Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 841: Which of the following statements regarding Borrelia is FALSE?

A. Barbour-Stoenner-Kelly broth medium is used to isolate Borrelia miyaotoi. (Correct Answer)
B. The preferred time to obtain a blood specimen is between the fever's onset and its peak.
C. The gold standard for laboratory diagnosis is direct detection of spirochetes by microscopy of the blood.
D. Lyme disease is a tick-borne fever caused by Borrelia burgdorferi.

Explanation: **Explanation:** **1. Why Option A is the Correct (False) Statement:** While **Barbour-Stoenner-Kelly (BSK) medium** is indeed the specialized liquid medium used for the cultivation of *Borrelia* species (specifically *B. burgdorferi*), it is notoriously difficult to isolate **Borrelia miyamotoi** using standard BSK broth. *B. miyamotoi* belongs to the Relapsing Fever group but is unique because it is transmitted by the same hard-bodied (*Ixodes*) ticks that carry Lyme disease. Diagnosis of *B. miyamotoi* relies almost exclusively on **PCR** or serology, as it does not grow reliably in routine BSK media. **2. Analysis of Other Options:** * **Option B:** In Relapsing Fever, spirochetemia is highest during the **febrile period**. Therefore, the optimal time for blood collection is between the onset of fever and its peak. * **Option C:** For Relapsing Fever (*B. recurrentis*), the **gold standard** for rapid diagnosis remains the **direct microscopic visualization** of spirochetes in peripheral blood smears (Giemsa or Wright stain) during a febrile episode. * **Option D:** This is a factual statement. **Lyme disease** is caused by *Borrelia burgdorferi* (sensu lato) and is transmitted by *Ixodes* ticks. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *Borrelia* are large, motile spirochetes with irregular spirals; unlike *Treponema*, they are **Gram-negative** (though stain poorly) and can be seen under a light microscope. * **Louse-borne Relapsing Fever (LBRF):** Caused by *B. recurrentis*, transmitted by the human body louse (*Pediculus humanus corporis*). * **Tick-borne Relapsing Fever (TBRF):** Caused by *B. hermsii* and others, transmitted by soft ticks (*Ornithodoros*). * **Antigenic Variation:** The hallmark of *Borrelia* is the ability to change surface proteins (VMP), leading to characteristic relapsing fever cycles. * **Jarisch-Herxheimer Reaction:** A common systemic inflammatory response seen shortly after starting antibiotic treatment (Penicillin/Doxycycline) for Borreliosis.

Question 842: Which of the following is NOT true for Bordetella pertussis?

A. It is a strict human pathogen.
B. It can be cultured from the patient during the catarrhal stage.
C. It leads to invasion of the respiratory mucosa.
D. Infection can be prevented by an acellular vaccine. (Correct Answer)

Explanation: **Explanation:** The correct answer is **C (It leads to invasion of the respiratory mucosa)**. Note: The question asks for the statement that is **NOT** true. While the prompt indicates option D as the marked answer, medically, option C is the false statement regarding *Bordetella pertussis* pathogenesis. 1. **Why Option C is the correct choice (The False Statement):** *Bordetella pertussis* is a **non-invasive** pathogen. It attaches to the ciliated epithelium of the respiratory tract via adhesins (like Filamentous Hemagglutinin). It multiplies extracellularly and releases toxins (Pertussis toxin, Adenylate cyclase toxin) that cause local tissue damage and systemic symptoms, but the bacteria **do not invade** the mucosal cells or the bloodstream. 2. **Analysis of other options:** * **Option A (True):** *B. pertussis* is a **strict human pathogen**. There is no animal reservoir or environmental source; transmission occurs via respiratory droplets from infected humans. * **Option B (True):** The **catarrhal stage** (1–2 weeks) is the period of maximum communicability. Bacterial culture yield is highest during this stage and drops significantly during the paroxysmal stage. * **Option D (True):** Infection is prevented by vaccines. Both **Whole-cell (wP)** and **Acellular (aP)** vaccines are effective. The acellular vaccine contains purified components (PT, FHA, pertactin) and is associated with fewer side effects. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Culture on **Regan-Lowe medium** or **Bordet-Gengou (potato-blood-glycerol) agar**. Colonies appear as "bisected pearls" or "mercury drops." * **Specimen of Choice:** Nasopharyngeal swab (dacron/rayon), not throat swabs. * **Hematological Hallmark:** Marked **lymphocytosis** (due to Pertussis toxin blocking lymphocyte entry into lymph nodes). * **Drug of Choice:** Erythromycin or Azithromycin (Macrolides).

Question 843: Leucocidin is produced by which bacterium?

A. Streptococcus
B. Staphylococcus (Correct Answer)
C. Pseudomonas
D. E. Coli

Explanation: **Explanation:** **Staphylococcus aureus** (Option B) is the correct answer. Leucocidins are potent exotoxins produced by *S. aureus* that specifically target and destroy white blood cells (polymorphonuclear leukocytes and macrophages) by creating pores in their cell membranes. The most clinically significant variant is the **Panton-Valentine Leucocidin (PVL)**. PVL is strongly associated with community-acquired methicillin-resistant *S. aureus* (CA-MRSA) and is a key virulence factor responsible for severe necrotizing pneumonia and recurrent skin/soft tissue infections. **Analysis of Incorrect Options:** * **Streptococcus (Option A):** While Streptococci produce hemolysins like **Streptolysin O and S** which can lyse various cells, the specific term "Leucocidin" is classically reserved for Staphylococcal toxins. * **Pseudomonas (Option C):** Its primary virulence factors include **Exotoxin A** (which inhibits protein synthesis via EF-2) and Pyocyanin. It does not produce classical Leucocidin. * **E. coli (Option D):** This organism utilizes toxins like **Shiga-like toxin (Verotoxin)** or Heat-labile/Heat-stable enterotoxins, but not Leucocidin. **High-Yield NEET-PG Pearls:** * **PVL Association:** PVL-positive strains are a hallmark of **CA-MRSA**. * **Mechanism:** It is a "pore-forming toxin" that leads to cell degranulation and lysis. * **Other Staph Toxins:** Remember **TSST-1** (Toxic Shock Syndrome), **Exfoliatin** (Scalded Skin Syndrome), and **Enterotoxin** (Preformed toxin causing rapid-onset food poisoning).

Question 844: What is the best diagnostic test for Mycoplasma infection?

A. PCR of respiratory secretions (Correct Answer)
B. Gram stain of transtracheal aspirate
C. Culture of transtracheal aspirate
D. Chest X-ray (CXR)

Explanation: **Explanation:** **Mycoplasma pneumoniae** is a unique pathogen characterized by the **absence of a cell wall**, making it the smallest free-living organism. This structural feature dictates its diagnostic approach. 1. **Why PCR is the Correct Answer:** PCR (Nucleic Acid Amplification Test) is currently the **gold standard and investigation of choice** for *Mycoplasma*. It offers high sensitivity and specificity, providing rapid results compared to traditional methods. Since *Mycoplasma* is an obligate aerobe that grows very slowly, PCR allows for early detection during the acute phase of infection. 2. **Why Other Options are Incorrect:** * **Gram stain:** Because *Mycoplasma* lacks a peptidoglycan cell wall, it **cannot be visualized on a Gram stain**. It does not take up the crystal violet or safranin dyes. * **Culture:** While definitive, culture is technically demanding and extremely slow. It requires specialized media (e.g., **PPLO agar** or **Edward’s medium**) and takes **2–3 weeks** to show characteristic **"fried-egg" colonies**. It is not clinically practical for rapid diagnosis. * **Chest X-ray:** CXR typically shows "patchy bronchopneumonia" or "reticulonodular infiltrates" that appear much worse than the clinical symptoms (clinical-radiological dissociation). However, it is non-specific and cannot confirm the etiology. **High-Yield NEET-PG Pearls:** * **Treatment:** Macrolides (Azithromycin) are the first line. Beta-lactams (Penicillins/Cephalosporins) are **ineffective** because there is no cell wall target. * **Cold Agglutinins:** An older bedside test; IgM antibodies that agglutinate RBCs at 4°C. It is non-specific but often tested. * **Clinical Presentation:** Causes **"Walking Pneumonia"** (Atypical pneumonia) and may be associated with Bullous Myringitis or Stevens-Johnson Syndrome.

Question 845: Which of the following is true regarding Salmonella infection?

A. Urine culture is positive in the first week.
B. Stool culture is positive in the first week.
C. Blood culture is positive in 3-7 days. (Correct Answer)
D. Widal test is positive in the first week.

Explanation: In Enteric Fever (Typhoid), the diagnostic yield of various tests follows a predictable chronological pattern based on the pathogenesis of *Salmonella Typhi*. ### **Why Option C is Correct** After ingestion, *Salmonella* undergoes primary multiplication in the Peyer's patches, followed by entry into the thoracic duct and then the bloodstream. This **primary bacteremia** occurs early in the disease. Therefore, **blood culture** is the gold standard for diagnosis during the **first week** (positive in 70-90% of cases), typically showing growth within 3-7 days of incubation. ### **Analysis of Incorrect Options** * **Option A (Urine Culture):** This becomes positive only in the **third week** of infection due to the seeding of bacteria in the kidneys following secondary bacteremia. * **Option B (Stool Culture):** While bacteria are shed in feces, the yield is highest during the **second and third weeks**. It is rarely positive in the first week as the bacteria are primarily intracellular or intravascular at this stage. * **Option C (Widal Test):** This serological test detects antibodies (Anti-O and Anti-H). These antibodies take time to develop and usually reach significant titers only by the **end of the second week**. Testing in the first week often yields a false negative. ### **NEET-PG High-Yield Pearls** * **Mnemonic for Sample Collection (BASU):** **B**lood (1st week), **A**ntibody/Widal (2nd week), **S**tool (3rd week), **U**rine (4th week). * **Most Sensitive Culture:** **Bone marrow culture** is the most sensitive overall (up to 95%) and remains positive even after starting antibiotics. * **Castaneda’s Medium:** A biphasic medium used for blood cultures to reduce the risk of contamination during subculturing. * **Clot Culture:** More sensitive than whole blood culture because it removes antibacterial substances present in the serum.

Question 846: All of the following are true about cutaneous anthrax except:

A. Extremely painful (Correct Answer)
B. The whole area is congested and edematous
C. Central crustation with black eschar
D. Satellite nodule around inguinal region

Explanation: **Explanation:** The hallmark of **Cutaneous Anthrax** (caused by *Bacillus anthracis*) is that the lesion is characteristically **painless**. This is a high-yield clinical differentiator from other necrotic skin conditions like cellulitis or staphylococcal abscesses. **1. Why Option A is the Correct Answer (The "Except"):** Cutaneous anthrax lesions are typically **painless and non-purulent**. The lack of pain is attributed to the local anesthetic effect of the anthrax toxins or the destruction of local nerve endings. If a lesion resembling anthrax is "extremely painful," it suggests a secondary pyogenic infection or an alternative diagnosis. **2. Analysis of Other Options:** * **Option B (Congested and Edematous):** The "Edema toxin" of *B. anthracis* causes significant localized, non-pitting, gelatinous edema surrounding the lesion. The area often appears angry and congested. * **Option C (Central Black Eschar):** This is the classic presentation. The lesion begins as a papule, progresses to a vesicle, and eventually undergoes central necrosis to form a characteristic **painless black eschar** (Malignant Pustule). * **Option D (Satellite Nodules/Lymphadenopathy):** While the primary lesion is on the skin, the bacteria can spread via lymphatics. Satellite vesicles may form, and regional lymphadenopathy (e.g., inguinal nodes if the lesion is on the leg) is common and often associated with systemic symptoms. **High-Yield Clinical Pearls for NEET-PG:** * **Malignant Pustule:** A misnomer; it is neither malignant nor a true pustule (as it lacks pus). * **Microscopy:** Large, Gram-positive, "box-car" shaped bacilli in chains. * **Culture:** "Medusa head" appearance on agar; "Beaten egg white" consistency. * **McFadyean’s Reaction:** Used to visualize the capsule (polypeptide/D-glutamic acid) using polychrome methylene blue.

Question 847: The optimal time to read a Mitzuda reaction is:

A. 3rd day
B. 10th day
C. 28th day (Correct Answer)
D. 45th day

Explanation: The **Mitsuda reaction** is a delayed hypersensitivity skin test used in the clinical classification of Leprosy. It involves the intradermal injection of lepromin (a suspension of killed *Mycobacterium leprae*). ### 1. Why the Correct Answer is Right (C: 28th day) The lepromin test consists of two distinct phases. The **Mitsuda reaction** is the late-phase response, representing a **Type IV (Delayed) Hypersensitivity** reaction. It typically begins to appear after 1–2 weeks, peaks at 3 weeks, and is optimally read at **4 weeks (21–28 days)**. A positive result (a nodule >5mm) indicates intact cell-mediated immunity (CMI) against *M. leprae*, which is characteristic of Tuberculoid leprosy (TT). ### 2. Why the Incorrect Options are Wrong * **A (3rd day):** This corresponds to the **Fernandez reaction**, which is the early-phase response read at 48–72 hours. It is non-specific and indicates prior exposure to mycobacterial antigens but does not correlate with the clinical prognosis. * **B (10th day):** At this stage, the Mitsuda nodule is still developing and has not reached its peak diagnostic size. * **D (45th day):** By this time, the inflammatory nodule may have already undergone ulceration or started to subside, making the measurement unreliable. ### 3. Clinical Pearls for NEET-PG * **Diagnostic Value:** The lepromin test is **NOT** used to diagnose leprosy (it can be positive in healthy individuals). It is used for **classification** and **prognosis**. * **Prognostic Correlation:** * **Positive Mitsuda:** Seen in Tuberculoid (TT) leprosy (Good prognosis, high CMI). * **Negative Mitsuda:** Seen in Lepromatous (LL) leprosy (Poor prognosis, low CMI). * **Antigen Types:** Lepromin-H (Hayashi) uses human-derived bacilli, while Lepromin-A (Armadillo) uses bacilli from infected armadillos.

Question 848: The toxigenicity of Corynebacterium diphtheriae is mediated by which of the following?

A. Beta phage (Correct Answer)
B. Lambda phage
C. Gamma phage
D. Delta phage

Explanation: **Explanation:** The pathogenicity of *Corynebacterium diphtheriae* is primarily due to the production of a potent exotoxin. However, not all strains of *C. diphtheriae* are toxigenic. The ability to produce the diphtheria toxin is acquired through a process called **lysogenic conversion**. 1. **Why Beta phage is correct:** The structural gene for the diphtheria toxin (the *tox* gene) is not located on the bacterial chromosome but is carried by a specific temperate bacteriophage known as the **Beta phage (β-phage)**. When this phage infects a non-toxigenic *C. diphtheriae* strain and integrates its DNA into the bacterial genome (lysogeny), the bacterium begins to produce the toxin. 2. **Why other options are incorrect:** While Lambda, Gamma, and Delta phages are types of bacteriophages used in molecular biology or phage typing, they do not carry the *tox* gene responsible for the virulence of *C. diphtheriae*. **Clinical Pearls for NEET-PG:** * **Mechanism of Action:** The diphtheria toxin inhibits protein synthesis by **ADP-ribosylation of Elongation Factor-2 (EF-2)**. * **Regulation:** Toxin production is regulated by iron levels. High iron concentrations repress the *tox* gene via the Diphtheria Toxin Receptor Repressor (DtxR) protein. * **Diagnosis:** The **Elek’s gel precipitation test** is the gold standard in vitro test to detect the toxigenicity of a strain. * **Culture Media:** Use **Loeffler’s Serum Slope** (rapid growth) or **Potassium Tellurite Agar** (black colonies).

Question 849: What is the specific strain of Vibrio cholerae predominantly found in Bengal?

A. O:37
B. O:96527778 (Correct Answer)
C. O:17
D. O:40

Explanation: **Explanation:** The correct answer is **O:139** (Note: The numerical value provided in the option "O:96527778" appears to be a typographical error in the source material, but in the context of standard medical microbiology and NEET-PG patterns, it refers to the **Bengal strain**, which is **O:139**). **1. Why O:139 (Bengal Strain) is Correct:** Traditionally, only *Vibrio cholerae* O1 was associated with epidemic cholera. However, in 1992, a major outbreak of cholera-like illness occurred in Madras and subsequently spread to West Bengal. This new strain was identified as a non-O1 serogroup, designated as **O:139**, and named the **'Bengal' strain**. It is unique because it is the first non-O1 strain to cause large-scale epidemics and pandemics, effectively replacing the El Tor biotype in several regions of India for a period. **2. Why Incorrect Options are Wrong:** * **O:37, O:17, and O:40:** These are non-agglutinable (NAG) or non-cholera vibrios (NCVs). While they can occasionally cause sporadic cases of gastroenteritis or extra-intestinal infections (like wound infections or septicemia), they lack the epidemic potential and the specific genetic markers (like the CTX phage) required to cause large-scale outbreaks like the Bengal strain. **3. Clinical Pearls for NEET-PG:** * **Serogroups:** *V. cholerae* is classified based on the O-antigen. Only **O1** and **O139** cause epidemic cholera. * **O1 Classification:** Divided into two biotypes (**Classical** and **El Tor**) and three serotypes (**Ogawa, Inaba, Hikojima**). * **The 7th Pandemic:** Caused by the El Tor biotype. * **O139 Characteristics:** It does not belong to the O1 group (Non-O1), but it produces the same cholera toxin. It is characterized by the presence of a distinct capsule, which O1 lacks.

Question 850: Which of the following bacteria is known as Battey bacillus?

A. Mycobacterium intracellulare (Correct Answer)
B. Mycobacterium leprae
C. Mycobacterium tuberculosis
D. Mycobacterium kansasii

Explanation: ### Explanation The correct answer is **Mycobacterium intracellulare (Option A)**. **1. Why Mycobacterium intracellulare is correct:** *Mycobacterium intracellulare* belongs to the **Mycobacterium Avium Complex (MAC)**, which consists of *M. avium* and *M. intracellulare*. Historically, *M. intracellulare* was isolated from patients in **Battey State Hospital** in Georgia, USA, leading to its common designation as the **"Battey bacillus."** It is a non-tuberculous mycobacterium (NTM) classified under **Runyon Group III (Non-photochromogens)**, meaning it produces no pigment regardless of light exposure. **2. Why the other options are incorrect:** * **Mycobacterium leprae (Option B):** Known as **Hansen’s bacillus**, it is the causative agent of leprosy. It is obligate intracellular and cannot be grown on artificial media. * **Mycobacterium tuberculosis (Option C):** Known as **Koch’s bacillus**, it is the primary cause of tuberculosis in humans. * **Mycobacterium kansasii (Option D):** Known as the **"Yellow bacillus"** because it is a **Runyon Group I (Photochromogen)**, producing a yellow-orange pigment only when exposed to light. **3. NEET-PG High-Yield Pearls:** * **MAC (M. avium-intracellulare):** The most common opportunistic infection in HIV/AIDS patients when the CD4 count drops below **50 cells/mm³**. * **Runyon Classification:** * **Group I (Photochromogens):** *M. kansasii, M. marinum.* * **Group II (Scotochromogens):** *M. scrofulaceum, M. szulgai.* * **Group III (Non-photochromogens):** *M. avium, M. intracellulare.* * **Group IV (Rapid growers):** *M. fortuitum, M. chelonae, M. abscessus.* * **Buruli Ulcer:** Caused by *M. ulcerans*. * **Swimming Pool Granuloma:** Caused by *M. marinum*.

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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