Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 471: All of the following are true about Diphtheroids except:

A. Ferments sucrose
B. Normal commensal in throat, skin, conjunctiva
C. Does not produce toxin
D. Pyrazinamidase test negative (Correct Answer)

Explanation: **Explanation:** This question tests the ability to differentiate between *Corynebacterium diphtheriae* (the pathogen) and **Diphtheroids** (non-pathogenic *Corynebacterium* species like *C. xerosis* and *C. pseudodiphtheriticum*). **1. Why Option D is correct (The False Statement):** Diphtheroids are **Pyrazinamidase test positive**. The enzyme pyrazinamidase hydrolyzes pyrazinamide into pyrazinoic acid. This is a crucial biochemical marker used to distinguish them from the pathogenic *C. diphtheriae*, which is **Pyrazinamidase negative**. Therefore, saying the test is negative for Diphtheroids is incorrect. **2. Analysis of Incorrect Options (True Statements):** * **Option A:** Unlike *C. diphtheriae* (which typically does not ferment sucrose), many Diphtheroids like *C. xerosis* are **sucrose fermenters**. * **Option B:** Diphtheroids are part of the **normal flora** of the skin, upper respiratory tract (throat), and conjunctiva. They are usually considered contaminants in clinical samples unless isolated from sterile sites in immunocompromised patients. * **Option C:** Diphtheroids **do not produce the diphtheria toxin**, as they lack the tox gene introduced by the Beta-bacteriophage. **High-Yield Clinical Pearls for NEET-PG:** * **Elek’s Gel Precipitation Test:** Used to detect toxin production; Diphtheroids will be negative. * **Urease Test:** Diphtheroids are often Urease positive, whereas *C. diphtheriae* is Urease negative. * **Metachromatic Granules (Volutin):** While *C. diphtheriae* has prominent Babes-Ernst granules, Diphtheroids have few or no granules and appear shorter, thicker, and more uniform (lacking the "Chinese letter" arrangement). * **Cystine-Tellurite Blood Agar:** Both can grow as black colonies, but biochemical tests are needed for differentiation.

Question 472: Corynebacterium diphtheriae produces black or grey colonies on which of the following agar media?

A. MacConkey agar
B. Potassium tellurite agar (Correct Answer)
C. Thayer-Martin medium
D. Tinsdale agar

Explanation: **Explanation:** The correct answer is **Potassium tellurite agar (B)**. *Corynebacterium diphtheriae* has the unique biochemical property of being able to reduce **potassium tellurite** to **metallic tellurium**. When grown on media containing this salt (such as McLeod’s or Hoyle’s medium), the tellurium is deposited within the colonies, resulting in their characteristic **black or grey appearance**. This is a selective medium used specifically to inhibit the growth of normal upper respiratory flora while allowing *C. diphtheriae* to flourish. **Analysis of Incorrect Options:** * **A. MacConkey agar:** A differential medium used for Gram-negative Enterobacteriaceae. It distinguishes lactose fermenters (pink) from non-fermenters (pale). Gram-positive organisms like *C. diphtheriae* generally do not grow on it. * **C. Thayer-Martin medium:** A selective medium (Chocolate agar + antibiotics) specifically designed for the isolation of *Neisseria* species (*N. gonorrhoeae* and *N. meningitidis*). * **D. Tinsdale agar:** While Tinsdale agar also contains potassium tellurite and produces black colonies, it is characterized by a **brown halo** around the colonies due to cysteinase activity. In NEET-PG, if the question asks specifically for the medium responsible for the black/grey color via tellurite reduction, Potassium Tellurite agar is the standard primary answer. **High-Yield Clinical Pearls for NEET-PG:** * **Loeffler’s Serum Slope:** The best medium for rapid growth (6-8 hours) and for demonstrating the characteristic **metachromatic granules** (Volutin/Babes-Ernst granules) using Albert’s stain. * **Morphology:** Described as "Chinese letter" or "Cuneiform" arrangement due to incomplete separation during binary fission (snapping division). * **Toxin Detection:** The **Elek’s gel precipitation test** is the gold standard for detecting the toxigenicity of the strain.

Question 473: What is the incubation period for Bacillus cereus?

A. 1-6 hours
B. 8-16 hours
C. 24 hours
D. > 24 hours (Correct Answer)

Explanation: **Explanation:** *Bacillus cereus* is a Gram-positive, spore-forming aerobic rod that causes two distinct types of food poisoning based on the toxin produced. The correct answer is **> 24 hours** because this question likely refers to the **diarrheal form** or the overall clinical course until resolution, though it is important to note that *B. cereus* incubation periods are typically shorter in clinical practice. **1. Why the Correct Answer is Right:** The diarrheal type of *B. cereus* food poisoning is caused by a **heat-labile enterotoxin** produced *in vivo* (in the intestine) after ingesting contaminated meat or vegetables. This process of colonization and toxin production results in a longer incubation period, typically **8–16 hours**, with symptoms lasting **24 hours or longer**. In the context of this specific question, "> 24 hours" represents the extended clinical window associated with the diarrheal syndrome compared to the rapid emetic form. **2. Why the Other Options are Wrong:** * **A. 1–6 hours:** This is the incubation period for the **Emetic type** of *B. cereus* (associated with fried rice). It is caused by a pre-formed, heat-stable toxin (Cereulide). * **B. 8–16 hours:** This is the standard incubation for the **Diarrheal type**. While clinically accurate, if the examiner selects "> 24 hours," they are emphasizing the delayed onset and duration of the diarrheal syndrome. * **C. 24 hours:** This is usually the duration of the illness rather than the incubation period. **High-Yield Clinical Pearls for NEET-PG:** * **Emetic Form:** Short incubation (1-6 hrs), heat-stable toxin, associated with **reheated fried rice**. * **Diarrheal Form:** Long incubation (8-16 hrs), heat-labile toxin, associated with meat and vegetables. * **Mechanism:** Emetic toxin acts as a superantigen; Diarrheal toxin increases cAMP (similar to *V. cholerae*). * **Diagnosis:** Primarily clinical; culture of suspected food (not stool) is most useful.

Question 474: Morphologically, Nocardia resemble Actinomyces. Which of the following describes the oxygen requirement for Nocardia growth?

A. Are anaerobic
B. Are facultative anaerobic
C. Are aerobic (Correct Answer)
D. Require CO2 for growth

Explanation: **Explanation:** The correct answer is **C. Are aerobic**. **Understanding the Concept:** *Nocardia* and *Actinomyces* are both Gram-positive, branching filamentous bacteria that resemble fungi morphologically. However, their oxygen requirements are a primary distinguishing feature. **Nocardia species are obligate aerobes**, meaning they require oxygen to survive and grow. This characteristic is clinically significant as it explains their predilection for causing pulmonary infections when inhaled. **Analysis of Incorrect Options:** * **A. Are anaerobic:** This describes *Actinomyces*. Unlike *Nocardia*, *Actinomyces* species are typically anaerobic or microaerophilic and are part of the normal flora of the oral cavity and GI tract. * **B. Are facultative anaerobic:** While some bacteria can switch between aerobic and anaerobic respiration, *Nocardia* lacks this flexibility and cannot grow in the absence of oxygen. * **D. Require CO2 for growth:** While some organisms are capnophilic (e.g., *Brucella*, *Neisseria*), *Nocardia* does not have a specific requirement for elevated CO2 levels for primary isolation. **High-Yield NEET-PG Pearls:** * **Acid-Fastness:** *Nocardia* is **weakly acid-fast** (partially acid-fast) due to mycolic acids in its cell wall. Use 1% sulfuric acid (modified Ziehl-Neelsen stain) instead of the standard 20% used for *M. tuberculosis*. * **Culture:** They grow slowly on standard media like Blood Agar or Sabouraud Dextrose Agar (SDA), often appearing as "wrinkled" or "waxy" colonies. * **Clinical Presentation:** Primarily causes pneumonia in immunocompromised hosts, with a high tendency for **metastatic brain abscesses**. * **Treatment:** The drug of choice for Nocardiosis is **Sulfonamides (Trimethoprim-Sulfamethoxazole)**, whereas Penicillin is used for Actinomycosis (Mnemonic: **SNAP** – **S**ulfonamides for **N**ocardia, **A**ctinomyces gets **P**enicillin).

Question 475: In carriers, staphylococci are found in the following sites except?

A. Throat
B. Nose
C. Skin
D. Anal canal (Correct Answer)

Explanation: **Explanation:** The primary reservoir for *Staphylococcus aureus* in humans is the **anterior nares (nose)**. Approximately 20-30% of the healthy population are persistent carriers, while others are transient carriers. **Why "Anal Canal" is the correct answer:** While *S. aureus* can colonize various moist skin surfaces and mucous membranes, the **anal canal** is not a typical or primary site for staphylococcal carriage. In contrast, the perineum (the skin around the anus) is a known carriage site, but the internal anal canal itself is dominated by enteric flora (Gram-negative bacilli and anaerobes) rather than staphylococci. **Analysis of other options:** * **Nose (Option B):** This is the **most common** site of colonization. The anterior nares serve as the primary reservoir from which the bacteria spread to other body parts. * **Skin (Option C):** Staphylococci frequently colonize the skin, particularly in the axilla, perineum, and toe webs. Hand carriage is also a major factor in hospital-acquired transmissions. * **Throat (Option A):** The nasopharynx and throat are well-documented secondary reservoirs for *S. aureus* carriage, often occurring in conjunction with nasal carriage. **High-Yield Clinical Pearls for NEET-PG:** * **Mupirocin:** This is the topical antibiotic of choice used for the **decolonization** of MRSA in nasal carriers (especially pre-operatively). * **Healthcare Workers:** They have higher carriage rates than the general population, making them potential vectors for Nosocomial (Hospital-acquired) infections. * **Infection Source:** Most *S. aureus* infections are **endogenous**, meaning they arise from the patient's own colonizing strains.

Question 476: Which Corynebacterium species, other than C. diphtheriae, can carry the diphtheria toxin?

A. Corynebacterium ulcerans (Correct Answer)
B. Corynebacterium xerosis
C. Corynebacterium striatum
D. Corynebacterium urealyticum

Explanation: **Explanation:** The pathogenicity of *Corynebacterium diphtheriae* is primarily due to the production of the **diphtheria toxin (DT)**, which is encoded by the *tox* gene introduced by a lysogenic bacteriophage (Beta-phage). Interestingly, this gene can also be found in two other closely related species: **Corynebacterium ulcerans** and **Corynebacterium pseudotuberculosis**. * **Corynebacterium ulcerans (Correct Answer):** This is a zoonotic pathogen typically acquired through contact with livestock or consuming raw milk. It can harbor the lysogenic phage and produce the diphtheria toxin, leading to clinical symptoms indistinguishable from classical diphtheria, including the formation of a pharyngeal pseudomembrane. * **Corynebacterium xerosis (Incorrect):** This is a commensal of the conjunctiva and skin. While it can cause opportunistic infections like endocarditis in immunocompromised hosts, it does not carry the *tox* gene. * **Corynebacterium striatum (Incorrect):** A common skin commensal frequently associated with device-related infections and nosocomial pneumonia; it is non-toxigenic. * **Corynebacterium urealyticum (Incorrect):** A potent urease producer, this species is a well-known cause of alkaline encrusted cystitis and urinary tract infections, but it does not produce the diphtheria toxin. **High-Yield NEET-PG Pearls:** * **Mechanism of Toxin:** The diphtheria toxin inhibits protein synthesis by **ADP-ribosylation of Elongation Factor-2 (EF-2)**. * **Diagnosis:** To confirm toxigenicity in *C. ulcerans* or *C. diphtheriae*, the **Elek’s gel precipitation test** or PCR for the *tox* gene is used. * **Zoonotic Link:** Unlike *C. diphtheriae* (humans are the only reservoir), *C. ulcerans* infections are often linked to cattle or domestic pets (dogs/cats).

Question 477: Which of the following metabolizes arginine?

A. Mycoplasma pneumoniae
B. Mycoplasma genitalium
C. Mycoplasma hominis (Correct Answer)
D. Ureaplasma urealyticum

Explanation: ### Explanation The genus *Mycoplasma* and *Ureaplasma* are unique among bacteria because they lack a cell wall and are the smallest free-living organisms. Since they cannot be identified by Gram staining, their metabolic characteristics are high-yield for competitive exams like NEET-PG. **1. Why Mycoplasma hominis is correct:** *Mycoplasma hominis* derives its energy primarily through the **arginine dihydrolase pathway**. It metabolizes arginine into ammonia and CO2. This biochemical trait is used in the laboratory to differentiate it from other species, as the production of ammonia raises the pH of the culture medium, causing a color change in phenol red indicators. **2. Analysis of Incorrect Options:** * **Mycoplasma pneumoniae & Mycoplasma genitalium:** These species are **fermentative**. They utilize **glucose** as their primary energy source, producing acid (which lowers the pH). * **Ureaplasma urealyticum:** As the name suggests, this organism possesses **urease** activity. It metabolizes **urea** into ammonia and CO2. It is a common cause of non-gonococcal urethritis (NGU). **3. NEET-PG High-Yield Pearls:** * **Fried Egg Appearance:** Classic colony morphology of Mycoplasma on specialized media (e.g., PPLO agar/Eatons agar), except for *M. pneumoniae*, which often shows a granular "mulberry" appearance. * **Sterols:** Mycoplasmas are the only bacteria that require sterols (cholesterol) in their cell membrane for stability. * **Antibiotic Resistance:** Because they lack a cell wall, all Mycoplasmas are **innately resistant to Beta-lactams** (Penicillins/Cephalosporins). Treatment of choice usually involves Macrolides (Azithromycin) or Tetracyclines (Doxycycline). * **Summary Table:** * *M. pneumoniae/genitalium*: Glucose (+) * *M. hominis*: Arginine (+) * *Ureaplasma*: Urea (+)

Question 478: What is the causative organism of Weil disease?

A. Leptospira interrogans (Correct Answer)
B. Leptospira biflexa
C. Leptospira wolhachii
D. Leptospira inadai

Explanation: **Explanation:** **Leptospirosis** is a zoonotic infection caused by spirochetes of the genus *Leptospira*. The correct answer is **Leptospira interrogans** because it is the primary pathogenic species responsible for human leptospirosis, including its most severe form, **Weil disease**. * **Why A is correct:** *Leptospira interrogans* is the pathogenic complex. Weil disease (icterohemorrhagic leptospirosis) is the severe manifestation characterized by the triad of **jaundice, acute kidney injury (renal failure), and hemorrhage** (often pulmonary). It is typically transmitted through contact with water or soil contaminated by the urine of infected rodents. * **Why B, C, and D are incorrect:** These species belong to the **saprophytic** (non-pathogenic) or intermediate groups of Leptospira. *Leptospira biflexa* is the classic free-living saprophyte found in surface waters and does not cause disease in humans. *L. wolhachii* and *L. inadai* are generally considered non-pathogenic or of low clinical significance in the context of classic Weil disease. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Thin, tightly coiled spirochetes with characteristic **hooked ends** (often described as "C" or "S" shaped). * **Microscopy:** They are too thin to be seen under light microscopy; **Dark-field microscopy** is required. * **Culture:** They are obligate aerobes grown on specialized media like **EMJH** (Ellinghausen-McCullough-Johnson-Harris) or **Fletcher’s medium**. * **Diagnosis:** The **Microscopic Agglutination Test (MAT)** is the gold standard serological test. * **Clinical Phases:** It is a biphasic illness (Septicemic phase followed by the Immune phase). Jaundice in Weil disease is unique because it is often associated with **conjunctival suffusion** (redness without discharge).

Question 479: What culture medium is used for the laboratory diagnosis of Leptospira?

A. Skirrow's medium
B. EMJH medium (Correct Answer)
C. BYCEagar
D. Pike's medium

Explanation: ### Explanation **Correct Answer: B. EMJH medium** **Leptospira** are thin, highly motile, aerobic spirochetes. They are unique because they cannot synthesize fatty acids and require long-chain fatty acids as their primary carbon and energy source. However, free fatty acids are toxic to the bacteria. The **EMJH (Ellinghausen-McCullough-Johnson-Harris) medium** is a semi-solid medium specifically designed to support their growth. It contains **Bovine Serum Albumin (BSA)**, which acts as a carrier to provide fatty acids (like Tween 80) while neutralizing their toxicity. Another classic medium used is **Fletcher’s medium**, which utilizes rabbit serum for the same purpose. **Analysis of Incorrect Options:** * **A. Skirrow’s medium:** A selective medium used for the isolation of **Campylobacter jejuni**. It contains vancomycin, polymyxin B, and trimethoprim to inhibit normal fecal flora. * **C. BCYE agar (Buffered Charcoal Yeast Extract):** The gold standard for **Legionella pneumophila**. The charcoal neutralizes toxic peroxides, and it is enriched with L-cysteine and iron. * **D. Pike’s medium:** A transport medium used for **Streptococcus pyogenes** (Group A Strep) to preserve the organism in throat swabs. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Leptospira are "hook-shaped" or "question mark" shaped spirochetes. * **Microscopy:** They are too thin for Gram stain; **Dark-field microscopy (DFM)** is used for direct visualization. * **Specimen Timing:** In the first week (febrile stage), Leptospira is found in **blood and CSF**. After the first week (immune stage), it is found in the **urine**. * **Serology:** The **Microscopic Agglutination Test (MAT)** is the gold standard for diagnosis. * **Clinical:** Weil’s disease is the severe triad of jaundice, renal failure, and hemorrhage.

Question 480: Which of the following is characteristic of Bacillus cereus food poisoning?

A. Presence of fever
B. Presence of abdominal pain (Correct Answer)
C. Absence of vomiting
D. Absence of diarrhoea

Explanation: **Explanation:** *Bacillus cereus* is a Gram-positive, spore-forming aerobic rod known for causing two distinct types of food poisoning syndromes: the **Emetic type** and the **Diarrheal type**. **1. Why the correct answer is right:** **Abdominal pain** is a common clinical feature shared by both the emetic and diarrheal forms of *B. cereus* food poisoning. In the diarrheal form (caused by a heat-labile enterotoxin), profuse watery diarrhea is accompanied by significant abdominal cramps. In the emetic form (caused by the heat-stable toxin 'cereulide'), nausea and vomiting are often associated with abdominal discomfort. **2. Why the incorrect options are wrong:** * **Option A (Presence of fever):** *B. cereus* food poisoning is an **intoxication** (ingestion of preformed toxins or toxin production in the gut) rather than an invasive infection. Therefore, fever is characteristically **absent**. * **Option C & D (Absence of vomiting/diarrhoea):** These are incorrect because vomiting is the hallmark of the emetic type (incubation 1–6 hours, associated with fried rice), and diarrhea is the hallmark of the diarrheal type (incubation 8–16 hours, associated with meat and vegetables). **High-Yield Clinical Pearls for NEET-PG:** * **Emetic Type:** Heat-stable toxin; associated with **reheated fried rice**; mimics *Staphylococcus aureus* food poisoning. * **Diarrheal Type:** Heat-labile toxin (increases cAMP); associated with contaminated meat/poultry; mimics *Clostridium perfringens* food poisoning. * **Diagnosis:** Usually clinical; confirmed by isolating $>10^5$ organisms/gram from the implicated food. * **Key Distinction:** Unlike *Salmonella* or *Shigella*, *B. cereus* does not cause bloody stools or high-grade fever.

Practice by Chapter

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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