Bacteriology Practice Questions

Q: A 25-year-old male presented with high-grade fever, headache, and neck stiffness. On examination, neck rigidity and positive Kernig's sign were found. CSF analysis showed neutrophilic predominance, low glucose, and a positive Limulus Amebocyte Lysate assay. Which of the following is the likely pathogen?

Neisseria meningitidis. **Explanation:** The clinical presentation of high-grade fever, headache, neck stiffness, and a positive Kernig’s sign is classic for **Acute Bacterial Meningitis** [1]. The CSF analysis showing neutrophilic pleocytosis and low glucose further confirms a bacterial etiology [1]. The definitive clue in this question is the **positive Limulus Amebocyte Lysate (LAL) assay**. The LAL test is a highly sensitive method used to detect **endotoxins (Lipopolysaccharide)** produced by **Gram-negative bacteria**. Among the options provided, **Neisseria meningitidis** is the only Gram-negative organism. **Analysis of Options:** * **Neisseria meningitidis (Correct):** A Gram-negative diplococcus. It is a leading cause of meningitis in young adults and is the only pathogen among the choices that would yield a positive LAL assay due to its endotoxin content [1]. * **Streptococcus pneumoniae (Incorrect):** While it is the most common cause of bacterial meningitis in adults, it is a **Gram-positive** coccus and lacks endotoxin; therefore, the LAL assay would be negative [1]. * **Staphylococcus aureus (Incorrect):** A **Gram-positive** coccus usually associated with post-neurosurgical procedures or endocarditis; LAL assay negative. * **Listeria monocytogenes (Incorrect):** A **Gram-positive** rod typically seen in neonates, elderly, or immunocompromised patients; LAL assay negative. **NEET-PG High-Yield Pearls:** * **LAL Assay:** Derived from the blood of the Horseshoe crab (*Limulus polyphemus*); used specifically to detect Gram-negative endotoxin. * **Drug of Choice:** For Meningococcal meningitis, IV Ceftriaxone is the standard treatment [1]. * **Prophylaxis:** Rifampicin is the drug of choice for close contacts of a patient with *N. meningitidis* [1]. * **CSF Findings in Bacterial Meningitis:** ↑ Proteins, ↓ Glucose (<40 mg/dL), ↑ Neutrophils (Polymorphs) [1].

Q: Actinomycosis is commonly seen in which of the following bones?

Mandible. **Explanation:** **Actinomycosis** is a chronic, granulomatous infection caused by anaerobic, Gram-positive filamentous bacteria, most commonly *Actinomyces israelii*. These organisms are normal commensals of the oral cavity, gastrointestinal tract, and female genital tract. **Why the Mandible is Correct:** The most common clinical form of the disease is **Cervicofacial Actinomycosis** (accounting for ~50-60% of cases), often referred to as "Lumpy Jaw." The infection typically follows a breach in the oral mucosa due to dental caries, tooth extraction, or gingivitis. The bacteria spread by direct tissue contiguity rather than lymphatic or hematogenous routes. When the infection involves the bone, the **mandible** is the most frequently affected site, followed by the maxilla. It presents as a slow-growing, painless "woody" induration that eventually forms multiple external sinuses. **Why Other Options are Incorrect:** * **Tibia, Scapula, and Femur:** These are common sites for pyogenic osteomyelitis (usually caused by *Staphylococcus aureus*). While actinomycosis can rarely affect the extremities (Primary Actinomycosis of the Extremity) following trauma or human bites, it is statistically rare compared to the classic cervicofacial presentation. **High-Yield Clinical Pearls for NEET-PG:** * **Sulfur Granules:** The hallmark of actinomycosis is the presence of yellow, sand-like "sulfur granules" in the pus (actually masses of filamentous bacteria). * **Microscopy:** Shows Gram-positive branching filaments that are **non-acid fast** (distinguishing them from *Nocardia*, which is weakly acid-fast). * **Ray Fungus Appearance:** On histology, the colonies show a peripheral radiating arrangement of filaments. * **Treatment:** High-dose **Penicillin G** for a prolonged duration (6–12 months) is the treatment of choice.

Q: Staphylococcus aureus does not cause which of the following skin infections?

Ecthyma gangrenosum. **Explanation:** The correct answer is **Ecthyma gangrenosum** because it is a pathognomonic skin manifestation of **Pseudomonas aeruginosa** septicemia, typically seen in immunocompromised or neutropenic patients. It presents as a painless, necrotic ulcer with a central black eschar surrounded by an erythematous halo. It is caused by bacterial invasion of perivascular media and adventitia, leading to ischemic necrosis, rather than a primary *Staphylococcus aureus* infection. **Analysis of Incorrect Options:** * **Bullous Impetigo:** This is primarily caused by *Staphylococcus aureus* (specifically strains producing exfoliative toxin A). It is characterized by large, thin-walled bullae that rupture to leave a thin "varnish-like" crust. * **Botryomycosis:** Despite the name suggesting a fungal origin, this is a chronic granulomatous bacterial infection most commonly caused by **Staphylococcus aureus**. It presents with skin nodules and draining sinuses containing "sulfur granules" (similar to actinomycosis). * **Cellulitis:** While *Streptococcus pyogenes* is the most common cause, *Staphylococcus aureus* is a frequent cause of cellulitis, especially when associated with an open wound, abscess, or penetrating trauma. **NEET-PG High-Yield Pearls:** * **Ecthyma gangrenosum = Pseudomonas.** Do not confuse it with *Ecthyma*, which is a deep form of impetigo caused by *S. pyogenes* or *S. aureus*. * **Staphylococcal Scalded Skin Syndrome (SSSS):** Also caused by exfoliative toxins (ET-A, ET-B) which cleave **Desmoglein-1**. * **Golden-yellow pigment:** Produced by *S. aureus* (staphyloxanthin) acts as a virulence factor by neutralizing reactive oxygen species.

Q: Which organism is most commonly responsible for gas gangrene?

Clostridium perfringens. ***Clostridium perfringens*** - This **anaerobic, Gram-positive** rod is the most common cause of gas gangrene (or **myonecrosis**) due to its rapid production of potent toxins, especially $\alpha$-toxin (a **lecithinase**). - It is found ubiquitously in soil and the GI tract and typically infects deep penetrating wounds, leading to rapid tissue destruction and gas formation (crepitus). *Clostridium difficile* - This organism primarily causes **antibiotic-associated diarrhea** and **pseudomembranous colitis** due to the production of Toxin A (enterotoxin) and Toxin B (cytotoxin). - It is rarely involved in tissue infections or gas gangrene. *Clostridium tetani* - This bacterium is the causative agent of **tetanus**, a neurological disorder characterized by muscle spasms and locked jaw. - Tetanus symptoms are due to the production of the neurotoxin **tetanospasmin**, which blocks inhibitory neurotransmitters. *Clostridium botulinum* - This organism produces the potent **botulinum toxin**, which causes **flaccid paralysis** (botulism) by preventing the release of acetylcholine at the neuromuscular junction. - It is associated with food poisoning or infant botulism, not typically gas gangrene.

Q: A 30-year-old man develops profuse vomiting and diarrhea within 3 hours of consuming pre-packaged salad and milk at a picnic. There is no fever. Which of the following is the most likely causative organism?

Staphylococcus aureus. ***Staphylococcus aureus***- This organism produces a **heat-stable enterotoxin** that, when ingested (preformed toxin), causes rapid symptoms within 1 to 6 hours, fitting the **3-hour onset** without requiring bacterial multiplication in the host.- Symptoms are primarily profound **nausea and vomiting**, accompanied by diarrhea, and characteristically **without fever**, as this is a food intoxication rather than an invasive infection.*Clostridium perfringens*- Symptoms usually manifest after a longer incubation period of **8 to 16 hours**, as the toxin is produced *in vivo* after the bacteria are ingested.- The illness is dominated by **watery diarrhea** and abdominal cramps; vomiting is typically mild or absent.*Salmonella enterica*- This organism causes an **invasive infection** (or requires prolonged multiplication) and thus has a longer incubation period, typically **6 hours to 3 days**.- Gastroenteritis caused by *Salmonella* almost always presents with associated systemic symptoms, most notably significant **fever**.*Bacillus cereus*- *B. cereus* causes two syndromes: the **diarrheal form** (slower onset) and the **emetic form** (rapid 1–6-hour onset), which is mostly associated with contaminated **fried rice**.- While the emetic form fits the timeline, *Staphylococcus aureus* is the most common cause of rapid, non-febrile illness affecting both the stomach (vomiting) and intestines (diarrhea) related to cold, packaged foods like salads or milk.

Q: What is the most common causative organism of prosthetic heart valve endocarditis?

S. epidermidis. ***S. epidermidis*** - **S. epidermidis** is the most common single causative organism of prosthetic heart valve (PHV) endocarditis, especially in the **early post-operative period** (within 60 days to 1 year of surgery). - This organism is a **coagulase-negative Staphylococcus** known for its ability to produce a **glycocalyx (biofilm)**, which adheres strongly to foreign materials like prosthetic valves. *S. viridans* - Typically the leading cause of **subacute infectious endocarditis** on native, damaged valves, often following minor **dental procedures**. - While it can cause late PHV endocarditis, it is much less frequent than staphylococcal species, which dominate PHV infections. *S. aureus* - **S. aureus** is the most common cause of **acute native valve endocarditis** and frequently implicated in endocarditis in **IV drug users**. - It is a major cause of PHV endocarditis, especially **late-onset** (more than 1 year post-surgery), but overall, **S. epidermidis** is considered the most common pathogen when considering all PHV infections. *Enterococcus* - **Enterococcus species** (especially *E. faecalis*) account for approximately **5-10% of prosthetic valve endocarditis** cases, particularly in late-onset infections. - While significant, enterococci are far less common than staphylococcal species in PHV endocarditis.

Q: A 70-year-old man with a prosthetic valve was admitted with a murmur and blood culture positive for Gram-positive cocci. Choose the most common agent causing prosthetic valve infective endocarditis.

Staphylococcus epidermidis. ***Staphylococcus epidermidis***- This is the most common cause of **early** prosthetic valve endocarditis (PVIE), which typically occurs within 60 days following surgery, often due to contamination during the replacement procedure.- This organism is a **coagulase-negative Staphylococci** known for forming protective **biofilms** on the prosthetic surface, leading to severe device-related infections.*Streptococcus viridans*- This is the most common cause of **native valve** infective endocarditis (NVIE), frequently associated with dental procedures or poor oral hygiene.- While it can cause PVIE, it is typically responsible for late-onset cases, and overall less common than **Staphylococcal species** in prosthetic valve disease.*Staphylococcus aureus*- This highly virulent organism causes a rapidly destructive form of endocarditis (both NVIE and PVIE), often associated with **IV drug use** or healthcare-associated infections.- Although a major cause of PVIE, *S. epidermidis* is **statistically more frequent** overall in this setting, especially in early postoperative infections.*Enterococci*- Enterococcal endocarditis is often seen in older men or patients who have recently undergone **genitourinary or gastrointestinal** instrumentation or surgery.- While it causes both NVIE and PVIE, its incidence in prosthetic valve disease is lower than the **Staphylococcal species**.

Q: A group of students consumed fried rice in a restaurant. Six hours later, they start vomiting. What is the etiological agent?

Bacillus cereus. ***Bacillus cereus***- The short incubation period (1 to 6 hours) and the primary symptom being **vomiting** are highly characteristic of the **emetic-type** food poisoning caused by *B. cereus*. - The emetic form is typically associated with contaminated **cooked rice** (like fried rice) that has been improperly stored, allowing the bacteria to proliferate and produce the heat-stable toxin, **cereulide**. *Clostridium botulinum* - This bacterium causes **botulism**, a severe neuroparalytic syndrome (e.g., descending flaccid paralysis, diplopia), not typically an acute emetic gastroenteritis. - The incubation period for botulism is usually longer (12 to 36 hours), and it is most often linked to improperly canned or vacuum-packed foods. *Escherichia coli* - Diarrheagenic *E. coli* strains typically require a longer incubation period (usually 1 or more days) and cause symptoms dominated by **diarrhea** (either watery or bloody), with vomiting being a less prominent feature. - Common infectious sources are often fecally contaminated water or uncooked produce, rather than cooked and cooled rice. *Shigella dysentriae* - *S. dysentriae* causes **bacillary dysentery**, characterized by a longer incubation period (1 to 7 days) and symptoms including fever, severe abdominal cramping, and **bloody, mucoid stools** (dysentery). - While systemic symptoms might occur, the rapid onset of vomiting alone is not characteristic of Shigellosis.

Q: A firefighter with fever and cough has a culture showing gram-negative, oxidase-positive bacilli with fluorescent colonies under UV light. Which of the following organisms is most likely causing the infection?

Pseudomonas aeruginosa. ***Pseudomonas aeruginosa***- This organism is a non-lactose fermenting, **Gram-negative bacillus** that is definitively **oxidase-positive**.- It characteristically produces the siderophore **pyoverdine**, which is fluorescent under UV light, confirming the most likely etiology.*Klebsiella*- *Klebsiella* species are **oxidase-negative** (**distinguishing factor**) and typically highly encapsulated, often causing consolidation in pneumonia.- They are also strong **lactose fermenters** and do not produce fluorescent pigments.*Staphylococcus aureus*- *S. aureus* is a **Gram-positive coccus** (stains purple) arranged in clusters, not a Gram-negative bacillus (stains pink).- It is non-motile, shows no fluorescence, and is identified by being coagulase and catalase positive.*Serratia*- *Serratia marcescens* is generally **oxidase-negative** (unlike the clinical finding) and belongs to the Enterobacteriaceae family.- Under room temperature, *Serratia* often produces a characteristic **red pigment** (**prodigiosin**), not a fluorescent pigment.

Question 1

A 25-year-old male presented with high-grade fever, headache, and neck stiffness. On examination, neck rigidity and positive Kernig's sign were found. CSF analysis showed neutrophilic predominance, low glucose, and a positive Limulus Amebocyte Lysate assay. Which of the following is the likely pathogen?

Accepted Answer

Neisseria meningitidis

Answer

Staphylococcus aureus

Answer

Streptococcus pneumoniae

Answer

Listeria monocytogenes

Question 2

A 47-year-old man suddenly develops high fever and hypotension. He has a generalized erythematous macular rash, and over the next day, develops gangrene of his left leg. Which of the following is the most likely organism?

Accepted Answer

Streptococcus group A

Answer

Corynebacterium diphtheriae

Answer

Streptococcus group C

Answer

Neisseria gonorrhoeae

Question 3

Actinomycosis is commonly seen in which of the following bones?

Accepted Answer

Mandible

Answer

Tibia

Answer

Scapula

Answer

Femur

Question 4

Staphylococcus aureus does not cause which of the following skin infections?

Accepted Answer

Ecthyma gangrenosum

Answer

Bullous impetigo

Answer

Botryomycosis

Answer

Cellulitis

Question 5

Which organism is most commonly responsible for gas gangrene?

Accepted Answer

Clostridium perfringens

Answer

Clostridium difficile

Answer

Clostridium tetani

Answer

Clostridium botulinum

Question 6

A 30-year-old man develops profuse vomiting and diarrhea within 3 hours of consuming pre-packaged salad and milk at a picnic. There is no fever. Which of the following is the most likely causative organism?

Accepted Answer

Staphylococcus aureus

Answer

Bacillus cereus

Answer

Salmonella enterica

Answer

Clostridium perfringens

Question 7

What is the most common causative organism of prosthetic heart valve endocarditis?

Accepted Answer

S. epidermidis

Answer

S. aureus

Answer

S. viridans

Answer

Enterococcus

Question 8

A 70-year-old man with a prosthetic valve was admitted with a murmur and blood culture positive for Gram-positive cocci. Choose the most common agent causing prosthetic valve infective endocarditis.

Accepted Answer

Staphylococcus epidermidis

Answer

Staphylococcus aureus

Answer

Streptococcus viridans

Answer

Enterococci

Question 9

A group of students consumed fried rice in a restaurant. Six hours later, they start vomiting. What is the etiological agent?

Accepted Answer

Bacillus cereus

Answer

Clostridium botulinum

Answer

Shigella dysentriae

Answer

Escherichia coli

Question 10

A firefighter with fever and cough has a culture showing gram-negative, oxidase-positive bacilli with fluorescent colonies under UV light. Which of the following organisms is most likely causing the infection?

Accepted Answer

Pseudomonas aeruginosa

Answer

Staphylococcus aureus

Answer

Serratia

Answer

Klebsiella

Bacteriology — MCQs

Bacteriology — MCQs

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