Bacteriology — MCQs
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Which of the following is NOT a feature of Streptococcus pneumoniae?
Which organism is implicated in cellulitis following animal bites?
A 16-month-old boy presents with fever, irritability, and lethargy. Examination reveals nuchal rigidity. He is unvaccinated. CSF Gram stain shows an organism that grows on chocolate agar but not blood agar. The patient has received empirical antibiotics. Which of the following statements regarding the treatment of this condition is FALSE?
What are the recommended conditions for autoclaving?
Proteus mirabilis is associated with which type of renal calculi?
Which of the following statements about Campylobacter is FALSE?
Sulphur granules are seen in which condition?
The gene that encodes the toxin for cholera is transmitted by which of the following?
New York agar is used for the isolation of which organism?
Out of 20 children that went to a party, 11 developed abdominal pain, diarrhea with nausea and vomiting around 6 hours after food intake. Most likely causative agent?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 791: Which of the following is NOT a feature of Streptococcus pneumoniae?
- A. Bile insolubility (Correct Answer)
- B. Optochin sensitivity
- C. Gram-positive cocci
- D. Encapsulated
Explanation: **Explanation:** The correct answer is **A. Bile insolubility**. *Streptococcus pneumoniae* (Pneumococcus) is characterized by being **bile soluble**. **1. Why Option A is the correct answer:** *Streptococcus pneumoniae* possesses an intracellular autolytic enzyme called **amidase**. Surface-active agents like bile salts (e.g., sodium deoxycholate) lower the surface tension on the cell wall, which triggers the activation of these autolysins. This results in the lysis of the bacteria and the clearing of a turbid broth culture. Therefore, saying it is "bile insoluble" is factually incorrect, making it the right choice for this "NOT" question. **2. Why other options are incorrect:** * **B. Optochin sensitivity:** This is a hallmark diagnostic feature. *S. pneumoniae* is highly sensitive to Optochin (ethylhydrocupreine hydrochloride), showing a zone of inhibition ≥14 mm. This distinguishes it from *Viridans streptococci*, which are resistant. * **C. Gram-positive cocci:** Morphologically, they are Gram-positive, flame-shaped (lanceolate) diplococci. * **D. Encapsulated:** It possesses a prominent polysaccharide capsule, which is its most important virulence factor. This capsule is responsible for the **Quellung reaction** (capsular swelling). **High-Yield NEET-PG Pearls:** * **Culture:** Shows "draughtsman" or "checkerboard" appearance (central indentation due to autolysis) on blood agar. * **Inulin Fermentation:** It is the only Streptococcus that ferments inulin. * **Virulence:** The capsule is essential for pathogenicity; non-capsulated strains are avirulent. * **Transformation:** Griffith's experiment on bacterial transformation was performed using *S. pneumoniae*.
Question 792: Which organism is implicated in cellulitis following animal bites?
- A. Staphylococcus aureus
- B. Pseudomonas
- C. Pasteurella (Correct Answer)
- D. Klebsiella
Explanation: **Explanation:** The correct answer is **Pasteurella** (specifically *Pasteurella multocida*). **1. Why Pasteurella is correct:** *Pasteurella multocida* is a small, Gram-negative coccobacillus that exists as normal flora in the oropharynx of many animals, particularly **cats (70-90%) and dogs (20-50%)**. Following a bite or scratch, it is the most common pathogen to cause rapidly progressing cellulitis, often appearing within **3 to 24 hours** of the injury. A key clinical feature is the rapid onset of inflammation and potential progression to tenosynovitis or osteomyelitis. **2. Why the other options are incorrect:** * **Staphylococcus aureus:** While it is the most common cause of *general* skin and soft tissue infections (cellulitis/abscesses), it is not the specific organism associated with the initial presentation of animal bites. * **Pseudomonas:** This is typically associated with puncture wounds through the soles of shoes (leading to osteochondritis) or "hot tub folliculitis," but not primary animal bites. * **Klebsiella:** This is a common cause of pneumonia (in alcoholics) and UTIs, but it is not a primary pathogen in animal bite-related cellulitis. **3. NEET-PG High-Yield Pearls:** * **Drug of Choice:** The preferred treatment for *Pasteurella* infections is **Amoxicillin-Clavulanate** (Augmentin). It covers both *Pasteurella* and anaerobic co-pathogens. * **Morphology:** Look for **"Bipolar staining"** (safety-pin appearance) on Wayson or Giemsa stain. * **Cat vs. Dog:** While both carry the organism, cat bites are more likely to result in infection because their long, sharp teeth cause deep puncture wounds that "seed" the bacteria into bones and joints. * **Other Bite Organisms:** Remember *Eikenella corrodens* for human bites and *Capnocytophaga canimorsus* for sepsis following dog bites (especially in asplenic patients).
Question 793: A 16-month-old boy presents with fever, irritability, and lethargy. Examination reveals nuchal rigidity. He is unvaccinated. CSF Gram stain shows an organism that grows on chocolate agar but not blood agar. The patient has received empirical antibiotics. Which of the following statements regarding the treatment of this condition is FALSE?
- A. Corticosteroids have no role in the treatment of this disease.
- B. First-line therapy includes ceftriaxone or cefotaxime.
- C. The duration of therapy is 1-2 weeks.
- D. Fluoroquinolones can be used for treatment. (Correct Answer)
Explanation: ### Explanation **Diagnosis:** The clinical presentation (fever, nuchal rigidity), age (16 months), and lack of vaccination point to bacterial meningitis. The microbiological clue—growth on **chocolate agar** but not blood agar—is pathognomonic for ***Haemophilus influenzae* type b (Hib)**. This organism requires both Factor X (hemin) and Factor V (NAD), which are only available together in chocolate agar. **Why Option D is the Correct (False) Statement:** While fluoroquinolones (like ciprofloxacin) are used for **chemoprophylaxis** in close contacts of *N. meningitidis*, they are **not** used for the primary treatment of *H. influenzae* meningitis in children. Third-generation cephalosporins remain the gold standard due to superior CNS penetration and efficacy. **Analysis of Other Options:** * **Option A (Corticosteroids):** This is a nuanced point. While dexamethasone is standard for *S. pneumoniae* meningitis, its role in *H. influenzae* is specifically to **reduce the incidence of sensorineural hearing loss**. However, in many clinical guidelines, if the patient has already received empirical antibiotics, the benefit of starting steroids late is significantly diminished or absent. * **Option B (First-line therapy):** Ceftriaxone or Cefotaxime are the drugs of choice due to high beta-lactamase production (up to 40%) by *H. influenzae*. * **Option C (Duration):** The standard duration for uncomplicated *H. influenzae* meningitis is **7 to 10 days**. **High-Yield NEET-PG Pearls:** * **Satellitism:** *H. influenzae* grows on blood agar only near colonies of *S. aureus*, which provide the necessary Factor V. * **Quellung Reaction:** Positive for *H. influenzae* (capsulated strains). * **Prophylaxis:** Rifampicin is the drug of choice for chemoprophylaxis of household contacts of a Hib case. * **Vaccine:** The PRP (Polyribosylribitol phosphate) conjugate vaccine has drastically reduced the incidence of this disease.
Question 794: What are the recommended conditions for autoclaving?
- A. 115 degrees C for 20 minutes
- B. 121 degrees C for 15 minutes (Correct Answer)
- C. 118 degrees C for 15 minutes
- D. 124 degrees C for 15 minutes
Explanation: **Explanation:** **1. The Core Concept: Moist Heat Sterilization** Autoclaving is the most reliable method of sterilization, utilizing **moist heat under pressure**. The principle is that water boils at a higher temperature when pressure is increased. At a pressure of **15 psi** (pounds per square inch) above atmospheric pressure, the temperature of steam reaches **121°C**. This specific combination of temperature and time (15 minutes) is sufficient to cause irreversible denaturation of structural proteins and enzymes, effectively killing all vegetative forms of bacteria, fungi, viruses, and—most importantly—highly resistant **bacterial spores**. **2. Analysis of Options:** * **Option B (Correct):** 121°C for 15 minutes at 15 psi is the standard holding period for routine laboratory sterilization. * **Option A & C:** These temperatures are insufficient to guarantee the destruction of thermophilic spores (like *Geobacillus stearothermophilus*) within the specified timeframe. * **Option D:** While 124°C would sterilize the load, it is not the standard "recommended" protocol for routine autoclaving; higher temperatures (e.g., 134°C for 3 minutes) are typically reserved for "flash sterilization" or specific prion protocols. **3. High-Yield Clinical Pearls for NEET-PG:** * **Sterilization Control (Biological Indicator):** The efficacy of an autoclave is tested using spores of ***Geobacillus stearothermophilus*** (formerly *Bacillus stearothermophilus*). * **Chemical Indicator:** **Browne’s tubes** (color change from red to green) or **Bowie-Dick tape**. * **Prion Disinfection:** Requires higher parameters—134°C for 1-1.5 hours or immersion in 1N NaOH for 1 hour. * **Uses:** Ideal for culture media, surgical instruments, and infectious waste. It is **not** suitable for heat-sensitive plastics, sharp instruments (dulls them), or volatile liquids.
Question 795: Proteus mirabilis is associated with which type of renal calculi?
- A. Uric acid
- B. Struvite (Correct Answer)
- C. Cysteine
- D. Calcium Oxalate
Explanation: **Explanation:** The correct answer is **Struvite (Option B)**. **Mechanism:** *Proteus mirabilis* is a Gram-negative bacillus characterized by its production of the enzyme **urease**. This enzyme hydrolyzes urea into ammonia and carbon dioxide ($NH_2CONH_2 + H_2O \rightarrow 2NH_3 + CO_2$). The resulting ammonia increases the urine pH (alkalinization). In this alkaline environment, phosphate, magnesium, and ammonium ions precipitate to form **Struvite stones** (Magnesium Ammonium Phosphate). These stones often grow rapidly to fill the renal pelvis and calyces, forming the characteristic **"Staghorn calculus."** **Analysis of Incorrect Options:** * **A. Uric acid:** These stones form in **acidic urine** (low pH). They are radiolucent and associated with conditions like gout or high purine intake, not bacterial infections. * **C. Cysteine:** These are rare genetic stones caused by a defect in the transport of dibasic amino acids (COAL: Cysteine, Ornithine, Arginine, Lysine). They also favor an acidic environment. * **D. Calcium Oxalate:** This is the **most common** type of renal stone overall. While their formation is multifactorial (hypercalciuria, dehydration), they are not specifically caused by urease-producing bacteria. **High-Yield Clinical Pearls for NEET-PG:** * **Swarming Growth:** *Proteus* exhibits "swarming" motility on blood agar due to its peritrichous flagella. * **Dienes Phenomenon:** Used to distinguish different strains of *Proteus*. * **Culture Media:** MacConkey agar shows Non-Lactose Fermenting (NLF) colonies; Phenylalanine Deaminase (PPA) test is positive. * **Staghorn Calculus:** While *Proteus* is the most common cause, other urease-positive organisms like *Klebsiella*, *Staphylococcus saprophyticus*, and *Ureaplasma* can also lead to struvite stones.
Question 796: Which of the following statements about Campylobacter is FALSE?
- A. Humans are the only reservoirs (Correct Answer)
- B. The source of infection is poultry
- C. Campylobacteriosis is often due to C. jejuni
- D. C. jejuni infection can lead to Guillain-Barré syndrome (GBS) as a sequela
Explanation: **Explanation:** **1. Why Option A is the correct (False) statement:** *Campylobacter* species, particularly *C. jejuni*, are **zoonotic** pathogens. Humans are **accidental hosts**, not the only reservoir. The primary reservoirs are a wide range of animals, including domestic poultry, cattle, pigs, birds, and even pets like dogs and cats. The bacteria exist as commensals in the gastrointestinal tracts of these animals. **2. Analysis of Incorrect Options (True statements):** * **Option B:** Consumption of undercooked **poultry** or cross-contamination from raw chicken is the most common source of human infection. * **Option C:** *C. jejuni* is responsible for the vast majority (approx. 90%) of cases of human campylobacteriosis (enteritis), followed by *C. coli*. * **Option D:** *C. jejuni* is the most common antecedent infection associated with **Guillain-Barré syndrome (GBS)**. This occurs due to **molecular mimicry** between the bacterial lipooligosaccharides (LOS) and human gangliosides (GM1), leading to an autoimmune attack on peripheral nerves. **3. High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-negative, S-shaped or "sea-gull wing" appearance. * **Motility:** Shows characteristic **darting motility**. * **Culture:** It is **microaerophilic** (requires 5% $O_2$) and **thermophilic** (grows best at **42°C**). * **Selective Media:** Skirrow’s medium, Butzler’s medium, or Preston agar. * **Clinical Presentation:** Acute gastroenteritis often presenting with **bloody diarrhea** (dysentery) and pseudo-appendicitis. * **Complications:** GBS and Reactive Arthritis (Reiter’s syndrome).
Question 797: Sulphur granules are seen in which condition?
- A. Sulphur poisoning
- B. Actinomycosis (Correct Answer)
- C. H2SO4 poisoning
- D. Staphylococcal infection
Explanation: **Explanation:** **Actinomycosis** is a chronic granulomatous infection caused by anaerobic, Gram-positive filamentous bacteria, most commonly *Actinomyces israelii*. The hallmark of this infection is the formation of **Sulphur granules** (also known as "Bollinger’s granules"). These are not actually made of sulphur; they are yellowish, gritty micro-colonies of the bacteria embedded in a matrix of calcium phosphate and host tissue debris. When visualized under a microscope (crush preparation), they show a characteristic "sun-ray" appearance (Splendore-Hoeppli phenomenon) with peripheral radiating filaments. **Analysis of Options:** * **Option A & C (Sulphur/H2SO4 poisoning):** These are chemical toxicities. While they involve the element sulphur, they do not produce organized tissue "granules." This is a common distractor based on nomenclature. * **Option D (Staphylococcal infection):** While *Staphylococcus aureus* can cause "Botryomycosis" (which clinically mimics actinomycosis and produces granules), these are not referred to as sulphur granules. True sulphur granules are pathognomonic for Actinomycosis. **High-Yield Clinical Pearls for NEET-PG:** * **Most common site:** Cervicofacial ("Lumpy Jaw"), often following dental procedures or poor oral hygiene. * **Microscopy:** Gram-positive branching filaments that are **Non-Acid Fast** (unlike *Nocardia*, which is weakly acid-fast). * **Culture:** Grows on Brain Heart Infusion (BHI) agar, showing characteristic **"Molar Tooth" colonies**. * **Treatment:** High-dose Penicillin G is the drug of choice. * **Other granule-producing conditions:** Mycetoma (Madura foot) also produces granules, but they vary in color (black/white) depending on the causative agent (fungal vs. bacterial).
Question 798: The gene that encodes the toxin for cholera is transmitted by which of the following?
- A. Protozoa
- B. Bacteria
- C. Bacteriophage (Correct Answer)
- D. Fungus
Explanation: **Explanation:** The correct answer is **Bacteriophage**. The pathogenicity of *Vibrio cholerae* is primarily due to the production of the **Cholera Toxin (CT)**. The genes encoding this toxin (*ctxA* and *ctxB*) are not part of the native bacterial chromosome; instead, they are carried by a **lysogenic filamentous bacteriophage** known as **CTXφ (CTX phage)**. Through a process called **lysogenic conversion**, the CTX phage infects non-toxigenic strains of *Vibrio cholerae*, integrating its genetic material into the bacterial genome. This transforms a harmless bacterium into a virulent pathogen capable of causing epidemic cholera. **Why other options are incorrect:** * **Protozoa:** While *V. cholerae* can survive in aquatic environments by associating with zooplankton or copepods, protozoa do not transmit the toxin-encoding genes. * **Bacteria:** While bacteria can exchange genetic material via conjugation (plasmids), the specific cholera toxin gene is classically associated with phage transduction. * **Fungus:** Fungi have no role in the genetic transmission or pathogenesis of *Vibrio cholerae*. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Cholera toxin causes permanent activation of **adenylate cyclase**, leading to increased **cAMP** levels. This results in the hypersecretion of water and electrolytes into the intestinal lumen (Rice-water stools). * **Receptor:** The B-subunit of the toxin binds to the **GM1 ganglioside receptor** on enterocytes. * **Other Phage-Encoded Toxins (Mnemonic: ABCD):** * **A:** Group A *Streptococcus* (Pyrogenic exotoxin/Erythrogenic toxin) * **B:** *Botulinum* toxin (certain types) * **C:** *Cholera* toxin * **D:** *Diphtheria* toxin (via the Beta-phage) * **S:** *Shiga* toxin (in EHEC)
Question 799: New York agar is used for the isolation of which organism?
- A. Salmonella
- B. Clostridia
- C. Neisseria (Correct Answer)
- D. Bacillus Anthracis
Explanation: **Explanation:** **New York City (NYC) Agar** is a specialized selective medium primarily designed for the isolation of pathogenic **Neisseria** species, specifically *N. gonorrhoeae* and *N. meningitidis*. It is also capable of supporting the growth of genital mycoplasmas (*Mycoplasma hominis* and *Ureaplasma urealyticum*). The medium consists of a clear agar base supplemented with lysed horse erythrocytes, horse serum, and yeast dialysate. Its selectivity is derived from a specific cocktail of antibiotics: * **Vancomycin:** Inhibits Gram-positive bacteria. * **Colistin:** Inhibits most Gram-negative rods (except Neisseria). * **Nystatin:** Inhibits fungi/yeast. * **Trimethoprim:** Inhibits the swarming of Proteus. **Analysis of Incorrect Options:** * **Salmonella:** Isolated using enteric media like Wilson & Blair (Bismuth Sulphite Agar), DCA, or XLD agar. * **Clostridia:** Being anaerobes, they require media like Robertson’s Cooked Meat (RCM) broth or Blood Agar with aminoglycosides (e.g., Neomycin Blood Agar). * **Bacillus anthracis:** Typically grown on PLET medium (Polymyxin, Lysozyme, EDTA, Thallous acetate), which is highly selective for *B. anthracis*. **High-Yield Clinical Pearls for NEET-PG:** * **Thayer-Martin Medium:** The other classic selective medium for Neisseria; it is essentially Chocolate Agar with VCN (Vancomycin, Colistin, Nystatin) inhibitors. * **NYC Agar vs. Thayer-Martin:** NYC agar is often preferred because it is transparent (allowing earlier detection of colonies) and supports the growth of *Mycoplasma*. * **Transport:** Neisseria are fastidious and sensitive to cold; specimens should never be refrigerated and are ideally transported using **Stuart’s or Amies medium**.
Question 800: Out of 20 children that went to a party, 11 developed abdominal pain, diarrhea with nausea and vomiting around 6 hours after food intake. Most likely causative agent?
- A. Rotavirus
- B. Staphylococcus (Correct Answer)
- C. Streptococcus
- D. Clostridium perfringens
Explanation: **Explanation:** The clinical presentation points toward **Staphylococcal Food Poisoning**, a classic "intoxication" rather than an infection. **Why Staphylococcus is correct:** The hallmark of *Staphylococcus aureus* food poisoning is the **short incubation period (1–6 hours)**. It is caused by the ingestion of pre-formed, heat-stable **Enterotoxin (Type A-E)**. Because the toxin is already present in the food (often creamy items, salads, or processed meats), symptoms like nausea, projectile vomiting, and abdominal cramps appear rapidly. Diarrhea may occur but is usually secondary to vomiting. **Why other options are incorrect:** * **Rotavirus:** This is a common cause of pediatric diarrhea, but it typically has a longer incubation period (1–3 days) and is characterized by watery diarrhea and fever rather than rapid-onset vomiting after a specific meal. * **Streptococcus:** While some strains can cause pharyngitis or skin infections, they are not standard causes of acute food poisoning outbreaks. * **Clostridium perfringens:** This also causes food poisoning, but the incubation period is longer (**8–16 hours**). It is caused by toxin production *in vivo* (inside the gut) and is characterized primarily by watery diarrhea and cramps, with vomiting being rare. **High-Yield Clinical Pearls for NEET-PG:** * **Shortest Incubation (1–6 hrs):** *S. aureus* and *Bacillus cereus* (Emetic type). * **Intermediate Incubation (8–16 hrs):** *Clostridium perfringens* and *Bacillus cereus* (Diarrheal type). * **Long Incubation (>16 hrs):** *Vibrio cholerae*, *Salmonella*, and *Shigella*. * **Key Trigger:** *S. aureus* toxins are **heat-stable** (resist boiling for 30 mins), meaning reheating contaminated food does not prevent the illness.
Practice by Chapter
Staphylococci
Practice Questions
Streptococci and Enterococci
Practice Questions
Neisseria and Moraxella
Practice Questions
Corynebacterium and Listeria
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Bacillus and Clostridium
Practice Questions
Enterobacteriaceae
Practice Questions
Vibrio, Aeromonas, and Plesiomonas
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Pseudomonas and Related Bacteria
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Haemophilus and HACEK Group
Practice Questions
Bordetella and Brucella
Practice Questions
Mycobacteria
Practice Questions
Spirochetes
Practice Questions
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