Bacteriology Practice Questions

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

It produces the mildest form of dysentery.. **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).

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

Endotoxemia. **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.

Q: A 28-year-old sexually active man presents with a new, painless genital ulcer on his penis that he noticed 1 week ago. The ulcer is 1 cm in size, with an eroded base and an indurated margin. Dark-field examination of the ulcer fluid confirmed the diagnosis. What is the most likely infecting organism?

Syphilis. ### Explanation The clinical presentation describes a classic **Primary Syphilis** infection caused by the spirochete ***Treponema pallidum*** [1]. **Why Syphilis is correct:** The hallmark of primary syphilis is the **chancre**. It typically presents as a single, **painless**, well-circumscribed ulcer with a clean base and **indurated (firm) margins**. The incubation period is usually 3 weeks (range 10–90 days). **Dark-field microscopy** is the gold standard for immediate diagnosis of primary syphilis, as it allows for the direct visualization of motile spirochetes from the ulcer exudate before serological tests (like VDRL/RPR) become positive [1]. **Why the other options are incorrect:** * **Toxoplasmosis:** Caused by *Toxoplasma gondii*, it typically presents as a systemic febrile illness with lymphadenopathy or CNS involvement in immunocompromised patients, not genital ulcers. * **Tetanus:** Caused by *Clostridium tetani*, it presents with neuromuscular symptoms like lockjaw (trismus) and muscle spasms following a contaminated wound; it does not cause genital lesions. * **Smallpox:** Caused by the Variola virus, it presents with a systemic centrifugal rash (pustules) and has been eradicated globally. **NEET-PG High-Yield Pearls:** * **Painful vs. Painless:** Syphilis (Chancre) is **painless**, whereas Chancroid (*Haemophilus ducreyi*) is **painful** ("You *do cry* with *ducreyi*"). * **Diagnosis:** VDRL/RPR are screening tests (non-specific); FTA-ABS/TPPA are confirmatory (specific). * **Treatment:** The drug of choice for all stages of syphilis is **Benzathine Penicillin G**. * **Jarisch-Herxheimer Reaction:** An acute febrile reaction occurring within 24 hours of starting penicillin treatment for syphilis due to the release of endotoxins from dying spirochetes.

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

Endotoxin. **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).

Q: Which of the following statements about Listeria is true?

It is motile by peritrichous flagella.. **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.

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

Protein elevated, sugar decreased. 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.

Q: Which of the following Clostridia species are saccharolytic?

Sporogenes. **Explanation:** Clostridia are classified based on their metabolic activity into **Saccharolytic** (carbohydrate-fermenting) and **Proteolytic** (protein-digesting) groups. **The Correct Answer: B. Sporogenes** *Clostridium sporogenes* is primarily a **proteolytic** organism. It is often referred to as the "non-toxigenic counterpart" of *C. botulinum*. In the laboratory, it is characterized by its ability to digest cooked meat (turning it black with a foul odor) rather than fermenting sugars. *Note: There appears to be a discrepancy in the provided key. In standard microbiological classification (e.g., Ananthanarayan), C. welchii (perfringens), C. septicum, and C. oedematiens are predominantly **saccharolytic**, while C. sporogenes and C. tetani are predominantly **proteolytic**. If the question asks for a saccharolytic species, A, C, and D are correct; if it asks for a proteolytic species, B is the correct answer.* **Analysis of Other Options:** * **A. Welchii (C. perfringens):** Highly saccharolytic. It produces "stormy fermentation" in litmus milk due to vigorous acid and gas production from lactose. * **C. Septicum:** Strongly saccharolytic; it ferments glucose, lactose, and maltose. It is a common cause of gas gangrene in patients with underlying malignancies. * **D. Oedematiens (C. novyi):** Saccharolytic; it ferments glucose and maltose (though not lactose). **High-Yield NEET-PG Pearls:** 1. **Stormy Fermentation:** Pathognomonic for *C. perfringens*. 2. **Nagler’s Reaction:** Used to detect Lecithinase (Alpha-toxin) produced by *C. perfringens*. 3. **Cooked Meat Medium (RCM):** Saccharolytic species turn the meat **pink**, while proteolytic species turn the meat **black** and produce a putrid smell. 4. **C. septicum** is strongly associated with **colonic carcinoma**.

Q: All of the following are virulence factors of pertussis, except?

Capsule. **Explanation:** The correct answer is **Capsule**. While *Bordetella pertussis* is a Gram-negative coccobacillus, it is generally considered **non-capsulated**. Its pathogenicity relies on a complex array of toxins and adhesins rather than a polysaccharide capsule. **Why other options are incorrect (Virulence Factors of *B. pertussis*):** * **Tracheal Cytotoxin (TCT):** A peptidoglycan fragment that specifically destroys ciliated epithelial cells and inhibits DNA synthesis, leading to the characteristic failure of the "mucociliary escalator." * **Pertussis Toxin (PT):** An A-B toxin that causes ADP-ribosylation of Gi proteins. This leads to increased cAMP levels, resulting in lymphocytosis (a classic diagnostic clue), histamine sensitization, and insulin secretion. * **Pertactin:** An outer membrane protein that acts as an **adhesin**, facilitating the attachment of the bacteria to the respiratory epithelium. It is a key component of the acellular pertussis (aP) vaccine. **High-Yield Clinical Pearls for NEET-PG:** * **Filamentous Hemagglutinin (FHA):** Another critical adhesin required for colonization. * **Lymphocytosis:** Unlike most bacterial infections that cause neutrophilia, Pertussis causes a marked absolute lymphocytosis due to the Pertussis toxin blocking lymphocyte entry into lymph nodes. * **Gold Standard Diagnosis:** Culture on **Regan-Lowe medium** or **Bordet-Gengou (potato-blood-glycerol) agar** (showing "mercury drop" or "bisected pearl" colonies). * **Vaccine:** The acellular vaccine (aP) contains PT, FHA, and Pertactin, which are less reactogenic than the whole-cell vaccine.

Q: A patient with alkaline urine which is cloudy with plenty of pus cells is suffering from which type of infection?

Proteus. ### Explanation The correct answer is **Proteus**. **Why Proteus is the correct answer:** The hallmark of a *Proteus* species infection (most commonly *P. mirabilis*) is the production of the enzyme **Urease**. This enzyme hydrolyzes urea into ammonia and carbon dioxide. The release of ammonia increases the pH of the urine, making it **alkaline** (pH > 7.0). Alkaline urine reduces the solubility of magnesium and phosphate, leading to the formation of **Struvite stones** (Magnesium Ammonium Phosphate) and "cloudy" urine due to crystalluria and pyuria (pus cells). **Why other options are incorrect:** * **E. coli:** This is the most common cause of UTIs. However, *E. coli* does not produce urease; therefore, the urine remains **acidic** or neutral. It does not typically cause the marked alkalinity seen with *Proteus*. * **Tuberculosis (Renal TB):** Renal TB is classically associated with **"Sterile Pyuria"**—the presence of pus cells in acidic urine with negative routine bacterial cultures. It does not cause alkaline urine. **High-Yield Clinical Pearls for NEET-PG:** * **Staghorn Calculi:** *Proteus* is the most common organism associated with large staghorn stones composed of Struvite. * **Motility:** *Proteus* exhibits characteristic **"Swarming growth"** on blood agar due to its peritrichous flagella. * **Dienes Phenomenon:** A method used to differentiate two strains of *Proteus* based on the formation of a visible line of inhibition where two different swarming strains meet. * **Weil-Felix Reaction:** Certain *Proteus* antigens (OX19, OX2, OXK) cross-react with Rickettsial antibodies, a classic (though less specific) diagnostic test.

Question 1

Which of the following is NOT true about Shigella dysenteriae?

Accepted Answer

It produces the mildest form of dysentery.

Answer

It invades the colonic mucosa.

Answer

It can cause Hemolytic Uremic Syndrome (HUS).

Answer

It produces Shiga toxin (verocytotoxin).

Question 2

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

Accepted Answer

Endotoxemia

Answer

Pseudomembrane formation

Answer

Cardiomyopathy

Answer

Polyneuropathy

Question 3

A 28-year-old sexually active man presents with a new, painless genital ulcer on his penis that he noticed 1 week ago. The ulcer is 1 cm in size, with an eroded base and an indurated margin. Dark-field examination of the ulcer fluid confirmed the diagnosis. What is the most likely infecting organism?

Accepted Answer

Syphilis

Answer

Toxoplasmosis

Answer

Tetanus

Answer

Smallpox

Question 4

What is the cause of skin lesions in meningococcal meningitis?

Accepted Answer

Endotoxin

Answer

Exotoxin

Answer

Allergic reaction

Answer

Direct vascular damage

Question 5

Which of the following statements about Listeria is true?

Accepted Answer

It is motile by peritrichous flagella.

Answer

It is a Gram-negative bacillus.

Answer

It is the commonest cause of community-acquired meningitis.

Answer

Only one serovar of Listeria is known.

Question 6

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

Accepted Answer

Protein elevated, sugar decreased

Answer

Protein decreased, sugar elevated

Answer

Chloride elevated, protein decreased

Answer

Protein decreased, chloride decreased

Question 7

Which of the following Clostridia species are saccharolytic?

Accepted Answer

Sporogenes

Answer

Welchii

Answer

Septicum

Answer

Oedematiens

Question 8

All of the following are virulence factors of pertussis, except?

Accepted Answer

Capsule

Answer

Tracheal cytotoxin

Answer

Pertussis toxin

Answer

Pertactin

Question 9

A 12-year-old boy presents with a productive cough and large volumes of foul-smelling sputum. Three years prior, he was diagnosed with pancreatic insufficiency, evidenced by recurrent gastrointestinal symptoms of steatorrhea. Sputum culture reveals colorless, oxidase-positive colonies with a fruity aroma. The function of which of the following proteins is most likely inhibited by the bacteria responsible for this boy's infection?

Accepted Answer

A GTP-binding protein involved in the elongation step of protein synthesis

Answer

A CFTR protein lacking a phenylalanine at position 508 in exon 10

Answer

A GTP-binding protein coupled with α2-adrenergic receptors

Answer

A GTP-binding protein coupled with β-adrenergic receptors

Question 10

A patient with alkaline urine which is cloudy with plenty of pus cells is suffering from which type of infection?

Accepted Answer

Proteus

Answer

E. coli

Answer

Tuberculosis

Answer

None of the above

Bacteriology — MCQs

Bacteriology — MCQs

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