Bacteriology — MCQs
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Klebsiella oxytoca is:
Nagler's reaction detects which of the following?
Burkholderia cepacia is intrinsically resistant to which of the following antibiotics?
A 45-year-old veterinary surgeon presented with a black eschar on the left hand. There was extensive swelling around the ulcer but it was not painful. There were few vesicular lesions surrounding the ulcer. He presented a history of injury to the hand while handling animals. What would be the presumptive diagnosis on the basis of history and culture smear of fluid from the vesicles?
Pseudo-hemoptysis is seen mostly with which of the following microorganisms?
Which among the following organisms is found as a commensal on the skin?
Shigellosis is best diagnosed by?
Which of the following statements about Gram-positive cocci is false?
A 5-year-old child presents with a pseudomembrane in the throat, fever, and sore throat. Which organism is likely responsible?
Corynebacterium diphtheriae is described as:
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 541: Klebsiella oxytoca is:
- A. Gram variable
- B. Indole positive (Correct Answer)
- C. Methyl red negative
- D. No drug resistance reported
Explanation: **Explanation:** *Klebsiella oxytoca* is a significant human pathogen belonging to the Enterobacteriaceae family. The primary diagnostic challenge in microbiology is differentiating it from the more common *Klebsiella pneumoniae*. **1. Why Indole Positive is Correct:** The biochemical hallmark that distinguishes *K. oxytoca* from *K. pneumoniae* is the **Indole test**. While *K. pneumoniae* is indole negative, *K. oxytoca* possesses the enzyme tryptophanase, which breaks down the amino acid tryptophan into indole. This makes "Indole positive" the definitive identifying feature for *K. oxytoca* in the laboratory. **2. Analysis of Incorrect Options:** * **Gram variable:** Incorrect. Like all members of the Enterobacteriaceae family, *K. oxytoca* is a classic **Gram-negative** rod. * **Methyl red negative:** Incorrect. *Klebsiella* species are typically **Methyl Red (MR) negative** and **Voges-Proskauer (VP) positive**. While this is a true characteristic of the genus, it does not differentiate *K. oxytoca* from other *Klebsiella* species, and the Indole test remains the specific "textbook" differentiator for this organism. * **No drug resistance reported:** Incorrect. *K. oxytoca* is notorious for carrying chromosomal beta-lactamases (K1) and can acquire Extended-Spectrum Beta-Lactamase (ESBL) and Carbapenemase genes, making it highly resistant to many antibiotics. **Clinical Pearls for NEET-PG:** * **IMViC Profile:** *K. pneumoniae* is (- - + +), whereas *K. oxytoca* is **(+ - + +)**. * **Clinical Association:** *K. oxytoca* is specifically associated with **Antibiotic-Associated Hemorrhagic Colitis (AAHC)**, often following penicillin treatment, due to the production of the cytotoxin *tilivalline*. * **Morphology:** Like *K. pneumoniae*, it is non-motile and produces large, mucoid colonies due to its prominent capsule.
Question 542: Nagler's reaction detects which of the following?
- A. Coagulase
- B. Hyaluronidase
- C. Lecithinase (Correct Answer)
- D. All the above
Explanation: **Explanation:** **Nagler’s reaction** is a biochemical test used for the rapid identification of ***Clostridium perfringens***. The reaction specifically detects the presence of **Alpha (α) toxin**, which possesses **lecithinase** (phospholipase C) activity. 1. **Why Lecithinase is correct:** *Clostridium perfringens* produces alpha toxin, which acts on the lecithin present in egg yolk agar. The lecithinase enzyme breaks down lecithin into phosphorylcholine and insoluble diglycerides. This results in a characteristic **opalescence (opacity)** around the colonies. The reaction is confirmed by adding antitoxin to one half of the plate; the antitoxin neutralizes the enzyme, and no opalescence appears on that side (a classic example of a toxin-antitoxin neutralization test). 2. **Why other options are incorrect:** * **Coagulase:** This enzyme is produced by *Staphylococcus aureus* to convert fibrinogen to fibrin. It is detected via the Tube or Slide Coagulase test, not Nagler’s. * **Hyaluronidase:** Also known as "spreading factor," it is produced by various bacteria (like *S. pyogenes*) to degrade connective tissue. It is not the target of the Nagler reaction. **High-Yield Clinical Pearls for NEET-PG:** * **Organism:** *Clostridium perfringens* (formerly *C. welchii*). * **Clinical Condition:** Most common cause of **Gas Gangrene** (Myonecrosis) and food poisoning. * **Culture Characteristic:** Shows **"Target Hemolysis"** (double zone of hemolysis) on blood agar. * **Other Tests:** Positive **Stormy fermentation** in litmus milk. * **Nagler's Key:** It is a **neutralization test** using antitoxin.
Question 543: Burkholderia cepacia is intrinsically resistant to which of the following antibiotics?
- A. Doxycycline
- B. TMP-SMX
- C. Polymyxin B (Correct Answer)
- D. Meropenem
Explanation: **Explanation:** *Burkholderia cepacia* complex (Bcc) is a group of Gram-negative, non-fermenting bacilli known for high levels of intrinsic resistance to multiple antibiotics. **1. Why Polymyxin B is the correct answer:** The hallmark of *Burkholderia cepacia* is its **intrinsic resistance to Polymyxins (Polymyxin B and Colistin)**. The underlying mechanism involves a modification of the bacterial lipopolysaccharide (LPS). Specifically, *B. cepacia* incorporates **4-amino-4-deoxy-L-arabinose (L-Ara4N)** into its lipid A. This reduces the negative charge of the outer membrane, preventing the positively charged polymyxin molecules from binding to the cell surface. **2. Analysis of Incorrect Options:** * **A & B (Doxycycline and TMP-SMX):** These are actually among the drugs of choice for treating *B. cepacia* infections. TMP-SMX is often considered the primary treatment option. * **D (Meropenem):** While many Gram-negative bacteria develop resistance to carbapenems, *B. cepacia* is generally susceptible to Meropenem, making it a viable clinical alternative. **3. High-Yield Clinical Pearls for NEET-PG:** * **Clinical Association:** *B. cepacia* is a significant pathogen in patients with **Cystic Fibrosis** and **Chronic Granulomatous Disease (CGD)**. * **Cepacia Syndrome:** A rapid, fatal necrotizing pneumonia and septicemia seen in Cystic Fibrosis patients. * **Laboratory Diagnosis:** It grows on **BCSA (Burkholderia cepacia selective agar)** or **OFPBL medium**, often producing a characteristic "dirt-like" or earthy odor. * **Oxidase Test:** Most strains are weakly oxidase-positive. * **Mnemonic:** Remember **"B.C. is resistant to Colistin"** (B. cepacia). Other Colistin-resistant organisms include *Proteus*, *Providencia*, *Serratia*, and *Burkholderia pseudomallei*.
Question 544: A 45-year-old veterinary surgeon presented with a black eschar on the left hand. There was extensive swelling around the ulcer but it was not painful. There were few vesicular lesions surrounding the ulcer. He presented a history of injury to the hand while handling animals. What would be the presumptive diagnosis on the basis of history and culture smear of fluid from the vesicles?
- A. Actinomycetoma
- B. Clostridium infection
- C. Cutaneous anthrax (Correct Answer)
- D. Extrapulmonary nocardiosis
Explanation: ### Explanation The clinical presentation described is classic for **Cutaneous Anthrax**, caused by *Bacillus anthracis*. **1. Why Cutaneous Anthrax is correct:** * **Occupational History:** The patient is a veterinary surgeon with a history of handling animals, making this a "Hide Porter’s disease" scenario. * **The Lesion:** The pathognomonic feature is a **painless, itchy papule** that rapidly evolves into a vesicle and then a **depressed black eschar** (necrotic center) surrounded by extensive **non-pitting edema**. * **Microscopy:** A culture smear from the vesicle fluid would typically show large, Gram-positive, non-motile, spore-forming bacilli in chains (Bamboo-stick appearance). **2. Why the other options are incorrect:** * **Actinomycetoma:** Presents as a chronic, localized, swelling with multiple discharging sinuses and "sulfur granules." It is usually painful in later stages and lacks the acute black eschar. * **Clostridium infection (Gas Gangrene):** While it involves trauma, it is characterized by **excruciating pain**, crepitus (gas in tissues), and systemic toxemia, unlike the painless lesion of anthrax. * **Extrapulmonary nocardiosis:** Usually presents as chronic abscesses or cellulitis in immunocompromised patients; it does not typically present as an acute black eschar with massive edema. **High-Yield Clinical Pearls for NEET-PG:** * **Virulence Factors:** Encapsulated (Poly-D-glutamic acid capsule) and produces Anthrax Toxin (Edema Factor, Lethal Factor, and Protective Antigen). * **McFadyean’s Reaction:** Used for presumptive identification (polychrome methylene blue staining shows purple capsules). * **Medusa Head Appearance:** Characteristic colony morphology on nutrient agar. * **String of Pearls Reaction:** Seen on agar containing penicillin. * **Treatment:** Ciprofloxacin or Doxycycline are the first-line agents.
Question 545: Pseudo-hemoptysis is seen mostly with which of the following microorganisms?
- A. Streptococcus
- B. E. coli
- C. Serratia Marcescens (Correct Answer)
- D. R.S.V
Explanation: **Explanation:** **Serratia marcescens** is the correct answer because certain strains of this Gram-negative bacillus produce a characteristic red pigment called **prodigiosin**. When these bacteria colonize the respiratory tract or contaminate sputum samples, the red pigment can discolor the phlegm, mimicking the appearance of blood. This phenomenon is clinically termed **"Pseudo-hemoptysis"** because, while the sputum appears red, it does not contain actual red blood cells. **Analysis of Incorrect Options:** * **Streptococcus:** While *S. pneumoniae* causes "rusty sputum" due to the breakdown of hemoglobin, it is considered true hemoptysis/blood-tinged sputum, not pseudo-hemoptysis caused by bacterial pigment. * **E. coli:** This is a common coliform that does not produce red pigments; it typically causes UTIs or neonatal meningitis and is not associated with pseudo-hemoptysis. * **R.S.V (Respiratory Syncytial Virus):** This virus causes bronchiolitis in infants. While it causes significant respiratory distress, it does not produce pigments that mimic blood. **High-Yield NEET-PG Pearls for Serratia marcescens:** 1. **Pigment:** Prodigiosin (red/pink pigment), typically produced at room temperature. 2. **Clinical Significance:** Often associated with nosocomial infections (UTIs, pneumonia, and sepsis), especially in patients with indwelling catheters. 3. **Historical Fact:** It was once used in biological warfare simulations (Operation Sea-Spray) because its red pigment made it easy to track. 4. **Red Diaper Syndrome:** It can also cause a red discoloration of diapers in infants, which can be mistaken for hematuria.
Question 546: Which among the following organisms is found as a commensal on the skin?
- A. Staphylococcus aureus (Correct Answer)
- B. Streptococcus pyogenes
- C. Clostridium
- D. Borrelia
Explanation: **Explanation:** The human skin serves as a primary habitat for various microorganisms, collectively known as the **normal flora or commensals**. These organisms reside on the skin surface without causing disease under normal conditions. **Staphylococcus aureus (Option A)** is a well-known commensal, primarily colonizing the **anterior nares** (most common site), but it is also frequently found on the skin, particularly in moist areas like the axilla and perineum. While *Staphylococcus epidermidis* is the most ubiquitous skin commensal, *S. aureus* is carried by approximately 20-30% of the healthy population as part of their transient or resident flora. **Analysis of Incorrect Options:** * **Streptococcus pyogenes (Group A Strep):** It is considered a primary pathogen, not a commensal. Its presence on the skin usually indicates an impending or active infection (e.g., impetigo or cellulitis). * **Clostridium:** These are anaerobic, spore-forming bacilli typically found in the **soil** or the **human gastrointestinal tract** (e.g., *C. difficile*). They are not part of the normal skin flora. * **Borrelia:** These are spirochetes transmitted via arthropod vectors (ticks/lice). They are strictly pathogenic and cause diseases like Lyme disease or Relapsing fever; they never exist as commensals. **High-Yield Clinical Pearls for NEET-PG:** * **Most common skin commensal:** *Staphylococcus epidermidis* (Coagulase-negative Staph). * **Nasal Carriage:** *S. aureus* colonization in the nostrils is a major risk factor for surgical site infections. * **Resident vs. Transient Flora:** Resident flora (e.g., *Corynebacterium*, *Propionibacterium acnes*) are permanent inhabitants, while transient flora (*S. aureus*) are lost and regained periodically. * **Hand Hygiene:** The primary goal of handwashing is to remove "transient flora" to prevent cross-contamination in clinical settings.
Question 547: Shigellosis is best diagnosed by?
- A. Stool examination
- B. Stool culture (Correct Answer)
- C. Sigmoidoscopy
- D. Enzyme immunoassay
Explanation: **Explanation:** **1. Why Stool Culture is the Correct Answer:** Stool culture remains the **gold standard** and the most definitive method for diagnosing Shigellosis. *Shigella* species are highly infectious, requiring as few as 10–100 organisms to cause disease. Culture allows for the definitive identification of the species (*S. dysenteriae, S. flexneri, S. boydii, or S. sonnei*) and, crucially, enables **Antimicrobial Susceptibility Testing (AST)**, which is vital due to increasing multidrug resistance. For optimal yield, the specimen should be collected early in the illness and processed immediately using selective media like **Deoxycholate Citrate Agar (DCA)** or **Xylose Lysine Deoxycholate (XLD) agar**. **2. Why Other Options are Incorrect:** * **Stool Examination:** While microscopy reveals "inflammatory diarrhea" (numerous pus cells/PMNs and RBCs), it cannot differentiate *Shigella* from other invasive pathogens like *EIEC* or *Campylobacter*. * **Sigmoidoscopy:** This is an invasive procedure. While it may show characteristic "punctate ulcers" and friable mucosa, it is not the primary diagnostic tool and is usually reserved for atypical cases or to rule out ulcerative colitis. * **Enzyme Immunoassay (EIA):** While rapid tests for Shiga toxin exist, they are less sensitive than culture for routine diagnosis and do not provide the necessary sensitivity patterns for treatment. **NEET-PG High-Yield Pearls:** * **Specimen of Choice:** Fresh stool is preferred; if a delay is expected, use **Sachs' buffered glycerol saline** as a transport medium (avoiding acidic environments). * **Biochemical Profile:** *Shigella* are Gram-negative, non-motile, non-capsulated, and **non-lactose fermenters (NLF)**. * **Catalase Test:** All *Shigella* are catalase-positive **except** *S. dysenteriae* type 1. * **Clinical Presentation:** Characterized by "Bacillary Dysentery" (frequent, small-volume stools with blood and mucus) and significant tenesmus.
Question 548: Which of the following statements about Gram-positive cocci is false?
- A. Staphylococcus saprophyticus causes urinary tract infections in females.
- B. Most enterococci are resistant to penicillin. (Correct Answer)
- C. Non-pathogenic strains are coagulase-negative.
- D. Neonatal meningitis-causing streptococci hydrolyze hippurate.
Explanation: ### Explanation **1. Why Option B is the Correct Answer (The False Statement):** While Enterococci (like *E. faecalis* and *E. faecium*) have a higher minimum inhibitory concentration (MIC) for penicillin compared to Streptococci, they are **not inherently resistant** to penicillin. Most strains of *E. faecalis* remain susceptible to penicillin and ampicillin. Resistance is more common in *E. faecium*, but as a genus, the statement that "most" are resistant is clinically inaccurate. Enterococci are, however, intrinsically resistant to cephalosporins. **2. Analysis of Incorrect Options (True Statements):** * **Option A:** *Staphylococcus saprophyticus* is a common cause of "honeymoon cystitis" (UTIs) in young, sexually active females. It is identified by its resistance to **Novobiocin**. * **Option C:** The **Coagulase test** is the primary tool used to differentiate *Staphylococcus aureus* (pathogenic/coagulase-positive) from Coagulase-Negative Staphylococci (CoNS), which are generally considered commensals or opportunistic pathogens. * **Option D:** Group B Streptococcus (*S. agalactiae*), a leading cause of neonatal meningitis and sepsis, is characterized by its ability to **hydrolyze hippurate** and a positive **CAMP test**. **3. NEET-PG High-Yield Pearls:** * **Enterococci:** Always remember they are **catalase-negative**, grow in **6.5% NaCl**, and show **Bile Esculin hydrolysis**. * **Treatment:** For serious Enterococcal infections (like endocarditis), a combination of a cell-wall active agent (Penicillin/Ampicillin) plus an Aminoglycoside (Gentamicin) is used for synergy. * **VRE:** Vancomycin-Resistant Enterococci (especially *E. faecium*) are a major hospital-acquired concern; Linezolid or Quinupristin-Dalfopristin are the drugs of choice. * **Hippurate Hydrolysis Positive:** *S. agalactiae*, *Campylobacter jejuni*, and *Gardnerella vaginalis*.
Question 549: A 5-year-old child presents with a pseudomembrane in the throat, fever, and sore throat. Which organism is likely responsible?
- A. Comma-shaped Gram-negative bacilli
- B. Club-shaped Gram-positive bacilli (Correct Answer)
- C. Spore-forming Gram-positive bacilli
- D. Filamentous Gram-positive bacilli
Explanation: ### Explanation The clinical presentation of a **pseudomembrane** in the throat (a thick, gray-white adherent layer), fever, and sore throat in a child is a classic hallmark of **Diphtheria**, caused by ***Corynebacterium diphtheriae***. **1. Why the Correct Answer is Right:** *Corynebacterium diphtheriae* are characterized morphologically as **Club-shaped Gram-positive bacilli**. They often arrange themselves in "Chinese letter" or "cuneiform" patterns (V or L shapes) due to incomplete separation during cell division (snapping division). They are non-motile, non-sporing, and non-capsulated. **2. Analysis of Incorrect Options:** * **Option A (Comma-shaped Gram-negative bacilli):** This describes ***Vibrio cholerae***, which causes profuse watery diarrhea ("rice-water stools"), not respiratory symptoms. * **Option C (Spore-forming Gram-positive bacilli):** This describes species like ***Bacillus*** (e.g., *B. anthracis*) or ***Clostridium*** (e.g., *C. tetani*). While *B. anthracis* is a bacillus, it does not typically present with a pharyngeal pseudomembrane. * **Option D (Filamentous Gram-positive bacilli):** This describes ***Actinomyces*** or ***Nocardia***. These organisms cause chronic granulomatous infections (like lumpy jaw or mycetoma) rather than acute pharyngitis. **3. NEET-PG High-Yield Pearls:** * **Virulence Factor:** Diphtheria toxin (an AB toxin) inhibited protein synthesis by ADP-ribosylation of **EF-2**. * **Special Stains:** **Albert’s stain**, Neisser’s stain, or Ponder’s stain are used to visualize **metachromatic granules** (Volutin/Babes-Ernst granules). * **Culture Media:** **Loeffler’s Serum Slope** (rapid growth) and **Potassium Tellurite Agar** (black colonies). * **Toxin Detection:** **Elek’s Gel Precipitation Test** is the gold standard for detecting toxigenicity. * **Complications:** Myocarditis (most common cause of death) and neurological palsies (e.g., palatal palsy).
Question 550: Corynebacterium diphtheriae is described as:
- A. Gram-positive cocci
- B. Gram-positive bacilli (Correct Answer)
- C. Gram-negative cocci
- D. Gram-negative bacilli
Explanation: ### Explanation **Correct Answer: B. Gram-positive bacilli** **1. Understanding the Correct Answer:** *Corynebacterium diphtheriae* is a classic example of a **Gram-positive, non-acid-fast, non-motile bacillus** (rod-shaped). Under the microscope, these bacteria exhibit a characteristic **pleomorphic** appearance, often described as "club-shaped" (wider at one end). A key diagnostic feature is their arrangement in **Chinese letter patterns** or cuneiform arrangements, which occurs due to incomplete separation during binary fission (snapping division). **2. Why the Other Options are Incorrect:** * **Option A (Gram-positive cocci):** This category includes organisms like *Staphylococcus* and *Streptococcus*, which appear as spherical clusters or chains, unlike the rod-shaped *C. diphtheriae*. * **Option C (Gram-negative cocci):** This group primarily includes *Neisseria* species (e.g., *N. meningitidis*). They have a different cell wall structure that stains pink/red. * **Option D (Gram-negative bacilli):** This large group includes Enterobacteriaceae (like *E. coli*). *C. diphtheriae* retains the crystal violet stain due to its thick peptidoglycan layer, making it Gram-positive (purple). **3. High-Yield Clinical Pearls for NEET-PG:** * **Metachromatic Granules:** Also known as **Volutin or Babes-Ernst granules**. These are polymerized polyphosphates that stain reddish-purple with **Albert’s stain**. * **Culture Media:** The gold standard is **Loeffler’s Serum Slope** (rapid growth) and **Potassium Tellurite Agar** (selective media where colonies appear grey-black). * **Pathogenesis:** The disease is caused by a powerful **exotoxin** (encoded by the *tox* gene via a beta-phage) which inhibits protein synthesis by inactivating **EF-2**. * **Clinical Hallmark:** Presence of a tough, leathery **pseudo-membrane** on the tonsils/pharynx; attempts to remove it result in bleeding.
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Staphylococci
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Streptococci and Enterococci
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Neisseria and Moraxella
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Corynebacterium and Listeria
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Bacillus and Clostridium
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Enterobacteriaceae
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Vibrio, Aeromonas, and Plesiomonas
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Pseudomonas and Related Bacteria
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Haemophilus and HACEK Group
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Bordetella and Brucella
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Mycobacteria
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Spirochetes
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