Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1571: A 45-year-old male presents with fever, cough, and anorexia. Sputum is positive for AFB. He is being treated with isoniazid, rifampicin, pyrazinamide, and ethambutol. What is the primary reason for administering multidrug therapy in tuberculosis?

A. To broaden the antimicrobial spectrum.
B. To delay the development of drug resistance. (Correct Answer)
C. To prevent toxin release from the organism.
D. To reduce drug toxicity.

Explanation: ### Explanation **1. Why Option B is Correct:** The primary rationale for Multidrug Therapy (MDT) in Tuberculosis (TB) is to **prevent the emergence of drug-resistant mutants**. *Mycobacterium tuberculosis* has a high rate of spontaneous chromosomal mutations. In a large cavitary lesion (containing $\approx 10^8$ to $10^9$ bacilli), there is a high statistical probability that a few bacilli are naturally resistant to any single drug (e.g., 1 in $10^6$ for Isoniazid). If monotherapy is used, these resistant mutants survive and multiply (selective pressure). By using multiple drugs with different mechanisms of action simultaneously, a mutant resistant to one drug will be killed by the others, effectively preventing the development of Multi-Drug Resistant TB (MDR-TB). **2. Why Other Options are Incorrect:** * **Option A:** Broadening the spectrum is a goal in empirical therapy for polymicrobial infections (e.g., intra-abdominal sepsis). In TB, we already know the specific pathogen; the drugs are chosen for their specific synergistic action against different metabolic states of *M. tuberculosis*. * **Option C:** *M. tuberculosis* does not produce classic exotoxins or endotoxins; its pathogenicity is related to its ability to survive within macrophages and induce a delayed-type hypersensitivity response. * **Option D:** MDT often *increases* the risk of toxicity (e.g., combined hepatotoxicity of INH, Rifampicin, and Pyrazinamide). Toxicity is managed by dosage adjustment, not by adding more drugs. **3. High-Yield Clinical Pearls for NEET-PG:** * **Bactericidal Drugs:** Isoniazid (INH), Rifampicin, Pyrazinamide, Streptomycin. * **Bacteriostatic Drug:** Ethambutol. * **Role of Pyrazinamide:** Most effective against intracellular bacilli in an acidic medium (within macrophages); crucial for "sterilizing" the lesion. * **MDR-TB Definition:** Resistance to at least **Isoniazid and Rifampicin**. * **DOTS (Directly Observed Treatment Short-course):** The strategy used to ensure compliance and further prevent resistance.

Question 1572: Chlamydiae are classified as which of the following?

A. Viruses
B. Fungi
C. Protozoa
D. Bacteria (Correct Answer)

Explanation: **Explanation:** **Chlamydiae** are classified as **obligate intracellular bacteria**. Although they were once thought to be viruses due to their small size and inability to grow on artificial media, they possess all the structural and metabolic hallmarks of bacteria. **Why Bacteria is the Correct Answer:** 1. **Cell Wall:** They possess a rigid cell wall containing peptidoglycan (though modified) and an outer membrane similar to Gram-negative bacteria. 2. **Nucleic Acids:** They contain both **DNA and RNA** (viruses contain only one). 3. **Reproduction:** They divide by **binary fission** rather than assembly. 4. **Metabolism:** They possess their own ribosomes and synthesize their own proteins and lipids. 5. **Antibiotic Sensitivity:** They are susceptible to antibacterial antibiotics (e.g., Azithromycin, Doxycycline) but not antivirals. **Why Other Options are Incorrect:** * **Viruses:** Viruses lack a cellular structure, do not have both DNA and RNA simultaneously, and cannot replicate via binary fission. * **Fungi & Protozoa:** These are **eukaryotic** organisms. Chlamydiae are **prokaryotic**, lacking a nuclear membrane and membrane-bound organelles. **High-Yield Clinical Pearls for NEET-PG:** * **Energy Parasitism:** Chlamydiae are "energy parasites" because they cannot synthesize their own ATP and must derive it from the host cell. * **Unique Life Cycle:** They exist in two forms: the **Elementary Body (EB)**, which is infectious and extracellular, and the **Reticulate Body (RB)**, which is the metabolically active, replicative intracellular form. * **Staining:** They are best visualized using **Giemsa or Castaneda stains**; they are poorly visualized on Gram stain. * **Serotypes:** *C. trachomatis* serotypes A, B, Ba, and C cause **Trachoma**, while D-K cause **NGU (Non-Gonococcal Urethritis)** and Inclusion Conjunctivitis. L1, L2, and L3 cause **Lymphogranuloma Venereum (LGV)**.

Question 1573: Which carbohydrate does gonococcus ferment?

A. Glucose (Correct Answer)
B. Maltose
C. Sucrose
D. Fructose

Explanation: **Explanation:** The differentiation of pathogenic *Neisseria* species is primarily based on their carbohydrate fermentation patterns (acid production from sugars). *Neisseria gonorrhoeae* (Gonococcus) is biochemically characterized by its ability to ferment **only Glucose**. **1. Why Glucose is correct:** *Neisseria gonorrhoeae* utilizes glucose oxidatively to produce acid, but it lacks the enzymes necessary to ferment other sugars. A helpful mnemonic for NEET-PG is: **G**onococcus ferments **G**lucose only. **2. Analysis of Incorrect Options:** * **Maltose:** This is the key differentiating factor. While *N. meningitidis* (Meningococcus) ferments both **G**lucose and **M**altose (**M**eningococcus = **M**altose), *N. gonorrhoeae* cannot ferment maltose. * **Sucrose & Fructose:** These sugars are typically fermented by non-pathogenic, commensal *Neisseria* species (e.g., *N. sicca*). Pathogenic species like Gonococcus and Meningococcus are sucrose-negative. **Clinical Pearls for NEET-PG:** * **Culture Media:** The gold standard for isolation is **Thayer-Martin medium** (a selective Chocolate agar containing Vancomycin, Colistin, and Nystatin). * **Gram Stain:** Appears as Gram-negative, kidney-shaped diplococci found within polymorphonuclear leukocytes (intracellular). * **Oxidase/Catalase:** All *Neisseria* species are **Oxidase positive** and **Catalase positive**. * **Key Distinction:** Unlike Meningococcus, Gonococcus does **not** have a polysaccharide capsule and does **not** ferment maltose.

Question 1574: Which organism causes Salmonella-like diarrhea?

A. Enteroaggregative E. coli
B. Enterotoxigenic E. coli
C. Enteropathogenic E. coli
D. Enterohemorrhagic E. coli (EHEC) (Correct Answer)

Explanation: ### Explanation The correct answer is **Enterohemorrhagic E. coli (EHEC)**. **Why EHEC is correct:** EHEC (specifically serotype O157:H7) produces **Shiga-like toxins (Verotoxins)** that cause damage to the intestinal mucosa, leading to **hemorrhagic colitis**. This clinical presentation—characterized by severe abdominal pain and bloody diarrhea with little or no fever—mimics the invasive nature of *Salmonella* and *Shigella* infections. While *Salmonella* typically causes inflammatory diarrhea through mucosal invasion, EHEC achieves a similar "bloody" clinical picture via potent cytotoxins that destroy endothelial cells. **Why the other options are incorrect:** * **Enteroaggregative E. coli (EAEC):** Associated with persistent, watery diarrhea in children and HIV patients. It uses "stacked-brick" adhesion but does not typically cause the gross hematochezia seen in Salmonella-like illness. * **Enterotoxigenic E. coli (ETEC):** The leading cause of **Traveler’s diarrhea**. It produces LT (heat-labile) and ST (heat-stable) toxins, resulting in profuse watery diarrhea similar to Cholera, not invasive/bloody diarrhea. * **Enteropathogenic E. coli (EPEC):** Primarily causes infantile diarrhea in developing countries. It acts via "attachment and effacement" (A/E) lesions but lacks the toxins required to cause hemorrhagic, Salmonella-like symptoms. **High-Yield Clinical Pearls for NEET-PG:** * **EHEC & HUS:** EHEC is the most common cause of **Hemolytic Uremic Syndrome (HUS)**, characterized by the triad of Microangiopathic Hemolytic Anemia, Thrombocytopenia, and Acute Renal Failure. * **Sorbitol Agar:** Unlike most E. coli, EHEC (O157:H7) does **not** ferment sorbitol. It is identified on **Sorbitol MacConkey (SMAC) agar** as colorless colonies. * **Antibiotic Warning:** Avoid using antibiotics for EHEC, as they may increase toxin release and precipitate HUS.

Question 1575: A 2-year-old child presents with fever, stiff neck, and irritability. A Gram stain smear of cerebrospinal fluid reveals small, pleomorphic, gram-negative coccobacillary organisms. What is the most appropriate procedure to establish an etiological diagnosis?

A. Culture the cerebrospinal fluid on chocolate agar and identify the organism by growth factors. (Correct Answer)
B. Culture the cerebrospinal fluid on mannitol-salt agar.
C. Perform a catalase test on the isolated organism.
D. Perform a coagulase test on the isolated organism.

Explanation: ### Explanation **1. Why Option A is Correct:** The clinical presentation (fever, stiff neck, irritability) in a 2-year-old is highly suggestive of **acute bacterial meningitis**. The Gram stain description of **"small, pleomorphic, gram-negative coccobacilli"** is the classic morphological hallmark of ***Haemophilus influenzae* type b (Hib)**. *H. influenzae* is a fastidious organism that requires two specific growth factors found in blood: **Factor V (NAD)** and **Factor X (Hemin)**. While these factors are inside RBCs, they are only available in **Chocolate Agar** (where RBCs are lysed by heat). Identification is confirmed using the **"X and V factor test"** or the **Satellitism phenomenon** around *S. aureus*. **2. Why Other Options are Incorrect:** * **Option B:** **Mannitol Salt Agar (MSA)** is a selective and differential medium used for *Staphylococcus aureus*. It inhibits most other bacteria due to high salt concentration. * **Option C & D:** **Catalase and Coagulase tests** are primary biochemical tests used to differentiate **Gram-positive cocci** (Staphylococci vs. Streptococci). They are irrelevant for the identification of Gram-negative coccobacilli like *Haemophilus*. **3. NEET-PG High-Yield Pearls:** * **Satellitism:** *H. influenzae* grows near *S. aureus* colonies on blood agar because *S. aureus* provides Factor V via hemolysis. * **Culture Media:** Chocolate agar is the medium of choice; it is essentially heated blood agar. * **Quellung Reaction:** Can be used for rapid identification of the capsule (Hib). * **Prevention:** The Hib conjugate vaccine has significantly reduced the incidence of this type of meningitis in children. * **Drug of Choice:** Third-generation cephalosporins (e.g., Ceftriaxone) are used for empirical treatment.

Question 1576: Crepitus in a wound is produced by which of the following microorganisms?

A. Staphylococcus aureus
B. Clostridium tetani
C. Clostridium welchii (Correct Answer)
D. Pseudomonas

Explanation: **Explanation:** **Correct Answer: C. Clostridium welchii (Clostridium perfringens)** The presence of **crepitus** (a crackling sensation felt on palpation) indicates the presence of gas within the soft tissues. This is the hallmark clinical sign of **Gas Gangrene** (Clostridial Myonecrosis). * **Mechanism:** *Clostridium welchii* is an anaerobic, Gram-positive spore-forming bacillus. In an anaerobic environment (like deep crush injuries), it ferments carbohydrates in the muscle tissue, producing significant amounts of insoluble gas (CO₂ and H₂). This gas dissects through tissue planes, leading to crepitus, severe pain, and rapid tissue necrosis. The primary virulence factor responsible for tissue destruction is the **Alpha toxin** (Lecithinase). **Why other options are incorrect:** * **A. Staphylococcus aureus:** Typically causes pyogenic (pus-forming) infections like abscesses or cellulitis. While it can cause necrotizing fasciitis in rare cases, it is not a primary gas-producer. * **B. Clostridium tetani:** While also an anaerobe, it does not cause tissue necrosis or gas production. It produces **Tetanospasmin**, a neurotoxin that leads to spastic paralysis (Lockjaw). * **C. Pseudomonas:** Known for causing opportunistic infections in burns and producing a characteristic blue-green pigment (Pyocyanin). It does not typically produce gas in tissues. **NEET-PG High-Yield Pearls:** * **Nagler’s Reaction:** Used for rapid identification of *C. perfringens* (detects lecithinase activity on egg yolk agar). * **Morphology:** Described as "Box-car shaped" bacilli; notably **non-motile** (unlike most Clostridia). * **Stormy Fermentation:** Characteristic reaction in litmus milk. * **Target Hemolysis:** Double zone of hemolysis on blood agar (inner complete zone by theta toxin, outer partial zone by alpha toxin).

Question 1577: Which of the following antigens is most immunogenic?

A. O antigen
B. H antigen (Correct Answer)
C. Vi antigen
D. Somatic antigen

Explanation: **Explanation:** The immunogenicity of an antigen is primarily determined by its chemical complexity. The **H antigen (Flagellar antigen)** is composed of **proteins (flagellin)**. Proteins are the most potent immunogens because they possess complex tertiary structures and are processed via the MHC-II pathway, leading to a robust T-cell dependent immune response. This results in high antibody titers and the formation of memory cells. **Analysis of Options:** * **B. H antigen (Correct):** Being proteinaceous in nature, it is highly immunogenic compared to polysaccharide-based antigens. * **A & D. O antigen / Somatic antigen (Incorrect):** These refer to the same structure—the outer membrane Lipopolysaccharide (LPS). While O antigens are important for serotyping, they are heat-stable polysaccharides. Polysaccharides are generally less immunogenic than proteins as they often trigger T-cell independent responses. * **C. Vi antigen (Incorrect):** This is a capsular polysaccharide (Virulence antigen) found in *Salmonella Typhi*. Like the O antigen, its carbohydrate nature makes it less immunogenic than the protein-based H antigen. **NEET-PG High-Yield Pearls:** * **Chemical Nature:** H = Protein (Heat labile); O = Phospholipid-Polysaccharide complex (Heat stable); Vi = Polysaccharide (Heat labile). * **Widal Test Application:** In Enteric fever, anti-H antibodies appear later but persist longer and reach higher titers than anti-O antibodies due to the superior immunogenicity of the H antigen. * **Hierarchy of Immunogenicity:** Proteins > Polysaccharides > Lipids/Nucleic acids.

Question 1578: Which of the following diseases is associated with a chronic carrier state?

A. Measles
B. Whooping cough
C. Typhoid (Correct Answer)
D. None of the above

Explanation: **Explanation:** The concept of a **chronic carrier** refers to an individual who continues to harbor and shed a pathogen for a prolonged period (usually more than one year) after the initial clinical recovery, serving as a persistent reservoir for infection. **Why Typhoid is Correct:** *Salmonella Typhi* is notorious for establishing a chronic carrier state. Following an acute infection, the bacteria can persist in the **gallbladder** (most common site) or the biliary tract, often associated with gallstones or chronic cholecystitis. These carriers are asymptomatic but intermittently shed the bacilli in their feces (**fecal carriers**) or urine (**urinary carriers**, often associated with *Schistosoma haematobium* infection). This is the classic mechanism behind the famous case of "Typhoid Mary." **Why Other Options are Incorrect:** * **Measles:** This is an acute viral infection. The virus is cleared by the immune system after the illness, and there is no known chronic carrier state. Immunity following infection is typically lifelong. * **Whooping Cough (*Bordetella pertussis*):** While the cough can last for weeks (the "100-day cough"), it is an acute respiratory infection. Humans are the only reservoir, but there is no recognized chronic carrier state; transmission occurs during the catarrhal and early paroxysmal stages. **High-Yield Clinical Pearls for NEET-PG:** * **Definition:** A chronic carrier in Typhoid is defined as someone shedding *S. Typhi* for **>1 year**. * **Investigation of Choice:** To identify a fecal carrier, **multiple stool cultures** are required. For screening, the **Vi agglutination test** is used (high titers suggest a carrier state). * **Treatment:** The drug of choice for eradicating the carrier state is **Ciprofloxacin** (for 6 weeks). If gallstones are present, a **cholecystectomy** may be necessary. * **Other Chronic Carriers:** Other examples include Hepatitis B, Hepatitis C, and HIV.

Question 1579: Which of the following is NOT a part of the normal microbial flora of the human body?

A. Staphylococcus aureus
B. Mycobacterium tuberculosis (Correct Answer)
C. Candida albicans
D. Corynebacterium

Explanation: **Explanation:** The distinction between **normal flora** and **obligate pathogens** is a high-yield concept in microbiology. Normal flora consists of microorganisms that reside on or within the body of a healthy person without causing disease under normal conditions. **Why Mycobacterium tuberculosis is the correct answer:** *Mycobacterium tuberculosis* is an **obligate pathogen**. It is never considered part of the normal flora. Its presence in the human body always indicates an infection (either latent or active disease). It is transmitted via respiratory droplets and is primarily an intracellular pathogen of macrophages. **Analysis of Incorrect Options:** * **Staphylococcus aureus:** A common commensal found in the **anterior nares** (nasal carriage in ~30% of the population) and on the skin. It only becomes pathogenic when the skin barrier is breached or immunity is compromised. * **Candida albicans:** The most common fungal commensal. It is part of the normal flora of the **gastrointestinal tract, mouth, and vagina**. It acts as an opportunistic pathogen (e.g., causing oral thrush or vaginal candidiasis). * **Corynebacterium:** Often referred to as "diphtheroids," these are ubiquitous members of the normal flora of the **skin and mucous membranes**. (Note: *C. diphtheriae* is the pathogen, but the genus itself is a major component of skin flora). **NEET-PG High-Yield Pearls:** 1. **Sterile Sites:** Blood, Cerebrospinal fluid (CSF), lower respiratory tract (below the larynx), and internal organs are normally **sterile**. 2. **Skin Flora:** *Staphylococcus epidermidis* is the most common resident organism of the skin. 3. **Colon Flora:** *Bacteroides fragilis* (anaerobe) is the most abundant organism in the colon, outnumbering *E. coli*. 4. **Vaginal Flora:** *Lactobacillus* (Doderlein’s bacilli) maintains an acidic pH, preventing overgrowth of pathogens.

Question 1580: Malignant pustule is/are seen in infection with which organism?

A. Treponema pallidum
B. Campylobacter granulomatis
C. Bacillus anthracis (Correct Answer)
D. Haemophilus ducreyi

Explanation: **Explanation:** **Bacillus anthracis** is the correct answer because it is the causative agent of **Cutaneous Anthrax**, which is clinically characterized by a lesion known as a **"Malignant Pustule."** Despite the name, it is neither malignant (cancerous) nor a true pustule. It begins as a painless papule that evolves into a vesicle containing bluish-black fluid, eventually rupturing to form a characteristic **painless, black necrotic eschar** surrounded by significant non-pitting edema. The term "malignant" historically referred to the fatal nature of the disease if it progressed to septicemia. **Analysis of Incorrect Options:** * **Treponema pallidum:** Causes Syphilis. The primary lesion is a **Chancre**, which is a painless, indurated ulcer, typically found on the genitalia. * **Campylobacter (Klebsiella) granulomatis:** Causes Granuloma Inguinale (Donovanosis). It presents as painless, beefy-red, velvety ulcers that bleed easily on contact. * **Haemophilus ducreyi:** Causes **Chancroid**, characterized by a **painful**, soft ulcer (soft chancre) often associated with painful inguinal lymphadenopathy (bubo). **High-Yield Clinical Pearls for NEET-PG:** * **Occupational Hazard:** Anthrax is known as **"Hide-porter’s disease"** or "Wool-sorter’s disease" due to exposure to infected animal products. * **Morphology:** *B. anthracis* is a Gram-positive, spore-forming, non-motile rod with a characteristic **"Medusa head"** appearance on agar. * **Virulence Factors:** It possesses a polypeptide capsule (Poly-D-glutamic acid) and a tripartite toxin (Lethal Factor, Edema Factor, and Protective Antigen). * **McFadyean’s Reaction:** Used for presumptive identification of *B. anthracis* in blood smears using polychrome methylene blue.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

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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