Bacteriology — MCQs

A couple, who did not know each other very well, dated and had sexual contact. Several weeks later, the man noticed a small, painless vesicle on his penis, which ruptured and then healed. Soon, his inguinal lymph nodes enlarged and discharged pus through multiple sinus tracts. Lymphogranuloma venereum (LGV) is a venereal disease caused by serotype L1, L2, or L3 of Chlamydia trachomatis. The differential diagnosis should include which of the following?

Q890Easy

Which of the following is used as a biological indicator for control in plasma sterilization?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 881: Which of the following is known as Preiz Nocard bacillus?

A. Corynebacterium diphtheriae
B. Corynebacterium pseudotuberculosis (Correct Answer)
C. Influenza bacillus
D. Salmonella

Explanation: ### Explanation **Corynebacterium pseudotuberculosis** is the correct answer. It is historically and medically known as the **Preisz-Nocard bacillus**, named after the scientists Hugo von Preisz and Edmond Nocard who first described it. #### Why it is correct: * **Microbiology:** It is a Gram-positive, pleomorphic rod. While it belongs to the same genus as the diphtheria bacillus, it primarily causes **Caseous Lymphadenitis (CLA)** in sheep and goats. * **Pathogenesis:** It produces a potent exotoxin called **Phospholipase D (PLD)**, which increases vascular permeability and aids in the lymphatic spread of the bacteria, leading to "cheesy" abscesses in lymph nodes. #### Why the other options are incorrect: * **A. Corynebacterium diphtheriae:** Known as the **Klebs-Löffler bacillus**. It is the causative agent of diphtheria and is characterized by the presence of volutin (metachromatic) granules. * **C. Influenza bacillus:** This refers to **Haemophilus influenzae**, a Gram-negative coccobacillus. It was originally (and incorrectly) thought to cause the flu before the influenza virus was discovered. * **D. Salmonella:** These are Gram-negative enteric bacilli. Specific strains like *S. Typhi* are known as the **Eberth bacillus**. #### NEET-PG High-Yield Pearls: * **Zoonosis:** Human infection with *C. pseudotuberculosis* is rare but can occur in occupationally exposed individuals (shearers, farmers), typically presenting as subacute granulomatous lymphadenitis. * **Differentiation:** Unlike *C. diphtheriae*, *C. pseudotuberculosis* is **urease positive**. * **Reverse CAMP Test:** It produces a positive "Reverse CAMP" reaction (inhibition of *Staphylococcus aureus* beta-hemolysis), a key laboratory identification feature.

Question 882: Which type of bacteria cannot survive in the absence of oxygen?

A. Obligate aerobe (Correct Answer)
B. Facultative anaerobe
C. Microaerophilic
D. Obligate anaerobe

Explanation: **Explanation:** **1. Why Obligate Aerobe is Correct:** Obligate aerobes are organisms that require oxygen for survival and growth because they rely exclusively on **aerobic respiration**. They use oxygen as the terminal electron acceptor in the electron transport chain to produce ATP. Without oxygen, their metabolic processes cease, leading to cell death. These bacteria possess enzymes like **Superoxide Dismutase (SOD), Catalase, and Peroxidase** to detoxify reactive oxygen species (ROS) produced during metabolism. **2. Why the Other Options are Incorrect:** * **Facultative Anaerobes:** These are versatile organisms. They prefer oxygen (using aerobic respiration) but can switch to fermentation or anaerobic respiration to survive and grow in the **absence** of oxygen (e.g., *E. coli*, *Staphylococci*). * **Microaerophilic:** These bacteria require oxygen to survive, but at **lower concentrations** (typically 5–10%) than what is found in the atmosphere. High oxygen levels can actually be toxic to them (e.g., *Campylobacter*, *Helicobacter pylori*). * **Obligate Anaerobes:** These bacteria **cannot survive** in the presence of oxygen. They lack the necessary enzymes (SOD and Catalase) to neutralize toxic oxygen free radicals (e.g., *Clostridium* species). **3. NEET-PG High-Yield Clinical Pearls:** * **Mnemonic for Obligate Aerobes:** "**N**agging **P**ests **M**ust **B**reathe" (**N**ocardia, **P**seudomonas, **M**ycobacterium tuberculosis, **B**acillus). * **Pseudomonas aeruginosa** is a classic obligate aerobe often implicated in burn wound infections and cystic fibrosis. * **Mycobacterium tuberculosis** predilects the **apex of the lung** because it is the most highly oxygenated area. * **Culture Tip:** Obligate aerobes grow at the very top of a thioglycollate broth tube where oxygen concentration is highest.

Question 883: Which of the following is a catalase-negative, beta-hemolytic Streptococcus?

A. Streptococcus Pneumoniae
B. Streptococcus Pyogenes (Correct Answer)
C. Streptococcus Agalactiae
D. Enterococcus

Explanation: **Explanation:** The identification of Gram-positive cocci in clinical microbiology follows a systematic algorithm based on biochemical and hemolytic properties. **1. Why Streptococcus pyogenes is correct:** * **Catalase Test:** All members of the genus *Streptococcus* are **catalase-negative**, distinguishing them from *Staphylococci* (catalase-positive). * **Hemolysis:** *S. pyogenes* (Group A Streptococcus/GAS) exhibits **Beta-hemolysis** (complete lysis of RBCs) on blood agar, appearing as a clear zone around colonies. * **Bacitracin Sensitivity:** It is uniquely sensitive to Bacitracin, a key diagnostic feature used to differentiate it from other beta-hemolytic streptococci. **2. Analysis of Incorrect Options:** * **Streptococcus pneumoniae:** While catalase-negative, it shows **Alpha-hemolysis** (partial green discoloration) and is bile soluble and optochin sensitive. * **Streptococcus agalactiae (GBS):** It is also a catalase-negative, beta-hemolytic Streptococcus. However, in NEET-PG questions, if both are present, *S. pyogenes* is the classic prototype for "Beta-hemolytic Strep." *S. agalactiae* is distinguished by being **CAMP test positive** and hippurate hydrolysis positive. * **Enterococcus:** Formerly classified as Group D Strep, they are catalase-negative but usually show **Gamma-hemolysis** (non-hemolytic). They are distinguished by their ability to grow in 6.5% NaCl and hydrolyze bile esculin. **High-Yield Clinical Pearls for NEET-PG:** * **M Protein:** The chief virulence factor of *S. pyogenes*; it is anti-phagocytic. * **ASO Titer:** Used to diagnose non-suppurative sequelae like Rheumatic Fever (follows pharyngitis only) and PSGN (follows skin or throat infections). * **PYR Test:** *S. pyogenes* is **PYR positive**, a rapid test used for definitive identification.

Question 884: Which of the following are primarily human pathogenic bacteria?

A. Salmonella typhi
B. S. paratyphi A
C. S. paratyphi B
D. All of the above (Correct Answer)

Explanation: ### **Explanation** The genus *Salmonella* is divided into two main categories based on their host range: **Typhoidal** and **Non-Typhoidal Salmonella (NTS)**. **1. Why the correct answer is "All of the above":** The underlying medical concept is **Host Specificity**. *Salmonella typhi*, *S. paratyphi A*, and *S. paratyphi B* (also known as *S. Schottmuelleri*) are strictly **human pathogens**. They have no known animal reservoir. These serotypes cause **Enteric Fever**, a systemic disease characterized by prolonged fever, malaise, and abdominal symptoms. Because humans are the only natural hosts and reservoirs, transmission occurs exclusively via the fecal-oral route through contaminated food or water handled by cases or chronic carriers. **2. Analysis of Options:** * **Salmonella typhi:** The most common cause of enteric fever worldwide. It is highly adapted to humans and causes the most severe clinical symptoms. * **S. paratyphi A & B:** These cause paratyphoid fever, which is clinically similar to typhoid fever but generally milder. Like *S. typhi*, they are restricted to human hosts. * *Note:* While *S. paratyphi B* is primarily human-restricted, some rare variants (var. Java) can be found in animals, but for the purpose of NEET-PG, the *paratyphi* group is classified as human pathogens. **3. Clinical Pearls for NEET-PG:** * **Reservoir:** Humans are the only reservoir; the **gallbladder** is the most common site of chronic carriage. * **Non-Typhoidal Salmonella (NTS):** Unlike the options above, NTS (e.g., *S. Typhimurium*, *S. Enteritidis*) are **zoonotic**, meaning they inhabit animals (poultry, cattle, reptiles) and cause self-limiting gastroenteritis in humans. * **Diagnosis:** The **Widal test** detects antibodies against the O and H antigens of these specific human pathogens. * **Vaccination:** Vaccines (like the Typhoid Conjugate Vaccine) are only available against *S. typhi*, not the *paratyphi* strains.

Question 885: Which bacterium possesses strains known as Et, Eh, and EP?

A. Escherichia coli (Correct Answer)
B. Mycobacterium
C. Shigella
D. Neisseria

Explanation: **Explanation:** The correct answer is **Escherichia coli**. The abbreviations **ET, EH, and EP** refer to specific pathotypes of *E. coli* categorized based on their virulence factors and clinical presentation. These are part of the diarrheagenic *E. coli* group: * **ET (ETEC):** Enterotoxigenic *E. coli* (causes Traveler’s diarrhea). * **EH (EHEC):** Enterohemorrhagic *E. coli* (associated with HUS and O157:H7). * **EP (EPEC):** Enteropathogenic *E. coli* (causes infantile diarrhea). Other strains include **EI** (Enteroinvasive) and **EA** (Enteroaggregative). **Why other options are incorrect:** * **Mycobacterium:** Classified based on growth rate (Runyon classification) and complex cell wall lipids (Mycolic acid), not these abbreviations. * **Shigella:** Classified into four serogroups (A, B, C, D) based on O-antigen (*S. dysenteriae, S. flexneri, S. boydii, S. sonnei*). * **Neisseria:** Differentiated into species like *N. meningitidis* and *N. gonorrhoeae* based on sugar fermentation (Maltose/Glucose) and capsular polysaccharides. **High-Yield Clinical Pearls for NEET-PG:** * **ETEC:** Produces Heat-labile (LT) and Heat-stable (ST) toxins. LT acts similarly to Cholera toxin (increases cAMP). * **EHEC:** Does not ferment sorbitol (on SMAC agar) and produces Shiga-like toxins (Verocytotoxin). * **EPEC:** Characterized by "Attaching and Effacing" (A/E) lesions on intestinal epithelium. * **EIEC:** Clinically indistinguishable from Shigellosis (dysentery).

Question 886: What is the most common organism causing upper respiratory tract infections in adults?

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

Explanation: ### Explanation **Correct Answer: D. Streptococcus pyogenes** **Reasoning:** Upper Respiratory Tract Infections (URTIs) encompass conditions like pharyngitis, tonsillitis, and sinusitis. While the majority of URTIs are viral (Rhinovirus, Coronavirus), **Streptococcus pyogenes** (Group A Beta-Hemolytic Streptococcus or GABHS) is the **most common bacterial cause** of acute pharyngitis and tonsillitis in both adults and children. It is clinically significant due to its potential to cause post-streptococcal sequelae like Rheumatic Fever and Glomerulonephritis. **Analysis of Incorrect Options:** * **A. Haemophilus influenzae:** While it is a common cause of secondary bacterial infections following a viral cold (like sinusitis or otitis media) and was a major cause of epiglottitis, it is less frequent than *S. pyogenes* in primary pharyngeal infections. * **B. Staphylococcus aureus:** This organism is a common commensal of the anterior nares. While it can cause skin and soft tissue infections or pneumonia, it is rarely a primary pathogen for acute pharyngitis. * **C. Streptococcus pneumoniae:** This is the most common cause of Community-Acquired Pneumonia (CAP), Otitis Media, and Sinusitis, but it is not the primary cause of bacterial pharyngitis/sore throat. **NEET-PG High-Yield Pearls:** * **Centor Criteria:** Used clinically to estimate the probability of GABHS pharyngitis (Fever, Tonsillar exudates, Tender anterior cervical lymphadenopathy, and Absence of cough). * **Gold Standard Diagnosis:** Throat culture on Blood Agar (shows pin-point colonies with a wide zone of beta-hemolysis). * **Drug of Choice:** Penicillin remains the treatment of choice as *S. pyogenes* has not yet developed clinical resistance to it. * **Rapid Test:** Lateral flow immunoassays (RADT) target the Lancefield Group A carbohydrate antigen.

Question 887: Which component of the bacterial cell wall is responsible for endotoxic activity?

A. Region I
B. Region II
C. Region III (Correct Answer)
D. Biovin antigen

Explanation: ***Region III*** - **Region III** corresponds to **Lipid A**, which is the endotoxic component of **lipopolysaccharide (LPS)** in gram-negative bacteria. - **Lipid A** triggers **fever**, **septic shock**, and **cytokine release** (TNF-α, IL-1, IL-6) by activating **Toll-like receptor 4 (TLR4)**. *Region I* - **Region I** represents the **O-antigen** (outermost polysaccharide chain) of LPS, which provides antigenic specificity. - It contributes to **serotyping** and **immune recognition** but lacks **endotoxic activity**. *Region II* - **Region II** corresponds to the **core polysaccharide** that connects the O-antigen to Lipid A. - It provides **structural stability** to LPS but does not possess **endotoxic properties**. *Biovin antigen* - **Boivin antigen** refers to the **complete LPS complex** extracted by phenol-water method, not a specific endotoxic component. - While it contains **Lipid A**, the term describes the entire **LPS structure** rather than the specific endotoxic region.

Question 888: What is the antibiotic of choice for streptococcal pharyngitis?

A. Penicillin (Correct Answer)
B. Ampicillin
C. Erythromycin
D. Vancomycin

Explanation: **Explanation:** **1. Why Penicillin is the Correct Answer:** Penicillin (specifically Penicillin V for oral use or Penicillin G Benzathine for IM use) remains the drug of choice for **Streptococcal pharyngitis** caused by *Streptococcus pyogenes* (Group A Streptococcus). The primary medical rationale is that *S. pyogenes* remains **100% universally susceptible** to penicillin; no clinical resistance has been documented to date. Furthermore, penicillin has a narrow spectrum, is cost-effective, and is proven to prevent the most dreaded non-suppurative complication: **Acute Rheumatic Fever (ARF).** **2. Why Other Options are Incorrect:** * **Ampicillin:** While effective, it offers a broader spectrum than necessary. In cases where pharyngitis is actually due to Infectious Mononucleosis (EBV) rather than Strep, ampicillin can trigger a characteristic maculopapular rash. * **Erythromycin:** This is a macrolide used only as an alternative for patients with a **penicillin allergy**. It is not first-line due to increasing patterns of resistance among Group A Strep. * **Vancomycin:** This is a potent "reserve" antibiotic used for serious infections like MRSA or resistant Enterococci. Using it for simple Strep throat is inappropriate and contributes to antibiotic resistance. **3. NEET-PG High-Yield Clinical Pearls:** * **Prevention:** Treatment of Strep pharyngitis prevents **Acute Rheumatic Fever**, but it does **NOT** prevent **Post-Streptococcal Glomerulonephritis (PSGN)**. * **Diagnosis:** The **Centor Criteria** are used clinically to estimate the probability of bacterial pharyngitis. * **Gold Standard:** Throat culture on Blood Agar (showing beta-hemolysis) remains the gold standard for diagnosis. * **Alternative:** For patients with non-anaphylactic penicillin allergy, **First-generation Cephalosporins** (e.g., Cephalexin) are preferred.

Question 889: A couple, who did not know each other very well, dated and had sexual contact. Several weeks later, the man noticed a small, painless vesicle on his penis, which ruptured and then healed. Soon, his inguinal lymph nodes enlarged and discharged pus through multiple sinus tracts. Lymphogranuloma venereum (LGV) is a venereal disease caused by serotype L1, L2, or L3 of Chlamydia trachomatis. The differential diagnosis should include which of the following?

A. Babesiosis
B. Chancroid (Correct Answer)
C. Mononucleosis
D. Psittacosis

Explanation: **Explanation:** The clinical presentation describes **Lymphogranuloma venereum (LGV)**, caused by *Chlamydia trachomatis* (serotypes L1, L2, L3). [5] LGV typically presents in three stages: a painless primary vesicle/ulcer, followed by painful inguinal lymphadenopathy (buboes) that may develop "multilocular" suppurative sinus tracts, and finally, chronic proctocolitis or elephantiasis. [4][3] **Why Chancroid is the correct differential:** **Chancroid**, caused by *Haemophilus ducreyi*, is a major differential for any genital ulcer disease associated with inguinal lymphadenopathy. [1] While the primary ulcer in Chancroid is painful (unlike LGV), both conditions are characterized by the formation of **suppurative inguinal buboes** that can rupture and form sinus tracts. [2] In the context of sexually transmitted infections (STIs) with significant inguinal involvement, Chancroid must be ruled out. **Why other options are incorrect:** * **Babesiosis:** A tick-borne parasitic infection (caused by *Babesia microti*) that presents with malaria-like symptoms (fever, hemolytic anemia) and is not an STI. * **Mononucleosis:** Caused by Epstein-Barr Virus (EBV); presents with generalized lymphadenopathy, pharyngitis, and splenomegaly, but not genital ulcers or suppurative inguinal sinus tracts. * **Psittacosis:** Caused by *Chlamydia psittaci*; it is a respiratory infection (atypical pneumonia) acquired from infected birds, not a venereal disease. **NEET-PG Clinical Pearls:** * **LGV "Groove Sign":** Inguinal and femoral lymph nodes separated by the inguinal ligament (pathognomonic but not always present). * **Chancroid:** Remember the mnemonic "**H. ducreyi** makes you **cry**" (painful ulcer). * **Syphilis:** Painless ulcer (chancre) with painless, non-suppurative lymphadenopathy. [1] * **Donovanosis (Granuloma Inguinale):** Caused by *Klebsiella granulomatis*; presents with beefy red, painless ulcers; characterized by **Donovan bodies** on biopsy. [1]

Question 890: Which of the following is used as a biological indicator for control in plasma sterilization?

A. Bacillus subtilis
B. Bacillus stearothermophilus (Correct Answer)
C. Clostridium tetani
D. Clostridium perfringens

Explanation: **Explanation:** The correct answer is **Bacillus stearothermophilus** (Option B). Biological indicators (BIs) are considered the "gold standard" for sterilization monitoring because they challenge the process using highly resistant bacterial spores. **Plasma sterilization** (specifically Hydrogen Peroxide Gas Plasma) utilizes low-temperature, vacuum-assisted technology to kill microbes via free radicals. **Bacillus stearothermophilus** (now often reclassified as *Geobacillus stearothermophilus*) is the specific indicator used for both **Plasma sterilization** and **Autoclaving (Steam under pressure)**. Its extreme thermophilic nature and resistance to oxidative stress make it the ideal surrogate to ensure all viable microorganisms, including spores, have been eliminated. **Analysis of Incorrect Options:** * **Bacillus subtilis (Option A):** Historically used as the indicator for **Hot Air Oven (Dry Heat)** and **Ethylene Oxide (ETO)** sterilization. While it is a spore-former, it is not the standard for plasma or steam. * **Clostridium tetani (Option C) & Clostridium perfringens (Option D):** While these are spore-forming anaerobes, they are pathogenic and lack the standardized, extreme resistance required to serve as reliable indicators for industrial or clinical sterilization validation. **High-Yield Clinical Pearls for NEET-PG:** * **Autoclave/Plasma:** *Bacillus stearothermophilus* * **Hot Air Oven/ETO:** *Bacillus subtilis* (var. *niger*) * **Ionizing Radiation (Gamma rays):** *Bacillus pumilus* * **Filtration:** *Brevundimonas diminuta* * **D-Value:** The time required to reduce the microbial population by 90% (1 log) under specific conditions.

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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