Bacteriology Practice Questions

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: A 70-year-old patient with a smoking history presents with high-grade fever, cough, confusion, and diarrhea. Chest X-ray shows bilateral infiltrates in bilateral lower lung fields. On sputum gram stain, no organisms were detected. Laboratory results reveal Na: 126mEq/L, AST:62, ALT:56, RBS:112 mg/dl, serum bilirubin of 0.8mg%, and a positive HIV test. Which of the following organisms is responsible?

Legionella. ***Legionella***- This clinical picture, characterized by **high fever**, **atypical pneumonia** (bilateral infiltrates), and systemic symptoms including **confusion** (CNS involvement), **diarrhea** (GI involvement), **hyponatremia** and mildly elevated **LFTs**, is highly suggestive of **Legionella pneumophila** (Legionnaires' disease) [1].- *Legionella* is an **atypical bacterium** that requires specialized culture media and therefore typically results in a Gram stain showing **no organisms detected** through standard methods [1].*Streptococcus*- *Streptococcus pneumoniae* (the most common cause of typical community-acquired pneumonia) typically presents with an abrupt onset, pleurisy, and a consolidation pattern (**lobar pneumonia**) on CXR, differentiating it from this diffuse, atypical presentation.- It is a readily detectable **Gram-positive diplococcus** on sputum Gram stain, which contrasts with the microbiological report of no organisms seen [1].*Pneumocystis jeroveci*- While highly relevant in HIV-positive individuals, **Pneumocystis pneumonia (PJP)** typically presents with subacute onset, severe **hypoxemia**, and diffuse **interstitial infiltrates**, not typically associated with prominent hyponatremia or severe diarrhea [2].- PJP is diagnosed via special staining (e.g., silver stain) on induced sputum or bronchoalveolar lavage, and it does not usually cause the multisystem failure (hyponatremia and transaminitis) characteristic of Legionnaires' disease [2].*Klebsiella*- **Klebsiella pneumoniae** typically causes destruction and necrosis leading to classic **lobar pneumonia** (often in the upper lobes), classically associated with **currant jelly sputum** in immunocompromised individuals, especially alcoholics.- *Klebsiella* is a large encapsulated **Gram-negative rod** that would be easily visible and identifiable on a standard sputum Gram stain.

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: Neutrophilic granulomatous mastitis is caused by which of the following organisms?

Corynebacterium kroppenstedtii. ***Corynebacterium kroppenstedtii*** - This organism is the most commonly identified cause of **idiopathic granulomatous mastitis (IGM)**, specifically the subtype characterized by **neutrophilic microabscesses** adjacent to **non-caseating granulomas**. - It is a **lipophilic diphtheroid** often isolated from breast aspirates or biopsies of patients with **chronic, relapsing breast inflammation**. *Corynebacterium minutissimum* - This bacterium is primarily associated with **erythrasma**, a superficial skin infection presenting as reddish-brown patches, especially in intertriginous areas. - It is generally not involved in causing deep-seated, chronic granulomatous inflammation or abscess formation within the breast parenchyma. *Mycobacterium tuberculosis* - *M. tuberculosis* causes **tuberculous mastitis**, which is histologically defined by the presence of **caseating granulomas** and large areas of necrosis. - This differs from neutrophilic granulomatous mastitis, which features predominantly **non-caseating granulomas** mixed with neutrophils and microabscesses. *Staphylococcus aureus* - *S. aureus* is the leading cause of **acute mastitis** and **breast abscesses**, characterized by rapid onset, pain, fever, and abundant acute inflammatory cells (pus). - It is typically associated with acute suppurative changes, not the chronic, indolent, granulomatous tissue reaction found in neutrophilic granulomatous mastitis.

Q: In a village, several people developed dysentery after consuming raw milk. On laboratory examination, gram-negative, curved rods with polymorphonuclear infiltration were found in stool samples. Which of the following is the most likely causative organism?

Campylobacter jejuni. ***Campylobacter jejuni*** - This organism is a **Gram-negative, curved or S-shaped rod** (seagull appearance) that causes inflammatory diarrhea (**dysentery**) and is the most common antecedent of **Guillain-Barré Syndrome**. - The association of dysentery with consumption of **raw milk** is a classic epidemiological feature of *C. jejuni* infection, explaining the outbreak. *Clostridium perfringens* - This is a **Gram-positive, spore-forming rod** and causes toxin-mediated food poisoning (Type A) characterized primarily by watery diarrhea, not typically dysentery. - The source is usually contaminated meat and gravies, and the stool sample would show a **Gram-positive rod**, not a curved Gram-negative rod. *Staphylococcus aureus* - This organism is a **Gram-positive coccus** that causes food intoxication with rapid onset vomiting being the prominent feature, not dysentery. - The pathology is preformed toxin-mediated, and stool analysis would reveal **cocci**, not curved rods, and lacks the PMN infiltration typical of dysentery. *Vibrio parahaemolyticus* - While it is a **Gram-negative curved rod**, it is strongly associated with the consumption of **raw or undercooked seafood**, especially shellfish. - Clinically, it usually causes watery diarrhea, and raw milk is not its typical vehicle of transmission for large outbreaks.

Q: Group A Streptococcus is the most common cause of bacterial pharyngitis in school-aged children. Which of the following bacterial components is primarily responsible for its attachment to fibronectin on the epithelial lining of the pharynx?

Lipoteichoic acid. ***Lipoteichoic acid*** - **Lipoteichoic acid (LTA)**, along with the **M protein**, serves as a key adhesin for Group A Streptococcus, mediating attachment by binding to **fibronectin** receptors on pharyngeal epithelial cells. - This adherence mechanism is essential for the initial colonization and subsequent establishment of **streptococcal pharyngitis**. *Lipoprotein* - **Lipoproteins** are structurally diverse cell components, but they are not recognized as the primary molecular component mediating the specific attachment of GAS to **fibronectin**. - They often reside in the cell membrane and typically function in **nutrient transport** or signaling pathways. *Capsule* - The **hyaluronic acid capsule** of Group A Streptococcus mainly functions as an **anti-phagocytic factor** against host immune cells, preventing ingestion. - While important for virulence, it is not the molecule that directly facilitates initial adherence to the **fibronectin** layer of the host epithelium. *Flagella* - **Flagella** are organelles primarily responsible for **bacterial motility** (movement). - *Streptococcus pyogenes* (Group A Streptococcus) is a **non-motile** organism and therefore lacks flagella.

Q: A patient presents with a history of chronic meningitis. Laboratory findings show Gram-positive, filamentous branching bacteria, Positive ZN stain, Growth on paraffin bait culture. Which of the following is the most likely causative organism?

Nocardia asteroides. ***Nocardia asteroides*** - This organism is characterized by **Gram-positive, filamentous, branching bacteria** which are also typically **weakly acid-fast** (Positive ZN stain), differentiating it from Actinomyces. - The ability to utilize paraffin as a sole carbon source and grow on **paraffin bait culture** is a classical diagnostic method specifically employed for identifying **Nocardia** species, fitting the clinical picture of chronic meningitis (Nocardiosis). *Actinomyces israelii* - Although it is a **Gram-positive, filamentous, branching bacterium**, *Actinomyces* is strictly **anaerobic** and is typically **non-acid fast** (ZN negative), which contradicts the lab finding. - It commonly causes chronic suppurative infections like lumpy jaw, often involving **sulfur granules**, and is not diagnosed via paraffin culture methods. *Mycobacterium tuberculosis* - This organism is a major cause of **chronic meningitis** and is **strongly acid-fast** (ZN positive), but its morphology is that of a simple rod (**bacillus**), not a **filamentous branching** structure. - It requires specific media like **Lowenstein-Jensen (LJ) medium** for culture and is not known to grow on paraffin bait. *Cryptococcus neoformans* - This is an encapsulated **yeast (fungus)**, not a bacterium, and therefore does not show **filamentous branching** or Gram-positive bacterial morphology. - Diagnosis is typically made via **India ink stain** or detection of **Cryptococcal antigen** in CSF, not ZN staining or paraffin bait culture.

Q: The Kanagawa phenomenon observed on Wagatsuma agar is characteristic of which of the following organisms?

Vibrio parahaemolyticus. ***Vibrio parahaemolyticus*** - The **Kanagawa phenomenon** describes the ability of certain strains of *Vibrio parahaemolyticus* to produce a **heat-stable direct hemolysin (TDH)**. - This hemolysin causes **beta-hemolysis** (complete clearing) when the organism is grown on a specialized high-salt medium like **Wagatsuma agar**, distinguishing pathogenic strains. *Vibrio cholerae* - This organism causes **cholera** and typically produces yellow colonies on **TCBS agar** (Thiosulfate Citrate Bile Salts Sucrose). - Its primary virulence mechanism is the production of **cholera toxin**, and it does not exhibit the specific Kanagawa phenomenon. *Staphylococcus aureus* - *S. aureus* produces several hemolysins (alpha, beta, delta) but is generally identified using tests like the **coagulase test** and growth on **Mannitol Salt Agar**. - The Kanagawa phenomenon on Wagatsuma agar is a specific test reserved for identifying potentially pathogenic *Vibrio* species. *Burkholderia pseudomallei* - This bacterium causes **melioidosis** and is typically identified by its characteristic **wrinkled colony morphology** on media like Ashdown agar. - It is a Gram-negative bacillus common in Southeast Asia and is not associated with the specific hemolytic action defined by the Kanagawa test.

Q: Which of the following is responsible for green color on antibiotic disk sensitivity testing?

Pyoverdine and pyocyanin. **Pyoverdine and pyocyanin** - The characteristic green color in *Pseudomonas aeruginosa* cultures, as seen in the antibiotic disk sensitivity testing agar, is due to the synergistic production of **pyoverdine** (a fluorescent yellow-green pigment and siderophore) and **pyocyanin** (a blue-green phenazine pigment). - These pigments are **excreted products** that contribute to the organism's distinctive appearance on culture media and are factors in its pathogenicity. *Pneumolysin* - **Pneumolysin** is a **cytotoxin** produced by *Streptococcus pneumoniae*, which is responsible for damaging host cells but does not produce green pigmentation in culture. - It is a key virulence factor in pneumococcal infections, contributing to inflammation and tissue injury, but its presence does not result in a green color on agar. *Optochin* - **Optochin** is an **antibiotic** used to differentiate *Streptococcus pneumoniae* (sensitive) from other alpha-hemolytic streptococci (resistant). - Its action is cidal or static against *S. pneumoniae*, creating a **zone of inhibition** around the disk, but it does not produce any color change on its own. *Alpha hemolysin* - **Alpha hemolysis** refers to the **partial lysis of red blood cells**, resulting in a greenish discoloration around bacterial colonies on blood agar, often seen with *Streptococcus pneumoniae* or viridans streptococci. - While it produces a greenish hue, the strong, pervasive green color observed across the entire agar in the context of antibiotic disk sensitivity testing for organisms like *Pseudomonas aeruginosa* is specifically due to the exopigments pyoverdine and pyocyanin, rather than solely alpha hemolysis.

Question 1

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 2

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 3

A 70-year-old patient with a smoking history presents with high-grade fever, cough, confusion, and diarrhea. Chest X-ray shows bilateral infiltrates in bilateral lower lung fields. On sputum gram stain, no organisms were detected. Laboratory results reveal Na: 126mEq/L, AST:62, ALT:56, RBS:112 mg/dl, serum bilirubin of 0.8mg%, and a positive HIV test. Which of the following organisms is responsible?

Accepted Answer

Legionella

Answer

Klebsiella

Answer

Streptococcus

Answer

Pneumocystis jeroveci

Question 4

Which of the following is an acid-fast organism?

Accepted Answer

Mycobacterium avium

Answer

Streptococcus pneumoniae

Answer

Escherichia coli

Answer

Actinomyces

Question 5

Neutrophilic granulomatous mastitis is caused by which of the following organisms?

Accepted Answer

Corynebacterium kroppenstedtii

Answer

Corynebacterium minutissimum

Answer

Staphylococcus aureus

Answer

Mycobacterium tuberculosis

Question 6

In a village, several people developed dysentery after consuming raw milk. On laboratory examination, gram-negative, curved rods with polymorphonuclear infiltration were found in stool samples. Which of the following is the most likely causative organism?

Accepted Answer

Campylobacter jejuni

Answer

Staphylococcus aureus

Answer

Vibrio parahaemolyticus

Answer

Clostridium perfringens

Question 7

Group A Streptococcus is the most common cause of bacterial pharyngitis in school-aged children. Which of the following bacterial components is primarily responsible for its attachment to fibronectin on the epithelial lining of the pharynx?

Accepted Answer

Lipoteichoic acid

Answer

Capsule

Answer

Flagella

Answer

Lipoprotein

Question 8

A patient presents with a history of chronic meningitis. Laboratory findings show Gram-positive, filamentous branching bacteria, Positive ZN stain, Growth on paraffin bait culture. Which of the following is the most likely causative organism?

Accepted Answer

Nocardia asteroides

Answer

Actinomyces israelii

Answer

Mycobacterium tuberculosis

Answer

Cryptococcus neoformans

Question 9

The Kanagawa phenomenon observed on Wagatsuma agar is characteristic of which of the following organisms?

Accepted Answer

Vibrio parahaemolyticus

Answer

Vibrio cholerae

Answer

Staphylococcus aureus

Answer

Burkholderia pseudomallei

Question 10

Which of the following is responsible for green color on antibiotic disk sensitivity testing?

Accepted Answer

Pyoverdine and pyocyanin

Answer

Pneumolysin

Answer

Optochin

Answer

Alpha hemolysin

Bacteriology — MCQs

Bacteriology — MCQs

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