT.

Question 447: Gastrointestinal enteritis necroticans is caused by?

A. Clostridium difficile
B. Clostridium perfringens (Correct Answer)
C. Clostridium botulinum
D. Campylobacter jejuni

Explanation: **Explanation:** **Enteritis necroticans**, also known as **Pigbel**, is a severe, life-threatening necrotizing inflammation of the small intestine (primarily the jejunum). It is caused by **Clostridium perfringens Type C**. 1. **Why Clostridium perfringens is correct:** The pathogenesis involves the production of the **Beta toxin** by *C. perfringens* Type C. This toxin is highly sensitive to proteolytic enzymes like trypsin. In populations with low protein intake (low trypsin levels) or those consuming foods containing trypsin inhibitors (like sweet potatoes), the toxin remains active, leading to mucosal necrosis, hemorrhage, and gangrene of the bowel. 2. **Why the other options are incorrect:** * **Clostridium difficile:** Primarily causes **Pseudomembranous colitis**, typically following antibiotic therapy. It affects the colon rather than causing the segmental small bowel necrosis seen in Pigbel. * **Clostridium botulinum:** Causes **Botulism**, a paralytic illness mediated by a neurotoxin that blocks acetylcholine release at the neuromuscular junction. It does not cause necrotizing enteritis. * **Campylobacter jejuni:** A common cause of bacterial gastroenteritis and bloody diarrhea. While it causes inflammation, it does not produce the specific necrotizing pathology associated with Enteritis necroticans. **High-Yield Clinical Pearls for NEET-PG:** * **C. perfringens Type A:** Most common cause of gas gangrene (myonecrosis) and food poisoning. * **C. perfringens Type C:** Specifically associated with Enteritis necroticans (Pigbel). * **Nagler’s Reaction:** Used for rapid identification of *C. perfringens* (detects Lecithinase/Alpha toxin). * **Morphology:** Large, Gram-positive, "box-car" shaped bacilli; non-motile; produces a "target hemolysis" (double zone) on blood agar.

Question 448: Brucella melitensis is commonly found in which animal?

A. Pig
B. Dog
C. Cattle
D. Goat (Correct Answer)

Explanation: **Explanation:** Brucellosis is a zoonotic infection caused by small, Gram-negative coccobacilli. The species of *Brucella* are highly host-specific, and identifying the animal reservoir is crucial for epidemiological diagnosis. **1. Why Goat is Correct:** *Brucella melitensis* is the most virulent species causing human brucellosis. Its primary reservoirs are **goats and sheep**. Transmission to humans typically occurs through the consumption of unpasteurized milk or cheese (dairy products) or through direct contact with the birth products of these animals. **2. Analysis of Incorrect Options:** * **Pig (Option A):** The primary reservoir for pigs is ***Brucella suis***. This species is known for causing chronic infections and localized abscesses in humans. * **Dog (Option B):** The primary reservoir for dogs is ***Brucella canis***. While it can infect humans, it is generally the least common and least virulent species. * **Cattle (Option C):** The primary reservoir for cattle (cows/buffaloes) is ***Brucella abortus***. It is a common cause of human brucellosis worldwide but is generally less severe than *B. melitensis*. **3. High-Yield Clinical Pearls for NEET-PG:** * **Most Common Species:** *B. melitensis* is the most common cause of human brucellosis globally and in India. * **Clinical Presentation:** Characterized by **Undulant fever** (wave-like fever), profuse sweating (smelling like mold/wet hay), and splenomegaly. * **Diagnosis:** * **Standard Agglutination Test (SAT):** Detects antibodies; a titer of >1:160 is significant. * **Castaneda’s Medium:** A specialized biphasic medium used for blood cultures to reduce the risk of laboratory-acquired infections. * **Culture Characteristics:** They are strict aerobes, catalase-positive, and oxidase-positive. *B. abortus* is unique as it requires 5-10% $CO_2$ for growth (capnophilic).

Question 449: All of the following are true regarding Pseudomonas aeruginosa, except:

A. Oxidase positive
B. Polar flagellate
C. Ferments glucose forming acid and gas (Correct Answer)
D. Obligate aerobe

Explanation: ### Explanation **Pseudomonas aeruginosa** is a non-fermentative, Gram-negative bacillus. Understanding its metabolic and morphological characteristics is crucial for differentiating it from members of the *Enterobacteriaceae* family. **1. Why Option C is the correct answer (The Exception):** *Pseudomonas aeruginosa* is a **non-fermenter**. Unlike *E. coli* or *Klebsiella*, it lacks the metabolic pathways to ferment carbohydrates under anaerobic conditions. Instead, it utilizes glucose **oxidatively** (using the Entner-Doudoroff pathway), producing acid only in the presence of oxygen. It never produces gas from glucose. This is why it appears as a "Non-Lactose Fermenter" (NLF) on MacConkey agar. **2. Analysis of Incorrect Options:** * **A. Oxidase positive:** This is a key biochemical hallmark. *Pseudomonas* produces cytochrome c oxidase, which distinguishes it from the oxidase-negative *Enterobacteriaceae*. * **B. Polar flagellate:** It is actively motile via one or more **polar flagella** (monotrichous or lophotrichous), showing a characteristic "darting motility." * **D. Obligate aerobe:** It strictly requires oxygen as the terminal electron acceptor for energy production. While it can grow anaerobically if nitrate is available (using it as an alternative electron acceptor), it is classically categorized as an obligate aerobe. **Clinical Pearls for NEET-PG:** * **Pigments:** Produces **Pyocyanin** (blue-green, unique to *P. aeruginosa*), Pyoverdin (fluorescent yellow), and Pyorubin (red). * **Odor:** Cultures have a characteristic **fruity/grape-like odor** (due to aminoacetophenone). * **Resistance:** Inherently resistant to many antibiotics; the drugs of choice include Piperacillin-Tazobactam, Ceftazidime, Carbapenems, and Polymyxins. * **Common Infections:** Ecthyma gangrenosum, Hot tub folliculitis, Otitis externa (Swimmer's ear), and Ventilator-associated pneumonia (VAP).

Question 450: A patient with H. pylori infection is treated with drugs. What is the best method to detect the presence of residual H. pylori infection in this person?

A. Rapid urease test
B. Urea breath test (Correct Answer)
C. Endoscopy and biopsy
D. Serum anti H. pylori titre

Explanation: **Explanation:** The **Urea Breath Test (UBT)** is the gold standard non-invasive test for documenting the **eradication** of *H. pylori* and detecting residual infection post-treatment. **Why Urea Breath Test is correct:** * **Mechanism:** It relies on the potent **urease activity** of *H. pylori*. The patient ingests labeled urea ($^{13}C$ or $^{14}C$). If the bacteria are present, urease splits the urea into ammonia and labeled $CO_2$, which is then detected in the exhaled breath. * **Clinical Utility:** It reflects the **active** status of the entire gastric mucosa, unlike biopsy-based tests which can suffer from sampling errors if the infection is patchy after antibiotic use. **Why other options are incorrect:** * **Rapid Urease Test (RUT):** While highly specific, it requires an invasive endoscopy and biopsy. It is the test of choice for *initial* diagnosis during endoscopy but is less preferred than UBT for post-treatment follow-up. * **Endoscopy and Biopsy:** This is invasive. Post-treatment monitoring is ideally done via non-invasive means unless the patient has "red flag" symptoms (e.g., weight loss, GI bleed). * **Serum anti-H. pylori titre:** Serology detects IgG antibodies. These antibodies persist for months to years even after successful eradication; therefore, serology **cannot** distinguish between an active and a past/treated infection. **High-Yield Clinical Pearls for NEET-PG:** * **Test of Choice for Eradication:** Urea Breath Test (performed at least 4 weeks after completing therapy). * **Stool Antigen Test:** Another reliable non-invasive test for both diagnosis and monitoring eradication. * **Pre-test Protocol:** Patients must stop **Proton Pump Inhibitors (PPIs)** for 2 weeks and **Antibiotics/Bismuth** for 4 weeks before UBT to avoid false-negative results. * **Culture:** Most specific but difficult; used primarily for antibiotic sensitivity testing in refractory cases.

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

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free