Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 671: Lyme's disease is caused by which bacterium?

A. Borrelia parkeri
B. Borrelia burgdorferi (Correct Answer)
C. Borrelia recurrentis
D. Borrelia hermsi

Explanation: **Explanation:** **Lyme disease** is a multisystem inflammatory disease caused by the spirochete **Borrelia burgdorferi**. It is primarily transmitted to humans through the bite of infected **Ixodes ticks** (hard ticks). The disease typically manifests in three stages: early localized (characterized by the pathognomonic *Erythema chronicum migrans* or bull’s-eye rash), early disseminated (neurological and cardiac involvement), and late disseminated (chronic arthritis). **Analysis of Options:** * **Borrelia burgdorferi (Correct):** This is the primary causative agent of Lyme disease in North America and Europe. Other related species like *B. garinii* and *B. afzelii* also cause the disease in Eurasia. * **Borrelia recurrentis:** This species causes **Epidemic Relapsing Fever**, which is transmitted by the **human body louse** (*Pediculus humanus corporis*). * **Borrelia hermsi & Borrelia parkeri:** These species are causative agents of **Endemic Relapsing Fever**, which is transmitted by **soft ticks** of the genus *Ornithodoros*. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** *Ixodes scapularis* (Deer tick). The tick must usually be attached for 36–48 hours to transmit the bacteria. * **Reservoir:** White-footed mouse (larval/nymph stage) and White-tailed deer (adult stage). * **Diagnosis:** Screening is done via **ELISA**, and confirmation is required via **Western Blot**. * **Treatment:** **Doxycycline** is the drug of choice for early stages. For neurological or cardiac manifestations, **Ceftriaxone** (IV) is preferred. In children <8 years or pregnant women, Amoxicillin is used.

Question 672: All of the following statements about El-Tor vibrios are true, except?

A. Humans are the only reservoir
B. Can survive in ice-cold water for 2-4 weeks
C. Killed by boiling for 30 seconds
D. Enterotoxin can have direct effects on other tissues besides intestinal epithelial cells (Correct Answer)

Explanation: **Explanation:** The question asks for the **incorrect** statement regarding *Vibrio cholerae* biotype El Tor. **1. Why Option D is the Correct Answer (The False Statement):** The cholera enterotoxin (Choleragen) is a classic **A-B subunit toxin**. Its mechanism is highly specific: the B-subunit binds to **GM1 ganglioside receptors**, which are primarily expressed on the surface of **intestinal epithelial cells (enterocytes)**. Once the A-subunit enters the cell, it causes permanent activation of adenylate cyclase, leading to increased cAMP and massive electrolyte/water secretion. The toxin does **not** have direct systemic effects on other tissues; the systemic manifestations of cholera (hypovolemic shock, renal failure) are secondary to profound dehydration and electrolyte loss, not direct toxemia. **2. Analysis of Incorrect Options (True Statements):** * **Option A:** Humans are indeed the only known natural reservoir for *V. cholerae*. There is no significant animal reservoir, though they can persist in aquatic environments associated with plankton. * **Option B:** El Tor vibrios are hardier than the Classical biotype. They are highly resistant to environmental stress and can survive in ice-cold water for several weeks, facilitating water-borne transmission. * **Option C:** Like most vegetative bacteria, *Vibrio cholerae* is heat-labile. Boiling water for even a few seconds is sufficient to kill the organism, making it a key preventive measure. **Clinical Pearls for NEET-PG:** * **El Tor vs. Classical:** El Tor is characterized by being **VP test positive**, **Hemolysis positive**, and **Polymyxin B resistant** (Classical is the opposite). * **Current Pandemic:** The ongoing 7th pandemic is caused by the El Tor biotype. * **Gold Standard Diagnosis:** Stool culture on **TCBS agar** (Thiosulfate Citrate Bile Salts Sucrose), where it forms large yellow colonies. * **Treatment:** The mainstay is aggressive rehydration (ORS/IV fluids). Doxycycline is the drug of choice to reduce shedding.

Question 673: An outbreak of streptococcal pharyngitis has occurred in a remote village. To perform epidemiological investigations, it is necessary to culture throat swabs from patients suffering from the disease. What is the transport medium of choice for this purpose?

A. Salt mannitol agar
B. Pike's medium (Correct Answer)
C. Stuart's medium
D. Cary Blair medium

Explanation: **Explanation:** The correct answer is **Pike’s medium**. **1. Why Pike’s Medium is Correct:** *Streptococcus pyogenes* (Group A Streptococcus) is the primary causative agent of bacterial pharyngitis. When transporting throat swabs, it is crucial to prevent the overgrowth of normal oral flora (like commensal Neisseria or Staphylococci) which can mask the presence of Streptococci. **Pike’s medium** is a selective enrichment transport medium containing blood agar supplemented with **crystal violet and sodium azide**. These additives inhibit the growth of Gram-negative bacteria and Staphylococci, respectively, while preserving the viability of *S. pyogenes* for up to several days. **2. Why Other Options are Incorrect:** * **A. Salt Mannitol Agar:** This is a selective and differential medium used for the isolation of *Staphylococcus aureus*, not a transport medium for Streptococci. * **C. Stuart’s Medium:** This is a universal transport medium used for various pathogens (like *Neisseria gonorrhoeae*). While it can be used for Streptococci, Pike’s is the specific "medium of choice" for epidemiological studies of Streptococcal pharyngitis due to its selective properties. * **D. Cary Blair Medium:** This is the transport medium of choice for **fecal samples** (enteric pathogens like *Vibrio cholerae*, *Salmonella*, and *Shigella*). **3. High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Diagnosis:** Throat culture on Blood Agar (shows Beta-hemolysis). * **ASO Titer:** Used for diagnosing non-suppurative complications (Rheumatic fever), not acute pharyngitis. * **Anti-DNase B:** The most sensitive test for diagnosing post-streptococcal glomerulonephritis (PSGN) following skin infections (impetigo). * **Transport Media Summary:** * *V. cholerae*: VR Medium or Cary Blair. * *N. gonorrhoeae*: Stuart’s or Amies medium. * *Bordetella pertussis*: Regan-Lowe medium.

Question 674: What is the most common organism causing cerebrospinal fluid shunt-related infections?

A. Streptococcus agalactiae
B. E. coli
C. Coagulase-negative Staphylococcus aureus (Correct Answer)
D. Enterococcus

Explanation: **Explanation:** The correct answer is **Coagulase-negative Staphylococcus aureus (CoNS)**, specifically **Staphylococcus epidermidis**. **Why CoNS is the correct answer:** Cerebrospinal fluid (CSF) shunt infections are primarily caused by skin flora introduced during the surgical procedure or via retrograde migration along the shunt tract. *Staphylococcus epidermidis* (a CoNS) is the most common culprit because of its unique ability to produce an **extracellular polysaccharide biofilm (slime layer)**. This biofilm allows the bacteria to adhere firmly to the synthetic material of the shunt (silicone/plastic) and protects them from both the host’s immune response and systemic antibiotics. **Analysis of Incorrect Options:** * **A. Streptococcus agalactiae (GBS):** This is a leading cause of neonatal meningitis (acquired during birth), but it is not typically associated with prosthetic device infections. * **B. E. coli:** While a common cause of neonatal meningitis and UTIs, it only accounts for a small percentage of shunt infections, usually in the immediate postoperative period or via hematogenous spread. * **C. Enterococcus:** These are enteric organisms. While they can cause healthcare-associated infections, they are far less common than skin commensals in the context of neurosurgical hardware. **Clinical Pearls for NEET-PG:** * **Most common organism:** *Staphylococcus epidermidis* (>50% of cases). * **Second most common:** *Staphylococcus aureus*. * **Pathogenesis:** Biofilm formation is the key virulence factor. * **Management:** Successful treatment usually requires **removal of the infected shunt** in addition to targeted intravenous antibiotics (often Vancomycin due to high rates of methicillin resistance in CoNS). * **Timing:** Most shunt infections occur within 6 months of placement or revision.

Question 675: Which Streptococcus toxin is responsible for connective tissue breakdown?

A. Hyaluronidase (Correct Answer)
B. Streptolysin O
C. Streptolysin S
D. Streptococcus pyogenic exotoxin

Explanation: **Explanation:** **Correct Answer: A. Hyaluronidase** Hyaluronidase is an enzyme secreted by *Streptococcus pyogenes* (Group A Strep) that degrades **hyaluronic acid**, a critical component of the ground substance in mammalian connective tissue. By breaking down this "intercellular cement," the enzyme facilitates the rapid lateral spread of the bacteria through tissue planes. This is why it is historically referred to as the **"Dreading Factor."** **Incorrect Options:** * **B. Streptolysin O:** This is an oxygen-labile, highly antigenic hemolysin that creates pores in cell membranes, leading to the lysis of RBCs, WBCs, and platelets. It is the basis for the ASO (Antistreptolysin O) titer. * **C. Streptolysin S:** This is an oxygen-stable, non-antigenic hemolysin responsible for the zone of beta-hemolysis seen on blood agar plates. * **D. Streptococcus pyogenic exotoxin (Spe):** Also known as erythrogenic toxins, these act as **superantigens**. They are responsible for the clinical manifestations of Scarlet Fever (rash) and Streptococcal Toxic Shock Syndrome (STSS). **High-Yield NEET-PG Pearls:** * **Spreading Factors:** Other streptococcal enzymes that aid spread include **Streptokinase** (fibrinolysin) which dissolves clots and **DNase (Streptodornase)** which liquefies viscous pus. * **Clinical Correlation:** The action of Hyaluronidase is a major reason why streptococcal skin infections like **cellulitis** are characterized by diffuse, spreading inflammation rather than localized abscesses (unlike *Staphylococcus aureus*). * **M Protein:** Remember that M protein is the chief virulence factor of *S. pyogenes*, primarily acting as an anti-phagocytic agent.

Question 676: Murine typhus is caused by which of the following pathogens?

A. Rickettsia prowazekii
B. Rickettsia conori
C. Rickettsia typhi (Correct Answer)
D. Coxiella burnetii

Explanation: **Explanation:** **Murine typhus** (also known as endemic typhus) is caused by **Rickettsia typhi**. It is primarily transmitted to humans via the **rat flea (*Xenopsylla cheopis*)**. The reservoir for this pathogen is the urban rat. Clinically, it presents with fever, headache, and a maculopapular rash that typically spreads from the trunk to the extremities (centrifugal spread). **Analysis of Options:** * **Rickettsia prowazekii (Option A):** Causes **Epidemic typhus**. It is transmitted by the **human body louse** (*Pediculus humanus corporis*). It is more severe than murine typhus and can recur years later as Brill-Zinsser disease. * **Rickettsia conori (Option B):** Causes **Boutonneuse fever** (Indian tick typhus/Mediterranean spotted fever). It is transmitted by the brown dog tick and is characterized by a pathognomonic black crust at the site of the bite called an **"eschar."** * **Coxiella burnetii (Option D):** Causes **Q fever**. Unlike other rickettsial diseases, it does not cause a rash, does not require an arthropod vector (transmitted via inhalation of aerosols), and is **Weil-Felix negative**. **High-Yield Clinical Pearls for NEET-PG:** * **Weil-Felix Test:** A heterophile agglutination test using *Proteus* antigens. *R. typhi* and *R. prowazekii* react with **OX-19**. * **Drug of Choice:** Doxycycline is the gold standard treatment for all rickettsial infections. * **Vector Mnemonic:** **E**pidemic = **L**ouse (EL), **E**ndemic (Murine) = **F**lea (EF).

Question 677: Which of the following is an Interferon-gamma release assay used to diagnose tuberculosis?

A. T-SPOT.TB (Correct Answer)
B. B-SPOT.TB
C. NIGRAs
D. C-SPOT.TB

Explanation: **Explanation:** **Interferon-Gamma Release Assays (IGRAs)** are modern *in vitro* blood tests used to identify *Mycobacterium tuberculosis* infection. They work on the principle that T-cells of an individual previously sensitized to TB antigens will produce **Interferon-gamma (IFN-γ)** when re-exposed to specific TB-defined antigens (ESAT-6 and CFP-10). **Why T-SPOT.TB is correct:** There are two commercially available IGRAs: 1. **QuantiFERON-TB Gold Plus (QFT-Plus):** Uses ELISA to measure the total concentration of IFN-γ in whole blood. 2. **T-SPOT.TB:** Uses the **ELISPOT** (Enzyme-Linked Immunospot) technique to count the actual number of individual effector T-cells secreting IFN-γ. It is highly sensitive and specific. **Why the other options are incorrect:** * **B-SPOT.TB and C-SPOT.TB:** These are distractors; no such standardized diagnostic tests exist for tuberculosis. * **NIGRAs:** This is a fabricated acronym. The correct acronym is **IGRA** (Interferon-Gamma Release Assay). **High-Yield Clinical Pearls for NEET-PG:** * **BCG Vaccination:** Unlike the Mantoux (Tuberculin Skin Test), IGRAs do **not** give false-positive results in individuals vaccinated with BCG, as the antigens used (ESAT-6/CFP-10) are absent in the *M. bovis* BCG strain. * **Latent vs. Active TB:** IGRAs **cannot** distinguish between Latent TB Infection (LTBI) and active Tuberculosis disease. * **Anergy:** IGRAs are preferred over skin tests in immunocompromised patients or those with malnutrition where skin reactivity may be suppressed.

Question 678: What is the most common causative organism for lobar pneumonia?

A. Staphylococcus aureus
B. Streptococcus pyogenes
C. Streptococcus pneumoniae (Correct Answer)
D. Haemophilus influenzae

Explanation: **Explanation:** **Streptococcus pneumoniae (Pneumococcus)** is the most common cause of community-acquired pneumonia (CAP) and specifically **lobar pneumonia** across all age groups. The underlying medical concept involves the organism's ability to reach the alveoli, where its polysaccharide capsule inhibits phagocytosis. This leads to an intense inflammatory exudate that spreads rapidly through the **Pores of Kohn**, resulting in the consolidation of an entire lobe (lobar distribution) rather than patchy involvement. **Analysis of Incorrect Options:** * **Staphylococcus aureus:** Typically causes **bronchopneumonia** (patchy consolidation), often following a viral prodrome like Influenza. It is notorious for causing complications like lung abscesses and pneumatoceles. * **Streptococcus pyogenes:** A rare cause of pneumonia; it more commonly causes pharyngitis, skin infections, and post-streptococcal sequelae (RHD/PSGN). * **Haemophilus influenzae:** A common cause of CAP, particularly in patients with underlying **COPD** or cystic fibrosis, but it usually presents as bronchopneumonia rather than classic lobar consolidation. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Presentation:** Sudden onset high-grade fever, productive cough with **"rusty sputum"** (due to altered blood in the alveolar exudate). * **Pathology Stages:** Congestion → Red Hepatization → Gray Hepatization → Resolution. * **Diagnosis:** Gram-positive lancet-shaped diplococci; **Quellung reaction** positive; Bile solubility positive; Optochin sensitivity positive. * **Vaccination:** PPSV23 (capsular polysaccharide) and PCV13 (conjugate) are used for prevention in high-risk groups.

Question 679: Which of the following statements about meningococcal meningitis is false?

A. The vaccine is not effective in children below 2 years of age.
B. The source of infection is mainly clinical cases. (Correct Answer)
C. The disease is more common in dry and cold months of the year.
D. Chemoprophylaxis of close contacts of cases is recommended.

Explanation: **Explanation:** **1. Why Option B is the Correct (False) Statement:** In Meningococcal meningitis (caused by *Neisseria meningitidis*), the primary source of infection is **asymptomatic carriers**, not clinical cases. For every one clinical case, there are typically 10 to 100 carriers in the community. The bacteria colonize the nasopharynx; while cases are infectious, they are usually isolated quickly, whereas carriers remain the hidden reservoir responsible for the continued transmission and endemicity of the disease. **2. Analysis of Other Options:** * **Option A (True):** The traditional **unconjugated polysaccharide vaccines** (targeting groups A, C, Y, W-135) are poorly immunogenic in children under 2 years because their immune systems do not respond well to T-cell independent antigens. (Note: Newer conjugate vaccines are effective in infants). * **Option C (True):** Meningococcal disease shows distinct seasonality. It is more common in **dry and cold months** (winter and spring). Dry weather irritates the nasopharyngeal mucosa, facilitating bacterial invasion. * **Option D (True):** Chemoprophylaxis is mandatory for **close contacts** (household members, daycare contacts) to eliminate the carrier state. **Rifampicin** is the drug of choice; alternatives include Ciprofloxacin or Ceftriaxone. **Clinical Pearls for NEET-PG:** * **Reservoir:** Humans are the only known reservoir. * **Portal of Entry:** Nasopharynx. * **Most common Serogroup in India:** Historically Serogroup A. * **Drug of Choice for Treatment:** Intravenous Ceftriaxone or Penicillin G. * **Waterhouse-Friderichsen Syndrome:** Severe complication involving adrenal hemorrhage and septicemia.

Question 680: Clostridium perfringens is cultured on egg yolk agar. What does this test primarily detect?

A. Hyaluronidase
B. Theta toxin
C. Alpha toxin (Correct Answer)
D. Gas production

Explanation: **Explanation:** The correct answer is **Alpha toxin**. The culture of *Clostridium perfringens* on egg yolk agar is known as the **Nagler’s Reaction**. **1. Why Alpha Toxin is correct:** *Clostridium perfringens* produces **Alpha (α) toxin**, which is biochemically a **phospholipase C (lecithinase)**. Egg yolk is rich in lecithin. When the bacteria grow on egg yolk agar, the alpha toxin splits lecithin into phosphorylcholine and insoluble diglycerides. This results in a characteristic **opalescence (opacity)** around the colonies. This reaction is specifically inhibited by adding anti-alpha toxin (antitoxin) to one half of the plate, confirming the toxin's identity. **2. Why other options are incorrect:** * **Hyaluronidase:** While *C. perfringens* produces this enzyme to spread through tissues, it does not react with lecithin in egg yolk agar. * **Theta toxin:** This is a hemolysin (perfringolysin O) responsible for the "inner zone" of complete hemolysis on blood agar, not the opalescence on egg yolk agar. * **Gas production:** While *C. perfringens* is famous for "stormy fermentation" (acid and gas production in litmus milk), egg yolk agar is used to detect enzymatic activity, not gaseous metabolic byproducts. **High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** Rapid biochemical test for presumptive identification of *C. perfringens*. * **Target Hemolysis:** On blood agar, *C. perfringens* shows a double zone of hemolysis (Inner zone: Theta toxin; Outer zone: Alpha toxin). * **Stormy Fermentation:** Characteristic finding in litmus milk media. * **Clinical Condition:** Most common cause of Gas Gangrene (clostridial myonecrosis) and food poisoning (Type A strains).

Practice by Chapter

Staphylococci

Practice Questions

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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