Bacteriology Practice Questions

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 of the following bacteria shows Nagler's reaction?

Clostridium perfringens. **Explanation:** **Nagler’s Reaction** is a biochemical test used for the rapid identification of **Clostridium perfringens**. The reaction detects the presence of **Alpha (α) toxin**, which is a **Lecithinase** (Phospholipase C). 1. **Why C is correct:** *Clostridium perfringens* produces Alpha toxin that acts on the lecithin present in egg yolk agar. When the bacteria are grown on this medium, the lecithinase breaks down lecithin into insoluble diglycerides, resulting in an **opaque/turbid halo** around the colonies. To confirm, one half of the plate is smeared with antitoxin; the absence of opalescence on that side (due to neutralization) confirms a positive Nagler’s reaction. 2. **Why other options are incorrect:** * **C. tetani:** Produces tetanospasmin (neurotoxin); it is non-hemolytic and does not produce lecithinase. * **C. botulinum:** Produces botulinum toxin; while some strains produce lipase, they do not typically show a positive Nagler’s reaction for lecithinase. * **C. septicum:** Primarily produces alpha-toxin that is a hemolysin (pore-forming toxin), not a lecithinase. It is known for "swarming" growth rather than Nagler's positivity. **High-Yield Clinical Pearls for NEET-PG:** * **Target Hemolysis:** *C. perfringens* shows a characteristic **double zone of hemolysis** on blood agar (inner zone of complete hemolysis by theta toxin, outer zone of incomplete hemolysis by alpha toxin). * **Stormy Fermentation:** Rapid carbohydrate fermentation in litmus milk leads to acid production and gas, breaking the clot. * **Clinical Condition:** Most common cause of **Gas Gangrene** (Myonecrosis) and food poisoning.

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 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: The toughness of bacterial cell walls is due to:

Peptidoglycan. ***Peptidoglycan***- This unique polymer forms a robust, mesh-like structural layer (the **murein layer**) that encases the bacteria, providing essential mechanical strength and rigidity to the cell wall. Its strong cross-links allow it to withstand extremely high internal **osmotic pressure**, preventing the cell from bursting (lysis). *Teichoic acid* - These are polymers found exclusively in **Gram-positive** bacteria, anchored either to the peptidoglycan or the plasma membrane (lipoteichoic acid). - They contribute to the cell wall's overall integrity, and act as antigens, but are not the primary element responsible for the physical **toughness**. *Lipopolysaccharides* - LPS molecules make up the outer leaflet of the **outer membrane** in **Gram-negative** bacteria. - They function primarily as an **endotoxin** (lipid A component) and an O-antigen, contributing to virulence and permeability, not the mechanical load-bearing structure. *Outer membrane* - This membrane is characteristic of **Gram-negative** bacteria and acts as a protective, selective barrier (excluding certain antibiotics and chemicals). - While providing protective resistance, the intrinsic mechanical **toughness** preventing osmotic lysis is conferred by the underlying **peptidoglycan** layer.

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.

Q: A firefighter was admitted with fever, cough, culture shown in the image reveals gram-negative, oxidase-positive bacilli. Colonies transilluminating while passing under UV light. Choose the correct agent.

Pseudomonas aeruginosa. ***Pseudomonas aeruginosa*** - This organism is a **gram-negative**, **oxidase-positive** bacillus, matching the description. The image shows growth with a characteristic green pigment (**pyocyanin** and **pyoverdin**). - The key identifier is the production of **pyoverdin**, a fluorescent siderophore, which causes the colonies to transilluminate under UV light. *Klebsiella* - *Klebsiella* is **oxidase-negative**, which rules it out based on the biochemical test results provided. - It typically produces large, **mucoid colonies** due to its prominent capsule and is a lactose fermenter, features not described here. *Staphylococcus aureus* - *Staphylococcus aureus* is a **gram-positive coccus**, not a gram-negative bacillus, making it incorrect based on the initial microscopy. - It is also **oxidase-negative** and characteristically forms opaque, often golden-yellow colonies, not greenish fluorescent ones. *Serratia* - *Serratia* is a gram-negative bacillus but is **oxidase-negative**, which is inconsistent with the findings. - Certain species like *Serratia marcescens* are known for producing a red pigment called **prodigiosin**, especially at room temperature, not a green fluorescent pigment.

Q: A soldier presents to a referral hospital one day after sustaining a high-velocity shrapnel injury to his right thigh. He complains of severe pain at the wound site. On examination, crepitus is present. Which of the following organisms is the most likely cause of his condition?

Clostridium perfringens. ***Clostridium perfringens*** - *Clostridium perfringens* is the most common cause (80-90%) of **gas gangrene** (clostridial myonecrosis), especially following severe penetrating trauma such as that caused by a high-velocity shrapnel injury, which creates **anaerobic** conditions in deep tissue. - The organism produces potent toxins, primarily **alpha toxin** (a lecithinase), which causes rapid tissue necrosis, hemolysis, and the production of gas that manifests clinically as **crepitus**. *Incorrect: Clostridium septicum* - This organism is primarily associated with **spontaneous, non-traumatic gas gangrene**, often occurring in patients with underlying conditions like **colonic malignancy**, diabetes, or neutropenia. - While it can cause traumatic gas gangrene, it is significantly less frequent than *C. perfringens* in established wound infections. *Incorrect: Clostridium difficile* - *C. difficile* is the principal cause of **antibiotic-associated diarrhea** and **pseudomembranous colitis**. - It causes localized disease in the colon by producing **Toxin A (enterotoxin)** and **Toxin B (cytotoxin)** and does not typically cause myonecrosis or gas gangrene in penetrating wounds. *Incorrect: Clostridium novyi* - *C. novyi* is a recognized cause of gas gangrene but is less common than *C. perfringens* in traumatic settings and is sometimes associated with **intravenous illicit drug use**. - It is known for producing large amounts of **edema** compared to other clostridial species, causing a characteristic "malignant edema" syndrome rather than the primarily necrotizing and gas-producing infection seen with *C. perfringens*.

Q: Which of the following is an acid-fast organism?

Mycobacterium avium. ***Mycobacterium avium*** - Mycobacteria are **classically acid-fast organisms** due to high mycolic acid content in their cell wall - They retain carbolfuchsin stain even after treatment with acid-alcohol (Ziehl-Neelsen or Kinyoun stain) - M. avium is an atypical mycobacterium commonly causing infections in immunocompromised patients (especially AIDS) *Streptococcus pneumoniae* - Gram-positive coccus, NOT acid-fast - Identified by Gram staining, not acid-fast staining *Escherichia coli* - Gram-negative bacillus, NOT acid-fast - Standard Gram staining is used for identification *Actinomyces* - Gram-positive, branching filamentous bacteria - May show partial acid-fastness in modified acid-fast staining, but NOT classically acid-fast like Mycobacterium - Actinomyces is NOT considered an acid-fast organism in standard microbiology

Q: Which of the following culture media is selective for the isolation of Mycobacterium tuberculosis?

Löwenstein-Jensen (LJ) medium. ***Löwenstein-Jensen (LJ) medium*** - This is the most common and classic solid medium used for the isolation of **Mycobacterium tuberculosis**. - It is a **selective medium** because it contains **malachite green**, which inhibits the growth of most contaminating bacteria, allowing the slow-growing mycobacteria to flourish. *Chocolate agar* - This enriched medium is primarily used for the isolation of fastidious organisms like **Haemophilus influenzae** and **Neisseria gonorrhoeae**, as it contains heated (lysed) blood. - It is not selective for mycobacteria and would be easily overgrown by other respiratory flora. *MacConkey agar* - This medium is **selective** for **Gram-negative bacilli** (e.g., Enterobacteriaceae) and is differential based on lactose fermentation. - It contains bile salts and crystal violet, which inhibit Gram-positive organisms, including mycobacteria. *Blood agar* - This is a **general-purpose enriched medium** used for the isolation and detection of hemolysis patterns of many pathogenic bacteria, such as **Streptococcus** species. - It is highly non-selective and unsuitable for isolating the slow-growing **M. tuberculosis** from contaminated clinical specimens.

Question 1

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

Accepted Answer

Ecthyma gangrenosum

Answer

Bullous impetigo

Answer

Botryomycosis

Answer

Cellulitis

Question 2

Which of the following bacteria shows Nagler's reaction?

Accepted Answer

Clostridium perfringens

Answer

Clostridium tetani

Answer

Clostridium botulinum

Answer

Clostridium septicum

Question 3

Which organism is most commonly responsible for gas gangrene?

Accepted Answer

Clostridium perfringens

Answer

Clostridium difficile

Answer

Clostridium tetani

Answer

Clostridium botulinum

Question 4

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 5

The toughness of bacterial cell walls is due to:

Accepted Answer

Peptidoglycan

Answer

Teichoic acid

Answer

Lipopolysaccharides

Answer

Outer membrane

Question 6

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

Question 7

A firefighter was admitted with fever, cough, culture shown in the image reveals gram-negative, oxidase-positive bacilli. Colonies transilluminating while passing under UV light. Choose the correct agent.

Accepted Answer

Pseudomonas aeruginosa

Answer

Staphylococcus aureus

Answer

Serratia

Answer

Klebsiella

Question 8

A soldier presents to a referral hospital one day after sustaining a high-velocity shrapnel injury to his right thigh. He complains of severe pain at the wound site. On examination, crepitus is present. Which of the following organisms is the most likely cause of his condition?

Accepted Answer

Clostridium perfringens

Answer

Clostridium novyi

Answer

Clostridium difficile

Answer

Clostridium septicum

Question 9

Which of the following is an acid-fast organism?

Accepted Answer

Mycobacterium avium

Answer

Streptococcus pneumoniae

Answer

Escherichia coli

Answer

Actinomyces

Question 10

Which of the following culture media is selective for the isolation of Mycobacterium tuberculosis?

Accepted Answer

Löwenstein-Jensen (LJ) medium

Answer

MacConkey agar

Answer

Chocolate agar

Answer

Blood agar

Bacteriology — MCQs

Bacteriology — MCQs

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