Bacteriology — MCQs

A 70-year-old female patient was readmitted to a local hospital with fever and chills following cardiac surgery. Blood cultures were taken and a gram-positive coccus grew from the blood cultures within 24 hours. Initial tests indicated that this isolate was resistant to penicillin. Which of the following is the most likely identification?

Q838Easy

Which characteristic differentiates Staphylococcus aureus from Staphylococcus epidermidis?

Q839Easy

Hot cold phenomenon is caused by which staphylococcal toxin?

Q840Medium

A 28-year-old male presents with high fever and signs suggestive of meningitis. On CSF microscopy, Gram-positive bacilli are seen. What is the most likely causative organism?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 831: A 44-year-old man develops intra-abdominal sepsis after undergoing difficult bowel resection and anastomosis. He is initially given ceftizoxime sodium, which is ineffective because of overgrowth of which of the following?

A. Pseudomonas (Correct Answer)
B. Staphylococcus aureus
C. Neisseria gonorrhoeae
D. Bacteroides fragilis

Explanation: **Explanation:** The correct answer is **Pseudomonas (Option A)**. **Why Pseudomonas is correct:** Ceftizoxime is a **third-generation cephalosporin**. While third-generation cephalosporins have expanded Gram-negative coverage compared to earlier generations, they are not all created equal. Ceftizoxime, specifically, has excellent activity against many Enterobacteriaceae and anaerobes (like *Bacteroides*), but it lacks significant activity against **Pseudomonas aeruginosa**. In a clinical setting of intra-abdominal sepsis, if a patient is treated with a drug that covers most flora but misses *Pseudomonas*, that organism can undergo "selective overgrowth," leading to treatment failure. **Why the other options are incorrect:** * **Staphylococcus aureus (B):** While third-generation cephalosporins have less Gram-positive activity than first-generation ones, they still possess some efficacy. However, the specific "overgrowth" scenario in post-surgical bowel sepsis typically points toward a resistant Gram-negative rod. * **Neisseria gonorrhoeae (C):** This is a primary pathogen of the urogenital tract, not a common cause of post-surgical intra-abdominal sepsis. * **Bacteroides fragilis (D):** Ceftizoxime is unique among many third-generation cephalosporins because it actually has **good anaerobic activity**, including against *Bacteroides*. Therefore, *Bacteroides* would likely be covered, not overgrow. **NEET-PG High-Yield Pearls:** * **Anti-Pseudomonal Cephalosporins:** Only **Ceftazidime** (3rd gen) and **Cefepime** (4th gen) have reliable activity against *Pseudomonas*. * **Ceftizoxime vs. Cefotaxime/Ceftriaxone:** Ceftizoxime is often noted in exams for its better anaerobic profile compared to its peers. * **Intra-abdominal Sepsis:** Usually polymicrobial (Gram-negative rods + Anaerobes). If a drug covers anaerobes but lacks anti-pseudomonal activity, *Pseudomonas* is the most likely culprit for persistent infection.

Question 832: Which of the following is NOT true for Bordetella pertussis?

A. It is a strict human pathogen.
B. It can be cultured from the patient during the catarrhal stage.
C. It leads to invasion of the respiratory mucosa.
D. Infection can be prevented by an acellular vaccine. (Correct Answer)

Explanation: ### Explanation The correct answer is **D**, but it is important to note that in the context of this question, Option D is technically a "true" statement, while **Option C is the "NOT true" statement.** *Bordetella pertussis* is a non-invasive pathogen. #### Why Option C is the correct answer (The False Statement): *Bordetella pertussis* does **not** invade the respiratory mucosa or the bloodstream. It is a surface-dwelling pathogen that attaches to the ciliated epithelial cells of the respiratory tract via adhesins (like filamentous hemagglutinin). It causes local tissue damage and systemic symptoms through the release of toxins (Pertussis toxin, tracheal cytotoxin), leading to ciliary stasis and necrosis, but the bacteria themselves remain superficial. #### Analysis of other options: * **Option A (True):** *B. pertussis* is a **strict human pathogen**. There is no known animal or environmental reservoir, which makes humans the sole source of infection. * **Option B (True):** The **catarrhal stage** (first 1–2 weeks) is the period of maximum infectivity. During this stage, the bacterial load is highest, making it the best time for successful culture (using Regan-Lowe or Bordet-Gengou medium). * **Option D (True):** Infection is indeed prevented by vaccines. Modern **acellular vaccines (aP)**, which contain purified components like pertussis toxin and filamentous hemagglutinin, are used (often as DTaP) to reduce side effects compared to the older whole-cell vaccine. #### NEET-PG High-Yield Pearls: * **Gold Standard Diagnosis:** Culture remains the gold standard, but **PCR** is now the preferred rapid diagnostic method. * **Specimen Collection:** Use **Dacron or calcium alginate swabs** (not cotton) via the nasopharyngeal route. * **Mercury Drop Appearance:** Colonies on Bordet-Gengou agar appear like "bisected pearls" or "mercury drops." * **Lymphocytosis:** Unlike most bacterial infections, Pertussis causes a marked **absolute lymphocytosis** due to Pertussis toxin blocking lymphocyte migration.

Question 833: In the laboratory, the screening procedure used to differentiate Salmonella and Shigella from other Enterobacteriaceae includes testing for their inability to:

A. Ferment glucose
B. Ferment lactose (Correct Answer)
C. Produce cytochrome oxidase
D. Reduce nitrates

Explanation: ### Explanation The primary screening method for differentiating major enteric pathogens like **Salmonella** and **Shigella** from the normal intestinal flora (like *E. coli*) is their inability to **ferment lactose**. **1. Why "Ferment Lactose" is Correct:** Most members of the *Enterobacteriaceae* family that constitute normal intestinal flora are **Lactose Fermenters (LF)**. In contrast, the major intestinal pathogens, *Salmonella* and *Shigella*, are characteristically **Non-Lactose Fermenters (NLF)**. When clinical samples (stool) are plated on differential media like **MacConkey Agar** or **Deoxycholate Citrate Agar (DCA)**: * **Lactose fermenters** produce acid, turning the colonies **pink**. * **Non-lactose fermenters** (Salmonella/Shigella) appear as **pale/colorless** colonies. This visual distinction is the first and most crucial step in screening for these pathogens. **2. Why Other Options are Incorrect:** * **A. Ferment glucose:** All members of the *Enterobacteriaceae* family, including *Salmonella* and *Shigella*, are capable of fermenting glucose. This does not help in differentiation. * **C. Produce cytochrome oxidase:** All *Enterobacteriaceae* are **Oxidase negative**. This test is used to differentiate them from other Gram-negative rods like *Pseudomonas* or *Vibrio*, not from each other. * **D. Reduce nitrates:** All *Enterobacteriaceae* have the ability to reduce nitrates to nitrites. **High-Yield Clinical Pearls for NEET-PG:** * **Exceptions to NLF:** *Shigella sonnei* is a **late lactose fermenter**. * **H2S Production:** To further differentiate the two NLFs, *Salmonella* generally produces H2S (black centers on HE agar), while *Shigella* does not. * **Motility:** *Salmonella* is motile (except *S. Gallinarum* and *S. Pullorum*), whereas *Shigella* is characteristically **non-motile**. * **Triple Sugar Iron (TSI) Agar:** Both will show a K/A (Alkaline slant/Acid butt) reaction, indicating glucose fermentation only.

Question 834: Staphylococcal food poisoning includes which of the following features?

A. Incubation period of 24 hours
B. Requires immediate antibiotics
C. Common with dairy products (Correct Answer)
D. Heat-labile enterotoxin

Explanation: Staphylococcal food poisoning is a classic example of **bacterial intoxication**, caused by the ingestion of pre-formed enterotoxins produced by *Staphylococcus aureus*. **Why Option C is correct:** *Staphylococcus aureus* thrives in foods with high salt or sugar content and those requiring significant manual handling. **Dairy products** (milk, cream, custards), processed meats, and potato salads are common vehicles. The bacteria are often introduced by asymptomatic human carriers (nasal or skin colonization) during food preparation. **Why other options are incorrect:** * **Option A:** The incubation period is characteristically **short (1–6 hours)** because the toxin is already present in the food. A 24-hour incubation is more typical of infections like *Salmonella*. * **Option B:** Treatment is **supportive** (rehydration). Since the symptoms are caused by a pre-formed toxin and not an active infection, antibiotics are ineffective and unnecessary. * **Option D:** The enterotoxin is **heat-stable** (resists boiling at 100°C for 30 minutes). While the bacteria themselves are killed by cooking, the toxin remains active, leading to illness. **NEET-PG High-Yield Pearls:** * **Mechanism:** The enterotoxin acts as a **Superantigen**, stimulating massive T-cell proliferation and cytokine release. It specifically acts on the vagus nerve to stimulate the vomiting center in the CNS. * **Clinical Feature:** Violent, projectile vomiting is the hallmark; fever is usually absent. * **Toxin Types:** Enterotoxin **A** is most commonly associated with food poisoning; Enterotoxin **B** is associated with pseudomembranous enterocolitis. * **Diagnosis:** Primarily clinical; however, the toxin can be detected in suspected food using ELISA or latex agglutination.

Question 835: Which of the following statements about tetanus are true?

A. Gram-negative spore-forming aerobe
B. Caused by tetanospasmin
C. Muscle enzymes are normal
D. Vegetative cell will die with tetracycline (Correct Answer)

Explanation: **Explanation:** *Clostridium tetani* is the causative agent of Tetanus. Understanding its microbiological characteristics and susceptibility is crucial for NEET-PG. **1. Why Option D is Correct:** *Clostridium tetani* exists in two forms: the dormant **spore** and the active **vegetative cell**. While spores are highly resistant to heat, chemicals, and most antibiotics, the vegetative cells are metabolically active and susceptible to protein synthesis inhibitors like **Tetracyclines** or cell-wall inhibitors like Penicillin/Metronidazole. In clinical practice, antibiotics are used to kill these vegetative cells to stop further toxin production. **2. Why the other options are Incorrect:** * **Option A:** *C. tetani* is a **Gram-positive**, spore-forming, **obligate anaerobe**. It is famously known for its "drumstick" appearance due to terminal spherical spores. * **Option B:** While tetanus is indeed caused by **Tetanospasmin** (a potent neurotoxin), this option is technically "true" in a general sense. However, in the context of standard PG-level MCQs, Option D is often the preferred "most specific" microbiological fact regarding antibiotic susceptibility, or Option B may be considered a "distractor" if the question focuses on cellular biology. (Note: Tetanospasmin causes spastic paralysis by inhibiting GABA/Glycine release). * **Option C:** In clinical tetanus, **Muscle enzymes (CPK/Creatine Phosphokinase)** are typically **elevated** due to intense, continuous muscle spasms and potential rhabdomyolysis, not normal. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Tetanospasmin acts on **Renshaw cells** in the spinal cord, blocking inhibitory neurotransmitters (GABA/Glycine), leading to spasticity. * **Clinical Signs:** Risus sardonicus (grimace), Trismus (lockjaw), and Opisthotonus (backward arching). * **Transport:** The toxin undergoes **retrograde axonal transport** to reach the CNS. * **Treatment:** Wound debridement is the most important step to remove the anaerobic niche. Metronidazole is currently the drug of choice over Penicillin (as Penicillin is a GABA antagonist and may worsen spasms).

Question 836: Pyoverdin is produced by which bacterium?

A. Pseudomonas aeruginosa (Correct Answer)
B. Clostridium perfringens
C. Bacillus stearothermophilus
D. Salmonella typhi

Explanation: **Explanation:** **Pseudomonas aeruginosa** is the correct answer. It is a non-fermenting Gram-negative bacillus known for producing several characteristic pigments. **Pyoverdin** (also called fluorescein) is a water-soluble, yellow-green pigment that fluoresces under ultraviolet light. It functions as a siderophore, helping the bacterium sequester iron from the environment. * **Why Option A is correct:** *P. aeruginosa* produces multiple pigments: Pyoverdin (yellow-green), Pyocyanin (blue-green, unique to *Pseudomonas*), Pyorubin (red), and Pyomelanin (brown). The combination of pyoverdin and pyocyanin gives the organism its classic "blue-green" appearance on culture media like Nutrient Agar or Mueller-Hinton Agar. * **Why Options B, C, and D are incorrect:** * **Clostridium perfringens:** Known for producing alpha-toxin (lecithinase) and showing "double zone of hemolysis" on blood agar, but does not produce fluorescent pigments. * **Bacillus stearothermophilus:** A thermophilic bacterium used as a biological indicator for autoclave sterilization; it does not produce pyoverdin. * **Salmonella typhi:** A Gram-negative enteric pathogen characterized by H2S production (black colonies on SSA) and "rose spots" clinically, but it lacks pigment production. **High-Yield Clinical Pearls for NEET-PG:** * **Pyocyanin** is the only pigment specific to *P. aeruginosa* and is a virulence factor that generates reactive oxygen species. * **Grape-like/Sweet musty odor:** Characteristic of *Pseudomonas* cultures (due to aminoacetophenone). * **Cetrimide Agar:** The selective medium used to enhance pigment production for identification. * **Clinical Association:** *Pseudomonas* is a leading cause of ventilator-associated pneumonia (VAP), ecthyma gangrenosum, and infections in cystic fibrosis and burn patients.

Question 837: A 70-year-old female patient was readmitted to a local hospital with fever and chills following cardiac surgery. Blood cultures were taken and a gram-positive coccus grew from the blood cultures within 24 hours. Initial tests indicated that this isolate was resistant to penicillin. Which of the following is the most likely identification?

A. Enterococcus (Correct Answer)
B. Group A streptococcus
C. Group B streptococcus
D. Neisseria species

Explanation: **Explanation:** The clinical presentation of a post-surgical patient with fever and chills, combined with the laboratory finding of **Gram-positive cocci (GPC)** resistant to penicillin, strongly points toward **Enterococcus**. 1. **Why Enterococcus is correct:** Enterococci (formerly Group D Streptococci) are notorious for their **intrinsic resistance** to many antibiotics, including most cephalosporins and, frequently, penicillins (due to low-affinity Penicillin-Binding Proteins). They are common causes of healthcare-associated infections, including endocarditis and bacteremia following invasive procedures or surgeries. Their ability to grow in 6.5% NaCl and hydrolyze bile esculin are key biochemical markers. 2. **Why the other options are incorrect:** * **Group A (S. pyogenes) and Group B (S. agalactiae) Streptococcus:** These remain **exquisitely sensitive to Penicillin G**. If a GPC is penicillin-resistant, these species are effectively ruled out in a standard exam context. * **Neisseria species:** These are **Gram-negative diplococci**, which contradicts the Gram stain finding (Gram-positive cocci) provided in the question. **High-Yield Clinical Pearls for NEET-PG:** * **Enterococcus faecalis** is the most common species isolated, but **Enterococcus faecium** is more likely to be multidrug-resistant (including Vancomycin resistance - VRE). * **Treatment:** For serious infections (like endocarditis), a combination of a cell-wall active agent (Penicillin/Ampicillin/Vancomycin) plus an **Aminoglycoside** is required for synergistic bactericidal action. * **Culture Characteristics:** They show **Gamma-hemolysis** (usually) on blood agar and can grow in harsh conditions (40% bile and 6.5% NaCl).

Question 838: Which characteristic differentiates Staphylococcus aureus from Staphylococcus epidermidis?

A. Coagulase-positive (Correct Answer)
B. Formation of white colonies
C. Common cause of urinary tract infection
D. Causes endocarditis of prosthetic valves

Explanation: The primary biochemical test used to differentiate *Staphylococcus aureus* from other Staphylococci is the **Coagulase test**. ### **Explanation of the Correct Answer** * **Coagulase-positive (Option A):** *Staphylococcus aureus* produces the enzyme coagulase, which converts fibrinogen to fibrin, causing plasma to clot. This is the gold-standard diagnostic feature that separates it from "Coagulase-Negative Staphylococci" (CoNS), such as *S. epidermidis* and *S. saprophyticus*. ### **Analysis of Incorrect Options** * **Formation of white colonies (Option B):** While *S. epidermidis* typically produces white colonies, *S. aureus* is known for its characteristic **golden-yellow colonies** (due to the carotenoid pigment staphyloxanthin). Therefore, colony color differentiates them in the opposite direction. * **Common cause of UTI (Option C):** While both can cause UTIs, *S. saprophyticus* is the specific CoNS famous for causing "honeymoon cystitis" in young women. *S. aureus* UTIs are less common and usually secondary to bacteremia. * **Causes endocarditis of prosthetic valves (Option D):** This is the classic clinical presentation of ***S. epidermidis***. It produces a polysaccharide **biofilm** (slime layer) that allows it to adhere to prosthetic material. While *S. aureus* causes aggressive endocarditis, it is more commonly associated with native valves or IV drug users. ### **NEET-PG High-Yield Pearls** * **Catalase Test:** All Staphylococci are Catalase-positive (differentiates them from Streptococci). * **Mannitol Fermentation:** *S. aureus* ferments mannitol (turning Mannitol Salt Agar yellow); *S. epidermidis* does not. * **Novobiocin Sensitivity:** *S. epidermidis* is Novobiocin **sensitive**, whereas *S. saprophyticus* is Novobiocin **resistant**. * **Protein A:** A key virulence factor of *S. aureus* that binds to the Fc portion of IgG, inhibiting phagocytosis.

Question 839: Hot cold phenomenon is caused by which staphylococcal toxin?

A. Alpha lysin
B. Beta lysin (Correct Answer)
C. Gamma lysin
D. Theta lysin

Explanation: **Explanation:** The **Hot-Cold Phenomenon** is a characteristic feature of **Staphylococcal Beta-lysin** (also known as sphingomyelinase C). This phenomenon refers to the observation that the toxin’s hemolytic activity is initiated at 37°C ("hot") but only becomes visible or complete after subsequent incubation at 4°C ("cold") or room temperature. **Mechanism:** Beta-lysin is an enzyme that degrades sphingomyelin in the red blood cell (RBC) membrane. At 37°C, the enzyme acts on the membrane but does not cause immediate lysis. When the temperature drops, the destabilized membrane undergoes structural contraction and collapses, leading to visible hemolysis. This is most commonly demonstrated using sheep or bovine RBCs. **Analysis of Options:** * **Option B (Correct):** Beta-lysin is the only staphylococcal toxin associated with the hot-cold effect. It is also the basis for the **CAMP test** (used to identify Group B Streptococci), where it acts synergistically with the CAMP factor. * **Option A:** **Alpha-lysin** is the most important virulence factor for *S. aureus*. It is a pore-forming toxin that causes rapid lysis of RBCs and platelets but does not exhibit the hot-cold effect. * **Option C:** **Gamma-lysin** is a bicomponent toxin associated with leukocidin activity. * **Option D:** **Theta-lysin** is not a standard staphylococcal toxin; *Clostridium perfringens* produces a Perfringolysin O (Theta toxin). **High-Yield Clinical Pearls for NEET-PG:** * **CAMP Test:** Beta-lysin producing *S. aureus* is used to detect *Streptococcus agalactiae*. * **Alpha-toxin:** Mediated by chromosomal genes; causes "pore formation." * **Panton-Valentine Leukocidin (PVL):** A key toxin in MRSA causing necrotizing pneumonia and skin infections. * **Toxic Shock Syndrome Toxin (TSST-1):** A superantigen that binds to MHC II and Vβ T-cell receptors.

Question 840: A 28-year-old male presents with high fever and signs suggestive of meningitis. On CSF microscopy, Gram-positive bacilli are seen. What is the most likely causative organism?

A. Streptococcus pneumoniae
B. Haemophilus influenzae
C. Listeria monocytogenes (Correct Answer)
D. Mycoplasma pneumoniae

Explanation: ### Explanation The correct answer is **Listeria monocytogenes**. **1. Why Listeria monocytogenes is correct:** The key to this question lies in the **Gram stain morphology**. *Listeria monocytogenes* is a unique cause of meningitis because it is a **Gram-positive bacillus (rod)**. While most common causes of bacterial meningitis are cocci or Gram-negative rods, *Listeria* stands out as a primary Gram-positive rod pathogen, especially in neonates, the elderly, and immunocompromised individuals. **2. Why the other options are incorrect:** * **Streptococcus pneumoniae (Option A):** While it is the most common cause of meningitis in adults, it appears as **Gram-positive cocci in pairs (diplococci)**, not bacilli. * **Haemophilus influenzae (Option B):** This is a **Gram-negative coccobacillus**. It would appear pink/red on a Gram stain, not purple. * **Mycoplasma pneumoniae (Option C):** This organism **lacks a cell wall** and therefore cannot be visualized on a Gram stain. Furthermore, it primarily causes atypical pneumonia and rarely causes meningitis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Motility:** *Listeria* exhibits characteristic **"Tumbling motility"** at 25°C and "Umbrella motility" in semi-solid agar. * **Cold Enrichment:** It can grow at low temperatures (4°C), a property used for selective isolation. * **Transmission:** Often associated with the consumption of unpasteurized milk, soft cheeses, and processed meats (deli meats). * **Treatment:** It is intrinsically resistant to cephalosporins. **Ampicillin** is the drug of choice for *Listeria* meningitis. * **Pathogenesis:** It is an intracellular pathogen that uses **"Actin tails"** (actin polymerization) to move from cell to cell, avoiding the host's immune system.

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

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