Bacteriology Practice Questions

Q: Which of the following staphylococcal proteins functions as a superantigen?

Exfoliative toxin. ### Explanation **Correct Answer: A. Exfoliative toxin** **Concept:** Superantigens are potent immunostimulatory molecules that bypass normal antigen processing. Instead of being presented in the MHC II groove, they bind directly to the **outer surface of MHC II** on antigen-presenting cells and the **Vβ region of T-cell receptors (TCR)**. This results in a massive, non-specific activation of T-cells (up to 20%), leading to a "cytokine storm" (IL-1, IL-2, TNF-α, and IFN-γ). Staphylococcal superantigens include: 1. **Toxic Shock Syndrome Toxin-1 (TSST-1)** 2. **Staphylococcal Enterotoxins** (A-E, cause food poisoning) 3. **Exfoliative Toxins (ETA, ETB):** These cause Staphylococcal Scalded Skin Syndrome (SSSS) by acting as superantigens and proteases that cleave **desmoglein-1**, leading to epidermal separation. --- ### Why the other options are incorrect: * **B. Cytolytic toxins:** (e.g., Alpha, Beta, Gamma, Delta toxins) These act by forming pores in the host cell membrane or through enzymatic action, leading to cell lysis, rather than T-cell activation. * **C. Protein A:** This is a surface protein that binds to the **Fc portion of IgG**, preventing opsonization and phagocytosis. It is an antiphagocytic virulence factor, not a superantigen. * **D. Leucocidin:** (e.g., Panton-Valentine Leucocidin) This is a pore-forming toxin that specifically targets and kills polymorphonuclear leukocytes (neutrophils) and macrophages. --- ### High-Yield Facts for NEET-PG: * **MHC Binding:** Superantigens bind to the **Vβ chain** of the TCR. * **SSSS vs. TEN:** In SSSS (caused by Exfoliative toxin), the cleavage occurs at the **stratum granulosum**, and the Nikolsky sign is positive. Unlike Toxic Epidermal Necrolysis (TEN), there is typically no mucosal involvement. * **Enterotoxin:** Staphylococcal enterotoxin is **heat-stable** and causes "pre-formed toxin" food poisoning (short incubation: 1–6 hours).

Q: A child attending preschool presents with multiple pustular skin lesions on the arms, some of which are crusted with a yellow exudate. Which of the following virulence factors of the most likely etiologic agent protects it from phagocytosis and provides serologic specificity?

M protein. ### Explanation The clinical presentation of pustular lesions with a "honey-colored" yellow crust in a preschool child is classic for **Impetigo**, most commonly caused by **Group A Streptococcus (GAS)**, also known as *Streptococcus pyogenes*. **Why M protein is the correct answer:** The **M protein** is the chief virulence factor of *S. pyogenes*. It is a hair-like projection from the cell wall that acts by: 1. **Antiphagocytic action:** It inhibits opsonization by interfering with the alternative complement pathway (degrading C3b). 2. **Serologic Specificity:** There are over 80 different serotypes of M protein, which determine the specific strain of GAS. This variability is why individuals can suffer from repeated GAS infections. **Analysis of Incorrect Options:** * **A. Erythrogenic toxin:** Also known as Pyrogenic Exotoxin, this is responsible for the rash in Scarlet Fever and Streptococcal Toxic Shock Syndrome, not for preventing phagocytosis. * **B. Hyaluronic acid capsule:** While this also provides antiphagocytic properties by mimicking human connective tissue (molecular mimicry), it is **not** used for serologic specificity because it is chemically identical to human host tissue and thus non-immunogenic. * **C. Lipoteichoic acid:** This is primarily involved in the **adhesion** of the bacteria to the pharyngeal epithelium (via fibronectin), not in preventing phagocytosis. **NEET-PG High-Yield Pearls:** * **M Protein & Sequelae:** Certain M types are "nephritogenic" (e.g., Type 12, 49) leading to PSGN, while others are "rheumatogenic" (e.g., Type 1, 3, 5, 6, 18) leading to Rheumatic Fever. * **Diagnosis:** Impetigo is usually diagnosed clinically. If cultured, GAS shows **Gram-positive cocci in chains** and is **Catalase negative** and **Bacitracin sensitive**. * **Major Virulence Factor:** If a question asks for the *primary* factor for both protection and typing in GAS, always choose M protein.

Q: Which disease is caused by Bartonella quintana?

Trench fever. **Explanation:** **Bartonella quintana** is a Gram-negative, fastidious bacterium primarily transmitted by the **human body louse** (*Pediculus humanus corporis*). It is the causative agent of **Trench fever** (Option A), a disease historically significant during World War I. It is characterized by a relapsing fever (often in 5-day cycles, hence the name "quintana"), headache, and severe pretibial (shin) pain. In modern clinical practice, it is also associated with bacillary angiomatosis and culture-negative endocarditis, particularly in homeless populations or those with poor hygiene. **Analysis of Incorrect Options:** * **B. Scrub typhus:** Caused by *Orientia tsutsugamushi* and transmitted by the bite of larval mites (chiggers). It typically presents with a characteristic black eschar at the bite site. * **C. Endemic typhus (Murine typhus):** Caused by *Rickettsia typhi* and transmitted to humans by rat fleas (*Xenopsylla cheopis*). * **D. Epidemic typhus:** Caused by *Rickettsia prowazekii*. While it shares the same vector as Trench fever (the human body louse), the causative organism is different. **High-Yield Clinical Pearls for NEET-PG:** * **Vector Match:** Both *Bartonella quintana* and *Rickettsia prowazekii* are transmitted by the **human body louse**. * **Bartonella species:** Remember *B. henselae* causes **Cat Scratch Disease**, while *B. bacilliformis* causes **Carrion’s disease** (Oroya fever/Verruga peruana) transmitted by the Sandfly (*Lutzomyia*). * **Bacillary Angiomatosis:** In HIV/immunocompromised patients, both *B. quintana* and *B. henselae* can cause these vascular proliferative lesions.

Q: A 23-year-old male patient presents to the OPD with nausea, vomiting, and four episodes of loose stools. On history taking, he reveals to have consumed food from a restaurant 3 hours back. What is the most likely etiologic organism?

Staphylococcus aureus. ### Explanation The key to solving this question lies in the **incubation period**. The patient developed symptoms (nausea, vomiting, and diarrhea) just **3 hours** after consuming restaurant food. **1. Why Staphylococcus aureus is correct:** *Staphylococcus aureus* food poisoning is caused by the ingestion of **pre-formed enterotoxins** (Type A-E) in contaminated food (often dairy, processed meats, or salads). Because the toxin is already present in the food, the onset of symptoms is rapid, typically occurring within **1 to 6 hours**. The clinical presentation is dominated by upper gastrointestinal symptoms like nausea and projectile vomiting, followed by diarrhea. **2. Why the other options are incorrect:** * **Bacillus cereus:** While it also causes rapid-onset vomiting (1–5 hours) via a pre-formed emetic toxin, it is classically associated with **reheated fried rice**. In the absence of a specific "rice" history, *S. aureus* is a more common general cause of rapid-onset restaurant-associated food poisoning. * **Salmonella:** This causes an infection (not just intoxication), requiring the bacteria to multiply in the gut. The incubation period is much longer, typically **12 to 72 hours**, and is often associated with fever. * **Vibrio cholerae:** This causes "rice-water stools" and severe dehydration. The incubation period is usually **1 to 3 days**, and vomiting typically occurs *after* the onset of profuse diarrhea. **Clinical Pearls for NEET-PG:** * **Shortest Incubation (1–6 hrs):** *S. aureus* and *B. cereus* (Emetic type). * **Intermediate Incubation (8–16 hrs):** *Clostridium perfringens* and *B. cereus* (Diarrheal type). * **Long Incubation (>16 hrs):** *Salmonella*, *Shigella*, and *Vibrio*. * **S. aureus Toxin:** It is **heat-stable** (resists boiling for 30 mins) and acts as a **superantigen**, stimulating the vagus nerve and the vomiting center in the brain.

Q: In a patient presenting with diarrhea due to Vibrio cholera, which of the following signs will be present?

Neutrophilia. **Explanation:** The hallmark of **Vibrio cholerae** infection is a non-inflammatory, secretory diarrhea mediated by the **cholera toxin (CT)**. The toxin stimulates adenylate cyclase, leading to increased cAMP levels, which results in the massive efflux of water and electrolytes into the intestinal lumen without causing mucosal invasion or cellular damage. **Why Neutrophilia is the correct answer:** Despite being a non-invasive disease, severe dehydration and hemoconcentration in cholera lead to a physiological stress response. This typically manifests as **leukocytosis with neutrophilia** in the peripheral blood. It is a systemic reaction to the acute volume depletion and stress, rather than a sign of pyogenic infection. **Why other options are incorrect:** * **Abdominal pain:** Cholera is classically described as **painless** "rice-water" stools. While mild muscle cramps (due to electrolyte loss) may occur, significant abdominal pain or tenesmus is absent because there is no mucosal inflammation. * **Presence of leukocytes in stool:** Since *V. cholerae* is **non-invasive**, the stool examination typically shows no RBCs or pus cells (leukocytes). This distinguishes it from invasive organisms like *Shigella* or *Salmonella*. * **Fever:** Fever is generally **absent** or very low-grade. The presence of high fever usually suggests a different, invasive etiology. **High-Yield Clinical Pearls for NEET-PG:** * **Stool appearance:** "Rice-water" stool with a fishy odor. * **Microscopy:** Darting motility (inhibited by specific antisera in the "Pfeiffer phenomenon"). * **Culture:** TCBS (Thiosulfate-Citrate-Bile Salts-Sucrose) agar is the selective medium; colonies appear **yellow** due to sucrose fermentation. * **Treatment:** The mainstay is aggressive **rehydration** (ORS/IV fluids). Doxycycline is the drug of choice to reduce the duration of shedding.

Q: Which of the following is commonly associated with Streptococcus infection?

Cellulitis. **Explanation:** **Streptococcus pyogenes (Group A Streptococcus)** is the most common cause of **Cellulitis**, a spreading inflammation of the subcutaneous and dermal layers of the skin. The underlying medical concept involves the production of spreading factors such as **Hyaluronidase** (which breaks down connective tissue) and **Streptokinase**. These enzymes allow the infection to spread rapidly through tissue planes, resulting in the characteristic diffuse, non-circumscribed redness and swelling seen in cellulitis. **Analysis of Options:** * **A. Cellulitis (Correct):** Classically caused by *S. pyogenes*. It presents as a diffuse, spreading infection unlike the localized abscesses typical of *Staphylococcus aureus*. * **B. Gangrene:** While *S. pyogenes* can cause Necrotizing Fasciitis ("flesh-eating disease"), Gas Gangrene is specifically associated with **Clostridium perfringens**. * **C. Pyoderma:** This is a general term for inflammatory skin diseases. While *Streptococcus* can cause Impetigo (a type of pyoderma), the term is more frequently associated with **Staphylococcus aureus** in clinical practice, or specific conditions like Pyoderma Gangrenosum. * **D. Urinary Tract Infection (UTI):** The most common cause is **E. coli**. Among Gram-positive cocci, *Enterococcus* or *Staphylococcus saprophyticus* are more common culprits than *Streptococcus*. **High-Yield Clinical Pearls for NEET-PG:** * **Erysipelas:** A superficial form of cellulitis with well-defined margins, almost exclusively caused by *S. pyogenes*. * **ASO Titer:** Useful for diagnosing post-streptococcal sequelae (like Glomerulonephritis) but is typically **not** elevated in skin infections. * **Drug of Choice:** Penicillin remains the treatment of choice for *S. pyogenes* due to its consistent sensitivity.

Q: Which of the following is a true statement about Enterococcus (formerly Streptococcus faecalis)?

Grows in 6.5% NaCl solution. **Explanation:** *Enterococcus* species (formerly classified as Group D Streptococci) are resilient organisms characterized by their ability to survive in harsh environmental conditions. **1. Why Option A is Correct:** The ability to grow in **6.5% Sodium Chloride (NaCl)** is a signature biochemical test used to differentiate *Enterococcus* from non-enterococcal Group D Streptococci (like *S. bovis*). Their cell membrane is uniquely adapted to withstand high osmotic pressure, making this a definitive laboratory marker. **2. Why the other options are Incorrect:** * **Option B:** While some species may ferment lactose, it is **not a defining characteristic** of the genus. They are primarily identified by their ability to ferment glucose and hydrolyze **Esculin** in the presence of 40% bile (Bile Esculin Test). * **Option C:** Enterococci are notably **heat-resistant**. They can survive heating at **60°C for 30 minutes**, a property known as "thermoduric" capability, which distinguishes them from most other Streptococci. * **Option D:** Enterococci are classified based on the **Lancefield Group D antigen**, which is a **glycerol teichoic acid** located in the cell membrane, not the cell wall. However, the primary classification for most Streptococci is based on the C-carbohydrate antigen of the cell wall; *Enterococcus* is the exception where the group antigen is membrane-associated. **High-Yield Clinical Pearls for NEET-PG:** * **PYR Test:** Enterococci are **PYR positive** (helpful for rapid identification). * **Catalase:** They are typically Catalase-negative (but may show "pseudocatalase" activity). * **VRE:** Vancomycin-Resistant Enterococci (VRE) are a major cause of nosocomial infections (UTI, endocarditis). * **Bile Solubility:** They are **Bile Esculin positive** but **Bile insoluble** (unlike *S. pneumoniae*).

Q: A 70-year-old man with a history of diabetes presents with severe pain in his right ear. The patient was diagnosed with external otitis. Further tests suggested that the patient suffered bone and nerve damage. Clinical laboratory analysis showed that the isolated microorganism produced a distinct blue pigment as well as an ADP-ribosylation toxin. What is the most likely causative agent?

Pseudomonas aeruginosa. **Explanation:** The clinical presentation describes **Malignant Otitis Externa (MOE)**, a severe, necrotizing infection of the external ear canal typically seen in elderly diabetic patients. **1. Why Pseudomonas aeruginosa is correct:** * **Clinical Presentation:** *P. aeruginosa* is the most common cause of MOE. It can invade the soft tissue and spread to the temporal bone (osteomyelitis) and cranial nerves (commonly CN VII), matching the "bone and nerve damage" described. * **Pigment Production:** It produces **Pyocyanin** (blue-green pigment) and Pyoverdin (fluorescent yellow-green). * **Virulence Factor:** It produces **Exotoxin A**, which acts via **ADP-ribosylation of Elongation Factor-2 (EF-2)**, inhibiting protein synthesis (identical mechanism to Diphtheria toxin). **2. Why the other options are incorrect:** * **Staphylococcus epidermidis:** Part of normal skin flora; usually associated with prosthetic valve endocarditis or catheter infections, not invasive otitis. * **Staphylococcus aureus:** A common cause of localized furuncles or cellulitis, but it does not produce blue pigment or an ADP-ribosylating toxin. * **Enterococcus faecalis:** Primarily causes UTIs and biliary tract infections; it is not a common cause of malignant otitis externa. **High-Yield NEET-PG Pearls:** * **Malignant Otitis Externa:** Always suspect in a diabetic patient with ear pain out of proportion to clinical findings. * **Pseudomonas Culture:** Characterized by a **fruity/grape-like odor** and growth at **42°C**. * **Toxin Mechanism:** Remember "P" for Pseudomonas and "P" for Protein synthesis inhibition (EF-2). * **Ecthyma Gangrenosum:** Another classic *Pseudomonas* presentation in neutropenic patients (black necrotic eschars).

Q: Which of the following is an important distinguishing characteristic of H. pylori as compared to Campylobacter species?

Urease production. **Explanation:** The correct answer is **C. Urease production.** While both *Helicobacter pylori* and *Campylobacter* species belong to the same family of epsilon-proteobacteria and share several morphological and biochemical traits, the **strong production of urease** is the hallmark feature that distinguishes *H. pylori*. 1. **Why Urease is the key:** *H. pylori* produces abundant urease, which hydrolyzes urea into ammonia and carbon dioxide. This creates a localized alkaline "cloud" that neutralizes gastric acid, allowing the bacteria to survive in the acidic stomach environment. This property is the basis for the **Urea Breath Test** and **Rapid Urease Test (RUT)** used in clinical diagnosis. *Campylobacter* species are typically urease-negative. 2. **Why other options are incorrect:** * **Oxidase and Catalase production (A & B):** Both *H. pylori* and *Campylobacter* are typically positive for oxidase and catalase. Therefore, these tests cannot be used to differentiate between them. * **Curved shape (D):** Both genera exhibit a characteristic curved, spiral, or "S-shaped" morphology (often described as "gull-wing" appearance for *Campylobacter*). **High-Yield Clinical Pearls for NEET-PG:** * **H. pylori:** Associated with Type B antral gastritis, Peptic Ulcer Disease (PUD), Gastric Adenocarcinoma, and MALToma. * **Culture Media:** *H. pylori* grows on Skirrow’s medium or Chocolate agar; *Campylobacter* requires selective media like Skirrow’s or Butzler’s medium. * **Temperature:** *Campylobacter jejuni* is thermophilic (grows best at 42°C), whereas *H. pylori* grows at 37°C. * **Motility:** Both are motile via polar flagella; *H. pylori* has multiple flagella at one pole (lophotrichous), while *Campylobacter* usually has a single flagellum (monotrichous).

Question 1

Which of the following staphylococcal proteins functions as a superantigen?

Accepted Answer

Exfoliative toxin

Answer

Cytolytic toxin

Answer

Protein A

Answer

Leucocidin

Question 2

A child attending preschool presents with multiple pustular skin lesions on the arms, some of which are crusted with a yellow exudate. Which of the following virulence factors of the most likely etiologic agent protects it from phagocytosis and provides serologic specificity?

Accepted Answer

M protein

Answer

Erythrogenic toxin

Answer

Hyaluronic acid capsule

Answer

Lipoteichoic acid

Question 3

Which disease is caused by Bartonella quintana?

Accepted Answer

Trench fever

Answer

Scrub typhus

Answer

Endemic typhus

Answer

Epidemic typhus

Question 4

A 23-year-old male patient presents to the OPD with nausea, vomiting, and four episodes of loose stools. On history taking, he reveals to have consumed food from a restaurant 3 hours back. What is the most likely etiologic organism?

Accepted Answer

Staphylococcus aureus

Answer

Bacillus cereus

Answer

Salmonella

Answer

Vibrio cholerae

Question 5

Which of the following is an example of an Enterobacteriaceae member that exhibits adherence and effacement (A/E) activity?

Accepted Answer

Pseudomonas

Answer

Klebsiella

Answer

Vibrio cholerae

Answer

Proteus

Question 6

In a patient presenting with diarrhea due to Vibrio cholera, which of the following signs will be present?

Accepted Answer

Neutrophilia

Answer

Abdominal pain

Answer

Presence of leukocytes in stool

Answer

Fever

Question 7

Which of the following is commonly associated with Streptococcus infection?

Accepted Answer

Cellulitis

Answer

Gangrene

Answer

Pyoderma

Answer

Urinary Tract Infection (UTI)

Question 8

Which of the following is a true statement about Enterococcus (formerly Streptococcus faecalis)?

Accepted Answer

Grows in 6.5% NaCl solution

Answer

Is a lactose fermenter

Answer

Is easily destroyed at 60°C for 30 minutes

Answer

Is classified on the basis of teichoic acid of the cell wall

Question 9

A 70-year-old man with a history of diabetes presents with severe pain in his right ear. The patient was diagnosed with external otitis. Further tests suggested that the patient suffered bone and nerve damage. Clinical laboratory analysis showed that the isolated microorganism produced a distinct blue pigment as well as an ADP-ribosylation toxin. What is the most likely causative agent?

Accepted Answer

Pseudomonas aeruginosa

Answer

Staphylococcus epidermidis

Answer

Staphylococcus aureus

Answer

Enterococcus faecalis

Question 10

Which of the following is an important distinguishing characteristic of H. pylori as compared to Campylobacter species?

Accepted Answer

Urease production

Answer

Oxidase production

Answer

Catalase production

Answer

Curved shape

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?