Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1891: Asymptomatic gonorrhoea in females is typically due to infections of which site?

A. Vagina
B. Urethra
C. Endocervix (Correct Answer)
D. Fundus

Explanation: **Explanation:** The **endocervix** is the primary site of infection for *Neisseria gonorrhoeae* in females. This is because the organism has a specific predilection for **columnar epithelium**, which lines the endocervical canal. In approximately 50–70% of cases, endocervical gonorrhea remains **asymptomatic**, acting as a major reservoir for transmission and increasing the risk of unrecognized progression to Pelvic Inflammatory Disease (PID). **Analysis of Options:** * **Vagina (Incorrect):** The adult vagina is lined by stratified squamous epithelium, which is resistant to gonococcal infection. Vaginal involvement (vulvovaginitis) is typically only seen in prepubertal girls where the epithelium is thin. * **Urethra (Incorrect):** While the urethra is frequently co-infected (causing dysuria), it is not the primary site of asymptomatic colonization in females. * **Fundus (Incorrect):** The fundus refers to the upper part of the uterus. While infection can ascend to the endometrium, the primary and most common site of initial infection is the cervix. **Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Culture on **Thayer-Martin medium** (a selective Chocolate agar containing Vancomycin, Colistin, and Nystatin). * **Most Sensitive Test:** Nucleic Acid Amplification Test (NAAT). * **Microscopy:** Gram-negative kidney-shaped diplococci found within polymorphonuclear leukocytes (intracellular). * **Fitz-Hugh-Curtis Syndrome:** A complication of gonococcal PID involving peri-hepatitis ("violin-string" adhesions). * **Treatment:** Current CDC/WHO guidelines recommend Ceftriaxone (IM) as the mainstay of therapy.

Question 1892: What is true about Mycoplasma?

A. Obligate intracellular organism
B. Penicillin is an effective treatment
C. Requires cholesterol for growth (Correct Answer)
D. Has a thick cell wall

Explanation: **Explanation:** **Mycoplasma** are the smallest free-living organisms. The defining characteristic of Mycoplasma is the **complete absence of a cell wall**. 1. **Why Option C is Correct:** Since Mycoplasma lack a cell wall, their cell membrane is their only protective barrier. To provide structural integrity and stability to this membrane, they incorporate **sterols (cholesterol)**. Unlike most bacteria, Mycoplasma cannot synthesize sterols and must acquire them from the external environment (e.g., PPLO agar supplemented with horse serum). 2. **Why Other Options are Incorrect:** * **Option A:** Mycoplasma are **extracellular** pathogens (except *M. penetrans*). They typically adhere to the respiratory or urogenital epithelium. * **Option B:** Penicillin acts by inhibiting cell wall synthesis (peptidoglycan). Because Mycoplasma **lack a cell wall**, they are **innately resistant** to all beta-lactam antibiotics (Penicillins, Cephalosporins). * **Option D:** As mentioned, they have **no cell wall**, making them highly pleomorphic (able to take many shapes) and resistant to Gram staining. **High-Yield Clinical Pearls for NEET-PG:** * **Culture:** They produce characteristic **"Fried Egg" colonies** on agar. * **Diagnosis:** *M. pneumoniae* causes **Atypical Pneumonia** (Walking Pneumonia). It is associated with **Cold Agglutinins** (anti-I antibodies) which can cause autoimmune hemolytic anemia. * **Treatment:** Drug of choice is **Macrolides** (Azithromycin) or Tetracyclines (Doxycycline). * **Staining:** They do not Gram stain; **Giemsa or Diene’s stain** is used instead.

Question 1893: Which of the following is NOT true about Shigella dysenteriae?

A. It invades the colonic mucosa.
B. It can cause Hemolytic Uremic Syndrome (HUS).
C. It produces Shiga toxin (verocytotoxin).
D. It produces the mildest form of dysentery. (Correct Answer)

Explanation: **Explanation:** The correct answer is **D**. *Shigella dysenteriae* (specifically Serotype 1) is the most virulent species of the genus and causes the **most severe** form of bacillary dysentery. It is associated with extensive mucosal ulceration, high fever, and life-threatening complications. In contrast, *Shigella sonnei* is responsible for the mildest form of the disease, often presenting as self-limiting watery diarrhea. **Analysis of other options:** * **A. Invades the colonic mucosa:** This is a hallmark of *Shigella* pathogenesis. The bacteria enter M cells in Peyer’s patches and spread laterally between colonic epithelial cells using actin tails (similar to *Listeria*), leading to inflammation and abscess formation. * **B. Can cause Hemolytic Uremic Syndrome (HUS):** *S. dysenteriae* Type 1 produces Shiga toxin, which can enter the bloodstream and damage glomerular endothelial cells. This leads to the triad of HUS: Microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. * **C. Produces Shiga toxin (verocytotoxin):** *S. dysenteriae* produces the potent Shiga toxin (Stx), which has one A subunit and five B subunits. It inhibits protein synthesis by inactivating the 60S ribosomal subunit. **High-Yield Clinical Pearls for NEET-PG:** * **Infective Dose:** Very low (10–100 organisms), making it highly communicable. * **Motility:** Non-motile (distinguishes it from *Salmonella*). * **Biochemicals:** Catalase positive (except *S. dysenteriae* Type 1), Oxidase negative, and Non-lactose fermenters (except *S. sonnei*, which is a late lactose fermenter). * **Species Severity:** *S. dysenteriae* (Most severe) > *S. flexneri* (Most common in India) > *S. boydii* > *S. sonnei* (Mildest/Most common in developed countries).

Question 1894: Which of the following is NOT a typical outcome of diphtheria infection?

A. Pseudomembrane formation
B. Cardiomyopathy
C. Endotoxemia (Correct Answer)
D. Polyneuropathy

Explanation: **Explanation:** The correct answer is **Endotoxemia** because *Corynebacterium diphtheriae* is a Gram-positive bacterium. Endotoxins (Lipopolysaccharides) are structural components found exclusively in the outer membrane of **Gram-negative** bacteria. Diphtheria causes pathology primarily through the release of a potent **Exotoxin**, which inhibits protein synthesis via ADP-ribosylation of Elongation Factor-2 (EF-2). **Analysis of Options:** * **Pseudomembrane formation:** This is the hallmark of respiratory diphtheria. The exotoxin causes local tissue necrosis, leading to an inflammatory exudate composed of fibrin, leukocytes, and dead epithelial cells that forms a tough, greyish-white membrane over the tonsils and pharynx. * **Cardiomyopathy:** The diphtheria toxin enters the bloodstream (toxemia) and has a high affinity for cardiac tissue. It can cause acute myocarditis, leading to heart failure or arrhythmias, typically occurring 1–2 weeks after the onset of local symptoms. * **Polyneuropathy:** The toxin also targets the nervous system by causing demyelination. This often manifests as soft palate paralysis, followed by oculomotor disturbances and peripheral polyneuropathy. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Diphtheria toxin is encoded by the **tox gene** introduced by a lysogenic bacteriophage (**Beta-phage**). * **Diagnosis:** Use **Albert’s stain** to visualize metachromatic granules (Volutin/Babes-Ernst granules). * **Culture:** Grows on **Löffler's serum slope** (rapid growth) and **Potassium Tellurite agar** (black colonies). * **Virulence Test:** The **Elek’s gel precipitation test** is used to detect toxin production. * **Treatment:** Immediate administration of **Diphtheria Antitoxin (ADS)** is the priority, as it only neutralizes unbound toxin.

Question 1895: What is the cause of skin lesions in meningococcal meningitis?

A. Exotoxin
B. Endotoxin (Correct Answer)
C. Allergic reaction
D. Direct vascular damage

Explanation: **Explanation:** The characteristic skin lesions in meningococcal meningitis (petechiae and purpura) are primarily caused by the **Endotoxin** of *Neisseria meningitidis*. 1. **Why Endotoxin is correct:** Unlike most Gram-negative bacteria that possess Lipopolysaccharide (LPS), *N. meningitidis* contains **Lipooligosaccharide (LOS)** in its outer membrane. During rapid bacterial proliferation and autolysis, large amounts of LOS are released into the bloodstream (endotoxemia). This triggers a massive inflammatory cascade, activating the complement system and coagulation pathways. This leads to **Disseminated Intravascular Coagulation (DIC)**, vascular endothelial damage, and capillary leakage, manifesting clinically as petechial rashes and purpura fulminans. 2. **Why other options are incorrect:** * **Exotoxin:** *Neisseria meningitidis* does not produce any exotoxins. Its virulence is derived from its capsule and LOS. * **Allergic reaction:** The lesions are a result of acute inflammatory and thrombotic processes, not a Type I or Type IV hypersensitivity reaction. * **Direct vascular damage:** While vascular damage occurs, it is a *secondary* effect mediated by the endotoxin-induced inflammatory surge and microthrombi, rather than direct mechanical invasion of the vessel walls by the bacteria alone. **High-Yield Clinical Pearls for NEET-PG:** * **Waterhouse-Friderichsen Syndrome:** Severe meningococcemia leading to bilateral adrenal hemorrhage and circulatory collapse. * **Virulence Factor:** The **Polysaccharide capsule** is responsible for grouping and resisting phagocytosis, but the **LOS (Endotoxin)** correlates with the severity of the disease and skin manifestations. * **Drug of Choice:** Ceftriaxone is the preferred treatment; Chemoprophylaxis for contacts is Rifampicin (or Ciprofloxacin/Ceftriaxone).

Question 1896: Which of the following statements about Listeria is true?

A. It is a Gram-negative bacillus.
B. It is motile by peritrichous flagella. (Correct Answer)
C. It is the commonest cause of community-acquired meningitis.
D. Only one serovar of Listeria is known.

Explanation: **Explanation:** *Listeria monocytogenes* is a unique Gram-positive, non-spore-forming bacillus with several high-yield characteristics relevant to clinical practice and exams. **Why Option B is Correct:** Listeria exhibits a characteristic temperature-dependent motility. At room temperature (**20–25°C**), it develops **peritrichous flagella**, resulting in a distinctive **"tumbling motility"** seen on hanging drop preparations. At 37°C, flagella production is downregulated, and it moves intracellularly via actin polymerization ("actin rockets"). **Why Other Options are Incorrect:** * **Option A:** Listeria is a **Gram-positive** bacillus (often described as coccobacillary). It can sometimes be confused with *Corynebacterium* (diphtheroids) or *S. pneumoniae*. * **Option C:** While it is a significant cause of meningitis in specific populations (neonates, elderly, and immunocompromised), the **most common** cause of community-acquired meningitis in the general adult population is *Streptococcus pneumoniae*. * **Option D:** There are **13 known serotypes** of *L. monocytogenes* based on somatic (O) and flagellar (H) antigens. Serotypes 1/2a, 1/2b, and 4b are responsible for the majority of human infections. **High-Yield Clinical Pearls for NEET-PG:** * **Cold Enrichment:** It can grow at temperatures as low as 4°C, a property used for selective isolation from contaminated food (e.g., unpasteurized cheese, cold cuts). * **CAMP Test Positive:** It produces a rectangular (not arrowhead) zone of hemolysis when streaked with *Staphylococcus aureus*. * **Catalase Positive:** This helps differentiate it from *Group B Streptococcus* (GBS), which is catalase-negative but also causes neonatal meningitis. * **Treatment:** **Ampicillin** is the drug of choice. It is inherently resistant to all cephalosporins.

Question 1897: Enteric fever is caused by which of the following organisms?

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

Explanation: **Explanation:** **Enteric fever** is a systemic infection caused by specific serotypes of the Gram-negative bacterium *Salmonella enterica*. The term "Enteric Fever" is a collective clinical designation that encompasses both **Typhoid fever** and **Paratyphoid fever**. 1. **Salmonella typhi:** This is the most common and virulent cause, responsible for classic Typhoid fever. 2. **Salmonella paratyphi A, B, and C:** These organisms cause Paratyphoid fever. While the clinical presentation is similar to typhoid fever, it is generally milder and has a shorter incubation period. Since both *S. typhi* and all three strains of *S. paratyphi* (A, B, and C) are etiologic agents of the clinical syndrome known as enteric fever, **Option D (All of the above)** is the correct answer. **Why individual options are insufficient:** Options A, B, and C are all individual causative agents. Selecting only one would be incomplete, as enteric fever is not restricted to a single species of *Salmonella*. **High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Fecal-oral route (contaminated food and water). * **Pathogenesis:** Organisms multiply in the **Peyer’s patches** of the terminal ileum and spread via the gallbladder and biliary tract. * **Clinical Signs:** Step-ladder pyrexia, relative bradycardia (**Faget’s sign**), rose spots, and hepatosplenomegaly. * **Diagnosis:** The **Widal test** (detects antibodies against O and H antigens) is usually positive after the first week. * **Gold Standard Diagnosis:** Bone marrow culture (highest sensitivity), though blood culture is the standard in the first week. * **Chronic Carrier State:** Defined as excretion of bacilli in stool for >1 year; the **gallbladder** is the primary reservoir.

Question 1898: Which of the following findings is true regarding Cerebrospinal Fluid (CSF) in pyogenic meningitis?

A. Protein elevated, sugar decreased (Correct Answer)
B. Protein decreased, sugar elevated
C. Chloride elevated, protein decreased
D. Protein decreased, chloride decreased

Explanation: In pyogenic (bacterial) meningitis, the CSF profile undergoes characteristic changes due to the inflammatory response and bacterial metabolism. ### **Explanation of the Correct Answer** **Option A (Protein elevated, sugar decreased)** is correct because: 1. **Elevated Protein:** Inflammation causes increased permeability of the blood-brain barrier (BBB) and the presence of bacterial proteins and inflammatory exudate, leading to a significant rise in CSF protein levels (typically >100 mg/dL). 2. **Decreased Sugar (Hypoglycorrhachia):** Bacteria and infiltrating polymorphonuclear leukocytes (neutrophils) consume glucose for metabolism. Additionally, there is impaired glucose transport across the BBB. CSF glucose is usually <40 mg/dL or less than 40% of simultaneous blood glucose. ### **Analysis of Incorrect Options** * **Option B:** Incorrect because sugar is consumed, not elevated, and proteins always rise during active infection. * **Options C & D:** Chloride levels in the CSF typically **decrease** in meningitis (especially in Tubercular Meningitis), but they are not the primary diagnostic markers compared to protein and glucose. Protein is never decreased in pyogenic infections. ### **NEET-PG High-Yield Pearls** * **Cell Count:** Pyogenic meningitis shows marked **Neutrophilic pleocytosis** (often >1000 cells/mm³). * **Tubercular Meningitis (TBM):** Characterized by **Lymphocytic** predominance, very high protein, and the most significant drop in **Chloride** levels. * **Viral (Aseptic) Meningitis:** Normal sugar, slightly elevated protein, and lymphocytic predominance. * **Normal CSF Glucose:** Usually 2/3rd (60-70%) of the plasma glucose level.

Question 1899: Which of the following clostridia species is non-saccharolytic?

A. Clostridium welchii
B. Clostridium sporogenes (Correct Answer)
C. Clostridium septicum
D. Clostridium oedematiens

Explanation: ### Explanation The classification of *Clostridium* species is often based on their metabolic activity, specifically their ability to ferment carbohydrates (**saccharolytic**) or break down proteins (**proteolytic**). **1. Why Clostridium sporogenes is correct:** *Clostridium sporogenes* is primarily **proteolytic and non-saccharolytic**. It lacks the enzymes necessary to ferment common sugars like glucose or lactose. Instead, it derives energy from the breakdown of amino acids (the Stickland reaction). In laboratory cultures, it is known for producing a putrid odor due to protein decomposition and is often considered a non-pathogenic relative of *C. botulinum*. **2. Why the other options are incorrect:** * **Clostridium welchii (C. perfringens):** This is a highly **saccharolytic** organism. It fermentatively produces large amounts of gas from lactose, leading to the characteristic "stormy fermentation" in litmus milk—a high-yield diagnostic feature for gas gangrene. * **Clostridium septicum:** This species is also **saccharolytic** and can ferment glucose, lactose, and maltose. It is frequently associated with atraumatic gas gangrene in patients with underlying colon cancer. * **Clostridium oedematiens (C. novyi):** This species is **saccharolytic**, fermenting glucose and maltose. It is a major causative agent of gas gangrene, characterized by profound edema. **3. NEET-PG High-Yield Pearls:** * **Non-saccharolytic Clostridia:** *C. sporogenes* and *C. tetani* (though *C. tetani* is weakly proteolytic, it is primarily non-saccharolytic). * **Stormy Fermentation:** Pathognomonic for *C. perfringens*. * **Nagler’s Reaction:** Used to detect the Lecithinase (Alpha toxin) produced by *C. perfringens*. * **Medusa Head Colonies:** Characteristic of *C. sporogenes* (similar to *B. anthracis*).

Question 1900: What constitutes the normal bacterial flora of the conjunctiva?

A. Corynebacterium xerosis (Correct Answer)
B. Escherichia coli
C. Streptococci
D. All of the above

Explanation: **Explanation:** The conjunctiva is a relatively protected environment due to the continuous flushing action of tears, which contain antimicrobial substances like **lysozyme**, lactoferrin, and IgA. Consequently, the normal flora is sparse. **1. Why Corynebacterium xerosis is correct:** The most common inhabitants of the healthy conjunctiva are Gram-positive bacteria. **Corynebacterium xerosis** (a non-pathogenic diphtheroid) and **Staphylococcus epidermidis** (Coagulase-negative Staphylococci) are the predominant commensals. These organisms are adapted to the ocular surface and rarely cause disease unless the local immunity is compromised or the ocular surface is breached. **2. Why the other options are incorrect:** * **Escherichia coli:** This is a Gram-negative coliform primarily found in the intestinal tract. Its presence in the conjunctiva is considered transient or indicative of fecal-oral contamination; it is not part of the resident normal flora. * **Streptococci:** While certain species (like *S. pneumoniae*) can occasionally be found in small numbers, they are more commonly associated with acute bacterial conjunctivitis rather than being established resident flora. * **All of the above:** Since *E. coli* is not a resident commensal of the eye, this option is incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **Lysozyme:** This enzyme in tears specifically targets the peptidoglycan layer of Gram-positive bacteria, keeping the bacterial population in check. * **Most common cause of Ophthalmia Neonatorum:** *Chlamydia trachomatis* (globally) and *Neisseria gonorrhoeae* (most severe). * **Contact Lens Wearers:** Be alert for *Pseudomonas aeruginosa* and *Acanthamoeba* keratitis. * **Post-operative Endophthalmitis:** Most commonly caused by the patient's own flora, specifically **Staphylococcus epidermidis**.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

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

Practice Questions

Spirochetes

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