Bacteriology Practice Questions

Q: True about the HACEK group of bacteria?

Require CO2 for growth. **Explanation:** The **HACEK group** is a collection of fastidious, slow-growing, **Gram-negative bacilli** that are part of the normal oropharyngeal flora. The group includes: * **H:** *Haemophilus* species (specifically *H. aphrophilus/paraphrophilus*) * **A:** *Aggregatibacter* species * **C:** *Cardiobacterium hominis* * **E:** *Eikenella corrodens* * **K:** *Kingella kingae* **Why Option D is Correct:** HACEK organisms are **capnophilic**, meaning they require an atmosphere enriched with **5–10% carbon dioxide (CO2)** for optimal growth. Because they are fastidious and slow-growing, blood cultures may need to be incubated for an extended period (though modern automated systems usually detect them within 5 days). **Why Other Options are Incorrect:** * **A & C:** HACEK organisms are **Gram-negative** (not positive) and are **facultative anaerobes** or microaerophiles, not obligate anaerobes. * **B:** *Coxiella burnetii* is the causative agent of Q fever and is an obligate intracellular bacterium; it is not part of the HACEK group. **High-Yield Clinical Pearls for NEET-PG:** 1. **Clinical Significance:** They are the most common cause of **culture-negative infective endocarditis** (in the context of subacute presentation) in non-IV drug users. 2. **Eikenella corrodens:** Associated with "clenched fist injuries" (human bites) and produces a characteristic **bleach-like odor** and "pitting" of the agar. 3. **Kingella kingae:** A common cause of septic arthritis and osteomyelitis in children. 4. **Treatment:** Most HACEK organisms are now treated with **Ceftriaxone** (3rd generation cephalosporins) as they may produce beta-lactamases.

Q: Which of the following Corynebacterium species produces a toxin similar to that of diphtheria toxin?

Corynebacterium ulcerans. ### Explanation **Correct Answer: C. Corynebacterium ulcerans** **Why it is correct:** *Corynebacterium ulcerans* and *Corynebacterium pseudotuberculosis* are the only two non-diphtherial species that can carry the **tox gene** (acquired via lysogenic bacteriophages). This gene allows them to produce a toxin that is functionally and immunologically identical to the **Diphtheria Toxin** produced by *C. diphtheriae*. * *C. ulcerans* is a zoonotic pathogen typically transmitted through raw milk or contact with infected farm animals/pets. It can cause a clinical presentation indistinguishable from classical diphtheria, including the formation of a **pharyngeal pseudomembrane** and systemic complications like myocarditis. **Analysis of Incorrect Options:** * **A. Corynebacterium auris:** Primarily associated with ear infections (otitis media); it does not produce diphtheria toxin. * **B. Corynebacterium jeikeium (jikei):** A common opportunistic pathogen in immunocompromised patients (especially those with intravascular catheters). It is notorious for being **multi-drug resistant** but does not produce the toxin. * **C. Corynebacterium striatum:** A commensal of the skin and nasopharynx that can cause device-related infections and endocarditis; it is non-toxigenic. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mechanism of Action:** Diphtheria toxin inhibits protein synthesis by **ADP-ribosylation of Elongation Factor-2 (EF-2)**. 2. **Diagnosis:** To confirm toxin production in *C. ulcerans*, the **Elek Test** or PCR for the *tox* gene is used. 3. **Treatment:** Infections with toxigenic *C. ulcerans* require **Diphtheria Antitoxin (DAT)** in addition to antibiotics (Erythromycin or Penicillin). 4. **Zoonosis:** Unlike *C. diphtheriae* (human-only reservoir), *C. ulcerans* is a **zoonotic** infection.

Q: All of the following are true about Listeria except?

Non-motile. **Explanation:** The correct answer is **C (Non-motile)** because *Listeria monocytogenes* is characteristically **motile**. It exhibits a unique **"tumbling motility"** at 25°C (room temperature) due to peritrichous flagella, while it remains non-motile at 37°C. In semi-solid agar, it produces a classic **"umbrella-shaped"** growth pattern. **Analysis of Options:** * **A. Catalase positive:** This is a true statement. *Listeria* is a Gram-positive bacillus that is catalase-positive, which helps differentiate it from *Streptococcus agalactiae* (GBS), which is catalase-negative but also causes neonatal sepsis. * **B. Anton test positive:** This is true. The Anton test is a diagnostic test where the instillation of the culture into a rabbit's conjunctival sac causes **purulent keratoconjunctivitis**. * **D. Causes chorioamnionitis:** This is true. *Listeria* has a predilection for the placenta. In pregnant females, it can lead to chorioamnionitis, premature labor, abortion, or **Granulomatosis infantiseptica** (disseminated abscesses/granulomas in the fetus). **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, non-sporing, coccobacilli in short chains. * **Cold Enrichment:** It can grow at temperatures as low as 4°C (psychrophilic), which is why it is associated with refrigerated foods like unpasteurized cheese and cold meats. * **Intracellular Pathogen:** It uses **Listeriolysin O** to escape phagosomes and **ActA protein** for "actin rockets" to move between cells. * **Drug of Choice:** Ampicillin (Note: It is inherently resistant to cephalosporins).

Q: What is true about Staphylococcus aureus?

Grows in 10% NaCl. ### Explanation **Correct Option: C. Grows in 10% NaCl** *Staphylococcus aureus* is a **haloduric** (salt-tolerant) organism. It can survive and grow in high salt concentrations, up to 10% NaCl. This physiological property is utilized in the laboratory for selective isolation. **Mannitol Salt Agar (MSA)**, which contains 7.5% NaCl, is the selective medium of choice; *S. aureus* not only grows on it but also ferments mannitol, turning the phenol red indicator yellow. **Analysis of Incorrect Options:** * **A. Microaerophilic:** This is incorrect. *Staphylococci* are **facultative anaerobes**. They grow best under aerobic conditions but can also grow in the absence of oxygen. Microaerophilic organisms (like *Campylobacter*) require reduced levels of oxygen. * **B. Produces lemon yellow colonies:** This is a common distractor. *S. aureus* typically produces **golden yellow** colonies (hence the name *aureus*, meaning gold) due to the pigment **staphyloxanthin**. Lemon yellow colonies are characteristic of *Micrococcus*. * **D. All are true:** Incorrect, as options A and B are false. **NEET-PG High-Yield Pearls:** * **Coagulase Test:** The definitive test to differentiate *S. aureus* (Positive) from Coagulase-negative Staphylococci (CoNS). * **Catalase Test:** Differentiates *Staphylococci* (Positive) from *Streptococci* (Negative). * **Protein A:** A major virulence factor that binds to the Fc portion of IgG, preventing opsonization and phagocytosis. * **Commonest Cause:** *S. aureus* is the leading cause of surgical site infections (SSI), osteomyelitis, and acute bacterial endocarditis in IV drug users.

Q: A 34-year-old pregnant female presented to the ER with fever, backache, headache, and myalgias. Blood cultures were collected and subjected to examination. Microbiological examination revealed positive motility at 25 degrees Celsius, catalase positivity, esculin positivity, and positive growth in 6.5% NaCl. Gram staining revealed the following characteristics of the organism isolated. Which of the following is the most common route of transmission of this organism?

Ingestion of contaminated food. ***Ingestion of contaminated food*** - The microbiological characteristics (motility at 25°C, **catalase positive**, **esculin positive**, growth in 6.5% NaCl) identify this as **Listeria monocytogenes**, which is primarily transmitted through **contaminated food**. - **Ready-to-eat foods** like deli meats, soft cheeses, and unpasteurized dairy products are common sources, making this especially concerning in **pregnancy** due to risk of **listeriosis**. *Aerosol person-to-person transmission* - **Listeria monocytogenes** is not transmitted through respiratory droplets or aerosols between individuals. - This route is typical for organisms like **Mycobacterium tuberculosis** or **influenza virus**, not for Listeria. *Feco-oral route* - While Listeria can be found in feces, **direct feco-oral transmission** is not the primary route of infection. - This route is more characteristic of **enteric pathogens** like **Salmonella** or **Shigella** species. *Sexually transmitted* - **Listeria monocytogenes** is not considered a **sexually transmitted pathogen**. - **Vertical transmission** from mother to fetus can occur, but this is through **hematogenous spread**, not sexual contact.

Q: A 65-year-old diabetic man presents with a severe productive cough producing thick, bloody sputum with a "currant-jelly" appearance. MacConkey agar reveals pink colonies, and routine laboratory media shows large mucoid colonies. Which of the following organisms is most likely responsible for this patient's pneumonia?

Klebsiella pneumoniae. **Explanation:** The clinical presentation and laboratory findings are classic for **Klebsiella pneumoniae**. **1. Why the Correct Answer is Right:** * **Clinical Presentation:** *Klebsiella* typically causes severe, necrotizing lobar pneumonia in patients with underlying conditions like **diabetes mellitus** or chronic alcoholism. The hallmark sign is **"currant-jelly" sputum**, which results from significant tissue necrosis and inflammation leading to blood-tinged, thick mucus. * **Laboratory Findings:** *Klebsiella* is a **Lactose Fermenter (LF)**, appearing as **pink colonies** on MacConkey agar. It possesses a prominent polysaccharide capsule, which manifests as **large, mucoid colonies** on routine media (e.g., Blood agar or Nutrient agar). **2. Why Incorrect Options are Wrong:** * **Enterobacter cloacae:** While it is a lactose fermenter, it is rarely associated with the specific "currant-jelly" sputum or the high degree of mucoid growth seen in *Klebsiella*. * **Escherichia coli:** Although an LF (pink colonies), it is an uncommon cause of primary lobar pneumonia; it more frequently causes UTIs or neonatal meningitis. * **Pseudomonas aeruginosa:** This is a **Non-Lactose Fermenter (NLF)**, producing pale/colorless colonies on MacConkey agar. It typically produces a characteristic fruity odor and blue-green pigment (pyocyanin). **3. High-Yield Clinical Pearls for NEET-PG:** * **Friedlander’s Bacillus:** The historical name for *K. pneumoniae*. * **String Test:** Positive for hypervirulent strains (a loop can pull a mucoid string >5mm from the colony). * **Imaging:** Often shows a "bulging fissure sign" on chest X-ray due to heavy inflammatory exudate. * **Biochemicals:** It is **Indole negative** (unlike *E. coli*), **Urease positive**, and **non-motile** (distinguishing it from other Enterobacteriaceae).

Q: Which of the following does not cause invasive diarrhea?

Bacillus cereus. **Explanation:** The mechanism of diarrhea is broadly classified into **secretory (non-invasive)** and **inflammatory (invasive)**. **Why Bacillus cereus is the correct answer:** *Bacillus cereus* causes diarrhea through the production of enterotoxins rather than mucosal invasion. It exhibits two distinct clinical syndromes: the **emetic type** (associated with fried rice and a preformed toxin, *cereulide*) and the **diarrheal type** (associated with meat/vegetables and an *in-vivo* produced heat-labile enterotoxin). The diarrheal toxin stimulates the adenylate cyclase-cAMP system, leading to intestinal secretion of water and electrolytes without invading the bowel wall. **Analysis of incorrect options:** * **Salmonella:** Non-typhoidal Salmonella species (like *S. enteritidis*) invade the intestinal mucosa of the distal small bowel and colon, leading to an inflammatory response and potential bacteremia. * **Shigella:** This is the prototype for invasive diarrhea. It invades the M cells of Peyer's patches and spreads laterally between epithelial cells using actin tails, causing mucosal ulceration and bloody stools (dysentery). * **Aeromonas:** Though often associated with water-borne transmission, *Aeromonas hydrophila* can cause an invasive, dysentery-like illness by directly invading the intestinal mucosa and producing cytotoxins. **NEET-PG High-Yield Pearls:** * **Invasive Organisms:** Remember the mnemonic **"SSYCE"**—*Shigella, Salmonella, Yersinia, Campylobacter,* and *EIEC (Enteroinvasive E. coli)*. * **B. cereus Emetic Type:** Short incubation (1–6 hours); heat-stable toxin; mimics *S. aureus* food poisoning. * **B. cereus Diarrheal Type:** Long incubation (8–16 hours); heat-labile toxin; mimics *C. perfringens* food poisoning.

Q: Phage typing is widely used for the intraspecies classification of which of the following bacteria?

Staphylococci. **Explanation:** **Phage typing** is a method used for the epidemiological surveillance and intraspecies classification of bacteria based on their susceptibility to specific bacteriophages. **Why Staphylococci is correct:** *Staphylococcus aureus* is the classic organism for which phage typing is the "gold standard" for strain differentiation. It is primarily used to trace the source of hospital-acquired outbreaks (nosocomial infections). The International Reference Center for Staphylococcal Phage Typing has established a standardized set of 22 phages (divided into four groups) to identify specific patterns or "phage types" of *S. aureus*. **Why other options are incorrect:** * **E. coli:** While phage typing exists for certain strains (like O157:H7), it is not the primary method for intraspecies classification. Serotyping (O and H antigens) and molecular methods (MLST) are more commonly used. * **Klebsiella pneumoniae:** Classification is predominantly done via **Capsular (K) typing** and **Bacteriocin typing**. * **Pseudomonas aeruginosa:** While phage typing is possible, **Pyocin typing** (bacteriocin typing) is the more traditional and widely recognized method for intraspecies differentiation of this organism. **High-Yield Clinical Pearls for NEET-PG:** * **Bacteriocin Typing:** Most commonly used for *Pseudomonas aeruginosa* (Pyocin) and *Shigella sonnei* (Colicin). * **Vi-phage typing:** Specifically used for *Salmonella Typhi* to track typhoid fever outbreaks. * **Modern Trend:** Phage typing is being rapidly replaced by molecular methods like **PFGE (Pulsed-field gel electrophoresis)** and **Whole Genome Sequencing (WGS)**, which offer higher discriminatory power.

Q: What is the most common organism causing osteomyelitis?

Staphylococcus Aureus. **Explanation:** **Staphylococcus aureus** is the most common cause of pyogenic osteomyelitis across almost all age groups and clinical scenarios. Its dominance is due to specific virulence factors, such as **surface adhesins** (MSCRAMMs) that allow it to bind to bone matrix components like collagen, and its ability to form **biofilms**, which protect the bacteria from host immune responses and antibiotics. **Analysis of Options:** * **Option A (Correct):** *S. aureus* is the overall leading cause. Even in specific populations like neonates or patients with Sickle Cell Disease (where *Salmonella* is a high-yield association), *S. aureus* remains a primary pathogen. * **Option B:** *Streptococcus pneumoniae* can cause osteomyelitis but is much rarer, usually occurring secondary to respiratory infections or in immunocompromised states. * **Option C:** *Haemophilus influenzae* was previously a common cause in children under age 5; however, its incidence has plummeted due to the widespread adoption of the **HiB vaccine**. * **Option D:** *Escherichia coli* and other Gram-negative bacilli are common causes of osteomyelitis in elderly patients, IV drug users, or following urinary tract infections, but they do not surpass *S. aureus* in frequency. **High-Yield Clinical Pearls for NEET-PG:** * **Sickle Cell Disease:** *Salmonella* is the most characteristic organism, but *S. aureus* is still statistically the most common. * **IV Drug Users:** High incidence of *Pseudomonas aeruginosa* (often affecting the S-joints: Sternoclavicular and Sacroiliac). * **Puncture wound through footwear:** Highly associated with *Pseudomonas aeruginosa*. * **Neonates:** Group B Streptococcus and *E. coli* are significant alongside *S. aureus*. * **Chronic Osteomyelitis:** Characterized by the **Involucrum** (new bone) and **Sequestrum** (dead bone).

Question 1

Bacterial endocarditis is most commonly caused by which organism?

Accepted Answer

Staphylococcus aureus

Answer

alpha-Hemolytic Streptococci

Answer

beta-Hemolytic Streptococci

Answer

Cardiobacterium

Question 2

True about the HACEK group of bacteria?

Accepted Answer

Require CO2 for growth

Answer

Anaerobes

Answer

Includes Coxiella burnetii

Answer

Gram-positive

Question 3

Which of the following Corynebacterium species produces a toxin similar to that of diphtheria toxin?

Accepted Answer

Corynebacterium ulcerans

Answer

Corynebacterium auris

Answer

Corynebacterium jikei

Answer

Corynebacterium striatum

Question 4

All of the following are true about Listeria except?

Accepted Answer

Non-motile

Answer

Catalase positive

Answer

Anton test positive

Answer

Causes chorioamnionitis in pregnant females

Question 5

What is true about Staphylococcus aureus?

Accepted Answer

Grows in 10% NaCl

Answer

Microaerophilic

Answer

Produces lemon yellow colonies

Answer

All are true

Question 6

A 34-year-old pregnant female presented to the ER with fever, backache, headache, and myalgias. Blood cultures were collected and subjected to examination. Microbiological examination revealed positive motility at 25 degrees Celsius, catalase positivity, esculin positivity, and positive growth in 6.5% NaCl. Gram staining revealed the following characteristics of the organism isolated. Which of the following is the most common route of transmission of this organism?

Accepted Answer

Ingestion of contaminated food

Answer

Aerosol person-to-person transmission

Answer

Feco-oral route

Answer

Sexually transmitted

Question 7

A 65-year-old diabetic man presents with a severe productive cough producing thick, bloody sputum with a "currant-jelly" appearance. MacConkey agar reveals pink colonies, and routine laboratory media shows large mucoid colonies. Which of the following organisms is most likely responsible for this patient's pneumonia?

Accepted Answer

Klebsiella pneumoniae

Answer

Enterobacter cloacae

Answer

Escherichia coli

Answer

Pseudomonas aeruginosa

Question 8

Which of the following does not cause invasive diarrhea?

Accepted Answer

Bacillus cereus

Answer

Salmonella

Answer

Aeromonas

Answer

Shigella

Question 9

Phage typing is widely used for the intraspecies classification of which of the following bacteria?

Accepted Answer

Staphylococci

Answer

E. coli

Answer

Klebsiella pneumoniae

Answer

Pseudomonas aeruginosa

Question 10

What is the most common organism causing osteomyelitis?

Accepted Answer

Staphylococcus Aureus

Answer

Streptococcus Pneumoniae

Answer

Haemophilus Influenzae

Answer

Escherichia Coli

Bacteriology — MCQs

Bacteriology — MCQs

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