Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 221: Carriers of Salmonella typhi can be detected by?

A. Widal test
B. Blood culture
C. Sputum culture
D. Culture of feces (Correct Answer)

Explanation: **Explanation:** The detection of *Salmonella typhi* carriers is crucial for public health, as these individuals harbor the bacteria without showing clinical symptoms. **Why Culture of Feces is Correct:** In chronic carriers (defined as excreting bacilli for more than one year), the bacteria typically colonize the **gallbladder** (in gallstones) or the biliary tract. From there, the bacilli are intermittently shed into the intestine and excreted in the feces. Therefore, **fecal culture** is the gold standard for identifying intestinal carriers. For urinary carriers (less common, often associated with *Schistosoma haematobium*), urine culture is used. **Analysis of Incorrect Options:** * **Widal Test:** This is a serological test used to diagnose **acute** infection by detecting antibodies (O and H). It is unreliable for carrier detection because antibody titers may be low or absent in chronic states. However, the **Vi agglutination test** is a better serological screening tool for carriers. * **Blood Culture:** This is the investigation of choice during the **first week** of acute typhoid fever (bacteremic phase). Carriers do not have active bacteremia, making blood cultures negative. * **Sputum Culture:** *S. typhi* is an enteric pathogen; it is not typically found in the respiratory tract or sputum. **NEET-PG High-Yield Pearls:** * **Most common site of colonization:** Gallbladder (Chronic carriers). * **Famous Case:** "Typhoid Mary" (Mary Mallon), a classic example of a fecal carrier. * **Screening Tool:** Vi antibody titer (if >1:10, it suggests a carrier state). * **Definitive Diagnosis:** Repeated fecal cultures (often requires enrichment media like Selenite F broth). * **Treatment for Carriers:** High-dose Ampicillin or Amoxicillin with Probenecid; Cholecystectomy may be required if gallstones are present.

Question 222: A hyperemic edema of the larynx and epiglottis that rapidly leads to respiratory obstruction in young children is most likely to be caused by which organism?

A. Klebsiella pneumoniae
B. Mycoplasma pneumoniae
C. Neisseria meningitidis
D. Haemophilus influenzae (Correct Answer)

Explanation: **Explanation:** The clinical presentation described—rapidly progressing hyperemic edema of the epiglottis leading to respiratory obstruction—is the hallmark of **Acute Epiglottitis**. This is a life-threatening pediatric emergency. **Why Haemophilus influenzae is correct:** Historically, **Haemophilus influenzae type b (Hib)** was the most common cause of acute epiglottitis in children. It is a Gram-negative coccobacillus that possesses a polyribosylribitol phosphate (PRP) capsule, which allows it to evade phagocytosis and cause invasive disease. The infection leads to severe inflammation of the supraglottic structures, resulting in the "cherry-red epiglottis" and the classic "Thumb sign" on a lateral neck X-ray. **Why other options are incorrect:** * **Klebsiella pneumoniae:** Primarily causes Friedlander’s pneumonia (characterized by "currant jelly sputum") and urinary tract infections, but is not a typical cause of acute epiglottitis. * **Mycoplasma pneumoniae:** An atypical bacterium that causes "Walking Pneumonia" and tracheobronchitis. It lacks a cell wall and typically presents with a subacute cough rather than acute upper airway obstruction. * **Neisseria meningitidis:** Primarily responsible for meningitis and meningococcemia (Waterhouse-Friderichsen syndrome). While it is a respiratory pathogen, it does not target the epiglottis. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Drooling, Dysphagia, and Distress (the "3 Ds"). * **Positioning:** Children often assume the **"Tripod position"** (leaning forward on hands) to maintain airway patency. * **Radiology:** The **Thumb sign** on lateral X-ray is diagnostic. * **Prevention:** The incidence has significantly decreased due to the **Hib conjugate vaccine**. * **Management:** The priority is securing the airway (intubation) before any invasive examination (like using a tongue depressor), which could trigger fatal laryngospasm.

Question 223: Which is the most sensitive test for Treponema?

A. VDRL
B. RPR
C. FTA-ABS (Correct Answer)
D. Kahn

Explanation: **Explanation:** The diagnosis of Syphilis (*Treponema pallidum*) is divided into non-specific (non-treponemal) and specific (treponemal) serological tests. **Why FTA-ABS is the Correct Answer:** The **Fluorescent Treponemal Antibody Absorption (FTA-ABS)** test is a specific treponemal test. It uses killed *T. pallidum* as the antigen to detect specific anti-treponemal antibodies in the patient's serum. It is considered the **most sensitive** test across all stages of syphilis, particularly in the very early (primary) stage and the late (tertiary) stage. Because it detects specific antibodies rather than reagin, it remains positive for life, even after successful treatment. **Why Other Options are Incorrect:** * **VDRL (Venereal Disease Research Laboratory) & RPR (Rapid Plasma Reagin):** These are non-treponemal tests that detect "reagin" antibodies against cardiolipin-lecithin-cholesterol antigen. While excellent for **screening** and monitoring treatment response (as titers fall after therapy), they are less sensitive than FTA-ABS in early and late stages and can yield Biological False Positives (BFP). * **Kahn Test:** An obsolete tube flocculation test (non-treponemal) similar to VDRL but less sensitive and no longer used in modern clinical practice. **High-Yield Clinical Pearls for NEET-PG:** * **Screening Test of Choice:** RPR or VDRL. * **Confirmatory Test of Choice:** FTA-ABS or TP-PA (Treponema pallidum Particle Agglutination). * **Neurosyphilis:** VDRL is the gold standard for testing **CSF** (highly specific). * **Prozone Phenomenon:** Can cause false-negative VDRL results in secondary syphilis due to excessively high antibody titers. * **Treatment Monitoring:** Only non-treponemal tests (VDRL/RPR) are used to follow up on treatment efficacy.

Question 224: The Xpert MTB/RIF test is used to detect which of the following?

A. Resistance to isoniazid
B. Multidrug-resistant TB
C. Rifampicin resistance (Correct Answer)
D. Drug response in MDR TB

Explanation: **Explanation:** The **Xpert MTB/RIF** is a cartridge-based nucleic acid amplification test (CBNAAT) that simultaneously detects *Mycobacterium tuberculosis* (MTB) and resistance to **Rifampicin**. It utilizes real-time PCR to amplify a 81-bp fragment of the **rpoB gene**, which is the "rifampicin resistance determining region." 1. **Why Rifampicin Resistance is Correct:** Rifampicin resistance is used as a **surrogate marker** for Multidrug-Resistant TB (MDR-TB). This is because over 90-95% of rifampicin-resistant strains are also resistant to isoniazid. The test identifies mutations in the *rpoB* gene within approximately 2 hours. 2. **Why other options are incorrect:** * **A & B:** While Xpert MTB/RIF helps *identify* potential MDR-TB cases, it does **not** directly test for Isoniazid resistance (which requires the *katG* or *inhA* gene targets, found in the newer Xpert MTB/XDR or Line Probe Assays). * **D:** Xpert MTB/RIF cannot be used to monitor **drug response** or treatment progress. This is because the test detects bacterial DNA, which can persist in the sputum of a patient even after the bacilli are dead (false positives). Sputum microscopy and culture remain the gold standards for monitoring treatment response. **High-Yield Clinical Pearls for NEET-PG:** * **WHO Recommendation:** Xpert MTB/RIF is the preferred initial diagnostic test for all individuals suspected of having TB. * **Sample:** It can be used on pulmonary (sputum) and extra-pulmonary samples (CSF, lymph nodes, etc.). * **Sensitivity:** It has much higher sensitivity than smear microscopy, especially in HIV-positive individuals. * **Next Gen:** The **Xpert Ultra** is a newer version with a lower limit of detection, improving diagnosis in paucibacillary cases (like pediatric or HIV-TB).

Question 225: How does tetanus toxin act?

A. Blocking gamma motor neurons
B. Blocking muscle end plate receptor
C. Blocking acetylcholine release
D. Presynaptic blocking (Correct Answer)

Explanation: **Explanation:** Tetanus toxin (Tetanospasmin), produced by *Clostridium tetani*, is a potent neurotoxin that causes spastic paralysis. The correct answer is **Presynaptic blocking** because the toxin acts at the presynaptic terminals of inhibitory interneurons (Renshaw cells) in the spinal cord. **Mechanism of Action:** The toxin is internalized via retrograde axonal transport to the CNS. It acts as a protease that cleaves **SNARE proteins** (specifically synaptobrevin). This cleavage prevents the fusion of synaptic vesicles with the presynaptic membrane, thereby **blocking the release of inhibitory neurotransmitters**—GABA (Gamma-aminobutyric acid) and Glycine. Without these inhibitory signals, motor neurons become overactive, leading to continuous muscle contraction and spasms. **Why other options are incorrect:** * **A. Blocking gamma motor neurons:** The toxin actually leads to the *disinhibition* (overactivity) of alpha motor neurons; it does not block the neurons themselves. * **B. Blocking muscle end plate receptor:** This describes the mechanism of Curare or neuromuscular blocking agents, which cause flaccid paralysis. * **C. Blocking acetylcholine release:** This is the mechanism of **Botulinum toxin**. While both toxins cleave SNARE proteins, Botulinum acts at the peripheral neuromuscular junction, leading to flaccid paralysis, whereas Tetanus acts centrally. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Trismus (lockjaw), Risus sardonicus (grimace), and Opisthotonus (archback). * **Transport:** Retrograde axonal transport to the spinal cord. * **Target:** Synaptobrevin (a V-SNARE). * **Management:** Wound debridement, Metronidazole (preferred over Penicillin), and Human Tetanus Immunoglobulin (HTIG).

Question 226: Which of the following is true regarding Leptospira interrogans?

A. Causes disease in rodents which is usually fatal
B. Mostly affects the kidney and liver (Correct Answer)
C. Causes a rise in IgM antibodies which disappears after the first week of the disease
D. All of the above

Explanation: **Explanation:** *Leptospira interrogans* is the causative agent of Leptospirosis, a zoonotic disease transmitted through contact with water or soil contaminated by the urine of infected animals (primarily rodents). **1. Why Option B is Correct:** Leptospirosis is characterized by widespread vasculitis. In its severe form, known as **Weil’s Disease**, the bacteria show a high tropism for the **liver and kidneys**. This leads to the classic triad of jaundice (liver dysfunction), acute kidney injury (interstitial nephritis/tubular necrosis), and hemorrhage. **2. Why Other Options are Incorrect:** * **Option A:** Rodents (especially rats) are the **natural reservoirs** and chronic carriers. In these hosts, the infection is **asymptomatic and persistent**, localized in the renal tubules. It is not fatal to them, which allows for prolonged shedding in urine. * **Option C:** IgM antibodies typically appear during the second week of illness (immune phase) and can persist for **several months to years**. They do not disappear after the first week; rather, the first week is the "leptospiremic phase" where the bacteria are found in the blood, and antibodies are just beginning to form. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Thin, tightly coiled spirochetes with **hooked ends** (resembling a question mark). * **Culture Media:** Requires specialized media like **EMJH** (Ellinghausen-McCullough-Johnson-Harris) or **Fletcher’s medium**. * **Gold Standard Test:** **Microscopic Agglutination Test (MAT)**. * **Clinical Sign:** **Conjunctival suffusion** (redness without discharge) is a pathognomonic early sign. * **Drug of Choice:** Doxycycline (prophylaxis/mild cases) or Penicillin G (severe cases).

Question 227: Which of the following is a toxin produced by Staphylococcus?

A. Hemolysin
B. Leucocidin
C. Enterotoxin
D. All of the above (Correct Answer)

Explanation: **Explanation:** *Staphylococcus aureus* is a highly pathogenic bacterium characterized by its diverse arsenal of virulence factors, including several potent exotoxins. The correct answer is **D (All of the above)** because *Staphylococcus* produces all these toxins to facilitate tissue invasion and evade the host immune system. 1. **Hemolysins (α, β, γ, δ):** These are membrane-damaging toxins. The most significant is **Alpha-hemolysin**, which creates pores in cell membranes, leading to the lysis of RBCs, platelets, and monocytes. 2. **Leucocidin:** Specifically the **Panton-Valentine Leucocidin (PVL)**, this toxin targets and destroys white blood cells (polymorphonuclear leukocytes). It is a key marker for community-acquired MRSA (CA-MRSA) and is associated with severe necrotizing pneumonia and skin infections. 3. **Enterotoxins:** These are heat-stable toxins (Types A-E, G-I) responsible for **Staphylococcal Food Poisoning**. They act as superantigens, stimulating massive T-cell proliferation and cytokine release. Type A is the most common cause of food poisoning. **Clinical Pearls for NEET-PG:** * **Superantigens:** *S. aureus* produces three main superantigens: Enterotoxin, Toxic Shock Syndrome Toxin-1 (TSST-1), and Exfoliative toxin. * **Food Poisoning:** Characterized by a short incubation period (1–6 hours) because the preformed toxin is ingested. * **Scalded Skin Syndrome (SSSS):** Caused by **Exfoliative (Epidermolytic) toxins** A and B, which cleave desmoglein-1 in the epidermis. * **Coagulase:** While an enzyme and not a toxin, it is the primary biochemical test used to differentiate *S. aureus* (positive) from other Staphylococci (CoNS).

Question 228: A VDRL reactive mother gave birth to an infant. Which of the following would help in determining the risk of transmission to the infant, except?

A. TPHA test on the serum sample of the mother (Correct Answer)
B. TPHA test on the serum sample of the infant
C. VDRL on the paired serum sample of the infant and mother
D. Time interval between the treatment of the mother and her delivery

Explanation: **Explanation:** The diagnosis of congenital syphilis requires differentiating between maternal antibodies transferred across the placenta and an active neonatal infection. **Why Option A is the correct answer:** The **TPHA (Treponema Pallidum Hemagglutination Assay)** is a treponemal-specific test. Once a person is infected with syphilis, treponemal tests usually remain positive for life, regardless of treatment or disease activity. Since the mother is already known to be VDRL reactive, performing a TPHA on her serum only confirms she had syphilis at some point; it provides **no information** regarding the current activity of the disease, the risk of transmission, or the success of her treatment. **Analysis of Incorrect Options:** * **Option B (TPHA on infant):** While maternal IgG crosses the placenta, a negative TPHA in the infant can help rule out transmission. However, its utility is limited compared to IgM-specific tests (like FTA-ABS 19S IgM). * **Option C (Paired VDRL):** This is the gold standard for initial assessment. A **four-fold increase** in the infant’s VDRL titer compared to the mother’s titer is diagnostic of congenital syphilis. * **Option D (Time interval):** Treatment must be completed at least **30 days prior to delivery** to be considered effective in preventing congenital syphilis. If the mother was treated just before delivery, the risk of transmission remains high. **Clinical Pearls for NEET-PG:** * **VDRL/RPR:** Non-specific (reaginic) tests; used for screening and monitoring treatment response (titers fall after treatment). * **TPHA/FTA-ABS:** Specific (treponemal) tests; remain positive for life (cannot be used to monitor treatment). * **Congenital Syphilis:** The most specific diagnostic finding is the detection of **IgM antibodies** in the infant (as IgM does not cross the placenta). * **Hutchinson’s Triad:** Interstitial keratitis, sensorineural deafness, and Hutchinson’s teeth (notched incisors).

Question 229: Pontiac fever is caused by which bacterium?

A. Legionella micdadei
B. Legionella pneumophila (Correct Answer)
C. Legionella adelaidensis
D. Legionella anisa

Explanation: **Explanation:** **Legionella pneumophila** is the primary causative agent of two distinct clinical syndromes: **Legionnaires' disease** (a severe pneumonia) and **Pontiac fever**. Pontiac fever is a mild, self-limiting, influenza-like illness characterized by fever, chills, and myalgia, but notably **without clinical or radiological evidence of pneumonia**. It has a high attack rate (>90%) and a short incubation period (24–48 hours), typically resolving spontaneously within 2–5 days without antibiotic treatment. **Analysis of Options:** * **Option B (Correct):** *Legionella pneumophila* (specifically Serogroup 1) is responsible for over 90% of all Legionellosis cases, including both the pneumonic form and Pontiac fever. * **Option A:** *Legionella micdadei* (Pittsburgh pneumonia agent) is the second most common species but is primarily associated with pneumonia in immunocompromised hosts, not the classic Pontiac fever outbreak. * **Options C & D:** *L. adelaidensis* and *L. anisa* are rare human pathogens. While *L. anisa* has been linked to occasional outbreaks of mild fever, *L. pneumophila* remains the definitive and most high-yield answer for Pontiac fever in competitive exams. **Clinical Pearls for NEET-PG:** * **Habitat:** Found in man-made water systems (AC cooling towers, showers, humidifiers). * **Staining:** Poorly visualized on Gram stain; requires **Silver (Dieterle) stain**. * **Culture:** Gold standard is **BCYE (Buffered Charcoal Yeast Extract) agar**, which requires L-cysteine and Iron. * **Diagnosis:** The **Urinary Antigen Test** is the rapid test of choice (detects Serogroup 1). * **Treatment:** Macrolides (Azithromycin) or Fluoroquinolones (Levofloxacin) are used for Legionnaires' disease; Pontiac fever requires only symptomatic care.

Question 230: Phage typing is done for which of the following bacteria?

A. Salmonella (Correct Answer)
B. Streptococcus
C. Shigella
D. Pseudomonas

Explanation: ### Explanation **Phage typing** is a phenotypic method used for the intra-species identification and epidemiological tracking of bacteria. It relies on the specificity of **bacteriophages** (viruses that infect bacteria) for certain surface receptors on specific bacterial strains. **Why Salmonella is Correct:** *Salmonella Typhi* and *Salmonella Paratyphi* are the classic examples where phage typing is used for epidemiological surveillance. The **Vi-phage typing** scheme is the gold standard for subtyping *S. Typhi*. It helps clinicians and epidemiologists trace the source of an outbreak (e.g., identifying a common "carrier" or contaminated food source) by distinguishing between strains that otherwise look identical on routine culture. **Analysis of Incorrect Options:** * **Streptococcus:** These are primarily classified using **Lancefield grouping** (based on C-carbohydrate antigen) or **Griffith typing** (M-protein). Phage typing is not a standard diagnostic or epidemiological tool for Streptococci. * **Shigella:** Classification is based on **serotyping** (O-antigen). While experimental phage typing exists, it is not used in clinical or public health practice. * **Pseudomonas:** While *Pseudomonas aeruginosa* can be typed using pyocin typing or molecular methods (like PFGE), phage typing is not the primary method used for this genus in the context of standard medical examinations. **High-Yield Clinical Pearls for NEET-PG:** * **Other bacteria where phage typing is used:** *Staphylococcus aureus* (specifically for identifying the source of hospital-acquired infections/MRSA) and *Vibrio cholerae*. * **Bacteriocin Typing:** Another method of intra-species typing (e.g., **Pyocin typing** for *Pseudomonas*, **Colicin typing** for *E. coli*). * **Modern Shift:** In contemporary practice, molecular methods like **PFGE** (Pulsed-field gel electrophoresis) and **WGS** (Whole Genome Sequencing) are replacing phage typing due to better reproducibility.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

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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

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Mycobacteria

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Spirochetes

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