Bacteriology Practice Questions

Q: How can Mycobacterium TB be differentiated from other mycobacteria?

Production of niacin. **Explanation:** The differentiation of *Mycobacterium tuberculosis* (MTB) from Non-Tuberculous Mycobacteria (NTM) is a high-yield topic in bacteriology. **1. Why Niacin Production is Correct:** All mycobacteria produce niacin (nicotinic acid) as a metabolic byproduct. However, most species possess an enzyme that converts niacin into niacin ribonucleotide. **MTB lacks this enzyme**, leading to an accumulation of free niacin in the culture medium. When tested with cyanogen bromide and aniline (Konno’s test), a positive result (canary yellow color) is highly characteristic of MTB. **2. Analysis of Incorrect Options:** * **Arylsulphatase test:** This test is primarily used to identify **Rapid Growers** (Runyon Group IV), specifically *M. fortuitum* and *M. chelonae*, which produce the enzyme within 3 days. MTB is typically negative. * **Coagulase test:** This is a biochemical test used to differentiate *Staphylococcus aureus* (positive) from Coagulase-negative Staphylococci (CoNS). It has no application in mycobacteriology. * **Bile solubility test:** This is the gold standard for identifying ***Streptococcus pneumoniae***, which lyses in the presence of bile salts. **3. NEET-PG Clinical Pearls:** * **Nitrate Reduction Test:** MTB is also positive for nitrate reduction, which helps differentiate it from *M. bovis* (negative). * **Catalase Test:** MTB is **catalase-negative at 68°C** (heat-labile catalase). Most NTMs remain catalase-positive at this temperature. * **Culture Media:** While LJ (Lowenstein-Jensen) medium is the traditional solid medium, **MGIT (Mycobacteria Growth Indicator Tube)** is the current gold standard for rapid liquid culture. * **Morphology:** MTB shows "serpentine cords" in liquid culture due to the presence of **Cord Factor**, a virulence factor that inhibits neutrophil migration.

Q: An infant presents with hemolytic uremic syndrome and thrombocytopenia. Stool culture reveals non-lactose fermenting colonies on Sorbitol MacConkey agar that also ferment glucose with gas. Which of the following bacteria is most likely isolated from the stool specimen?

E. coli O157/H7. ### Explanation The clinical presentation of **Hemolytic Uremic Syndrome (HUS)**—characterized by the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure—following a diarrheal illness is classic for **Enterohemorrhagic *E. coli* (EHEC)**, specifically the **O157:H7** serotype. **Why the correct answer is right:** * **Sorbitol MacConkey (SMAC) Agar:** While most *E. coli* strains ferment sorbitol, the O157:H7 strain is unique because it is a **non-sorbitol fermenter (NSF)**. It appears as colorless/pale colonies on SMAC agar, which is the gold standard for its identification. * **Biochemical Profile:** As a member of the *Enterobacteriaceae* family, *E. coli* is a **glucose fermenter** and typically produces **gas** (CO2 and H2) during fermentation. * **Pathogenesis:** It produces **Shiga-like toxins (Verotoxins)** which damage glomerular endothelial cells, leading to HUS. **Why the incorrect options are wrong:** * **Shigella:** While *S. dysenteriae* type 1 can cause HUS via Shiga toxin, it is **anaerogenic** (does not produce gas from glucose fermentation). * **Salmonella:** These are non-lactose fermenters and produce gas, but they are not typically associated with HUS or the specific NSF profile on SMAC agar used for screening. * **Aeromonas:** These are oxidase-positive (unlike *Enterobacteriaceae*) and are generally associated with watery diarrhea rather than the classic HUS presentation. **NEET-PG High-Yield Pearls:** * **EHEC/VTEC:** Most common cause of HUS in children. * **Transmission:** Often via undercooked ground beef (hamburger) or contaminated milk/water. * **Antibiotics:** Generally avoided in EHEC infections as they may increase toxin release and worsen the risk of HUS. * **Culture:** Look for "Colorless colonies on Sorbitol MacConkey" in the question stem to immediately point toward O157:H7.

Q: What is the drug of choice for meningococcal chemoprophylaxis?

Rifampicin. **Explanation:** The primary goal of chemoprophylaxis in meningococcal disease is to eradicate the nasopharyngeal carriage of *Neisseria meningitidis* in close contacts, thereby preventing the spread of the bacteria and the development of secondary cases. **Why Rifampicin is the Correct Answer:** **Rifampicin** is considered the traditional drug of choice for meningococcal chemoprophylaxis because it achieves high concentrations in salivary and nasopharyngeal secretions. It effectively eliminates the carrier state in approximately 90% of cases. The standard adult dose is 600 mg twice daily for 2 days. **Analysis of Incorrect Options:** * **A. Penicillin:** While Penicillin G is a treatment of choice for active meningococcal meningitis, it does not eliminate the nasopharyngeal carrier state because it does not reach sufficient concentrations in respiratory secretions. * **B. Gentamicin:** Aminoglycosides have poor penetration into the cerebrospinal fluid and respiratory secretions; they are not effective against *N. meningitidis*. * **C. Chloramphenicol:** This is used as an alternative for treating active meningitis in patients with severe penicillin allergy, but it is not used for prophylaxis due to its potential for serious bone marrow toxicity. **High-Yield Clinical Pearls for NEET-PG:** * **Alternative Agents:** While Rifampicin is the classic answer, **Ciprofloxacin** (500 mg single dose) and **Ceftriaxone** (250 mg IM single dose) are also highly effective and frequently used in clinical practice. * **Ceftriaxone** is the preferred prophylactic agent for **pregnant women**. * **Close Contacts:** Prophylaxis is indicated for household members, daycare contacts, and healthcare workers directly exposed to respiratory secretions (e.g., during intubation). * **Timing:** Prophylaxis should ideally be administered within 24 hours of identifying the index case.

Q: What is the most likely candidate responsible for the production of overwhelming septicemic shock complicating bacteremia with Neisseria meningitidis?

Lipopolysaccharide and outer membrane. ### Explanation **Correct Option: C (Lipopolysaccharide and outer membrane)** The primary mediator of septic shock in *Neisseria meningitidis* (Meningococcus) is its **Lipooligosaccharide (LOS)**, which is the neisserial equivalent of Lipopolysaccharide (LPS) found in the outer membrane. Unlike most Gram-negative bacteria, Meningococci shed excessive amounts of their outer membrane in the form of "blebs" during rapid growth. These blebs release massive quantities of LOS into the bloodstream. LOS acts as a potent **endotoxin**, triggering the release of pro-inflammatory cytokines (TNF-α, IL-1, and IL-6). This leads to the classic triad of meningococcemia: disseminated intravascular coagulation (DIC), massive hemorrhage into the adrenal glands (**Waterhouse-Friderichsen Syndrome**), and profound septic shock. **Why other options are incorrect:** * **A. Capsular polysaccharide:** This is the primary **virulence factor** responsible for preventing phagocytosis and is used for serotyping and vaccine production, but it does not trigger the cytokine storm required for shock. * **B. Pili:** These are essential for **initial attachment** and colonization of the nasopharyngeal epithelium. * **D. Low-molecular-weight outer membrane proteins (OMPs):** Proteins like PorA and PorB function as porins and are involved in nutrient transport and intracellular survival, but they lack the endotoxic activity of LOS. --- ### High-Yield NEET-PG Pearls * **LOS vs. LPS:** *Neisseria* lacks the "O-antigen" side chain found in enteric bacteria, hence it is called Lipooligosaccharide (LOS). * **Waterhouse-Friderichsen Syndrome:** Characterized by adrenal insufficiency, petechial rash, and shock. * **DOC:** Ceftriaxone is the drug of choice for treatment; Rifampicin is used for prophylaxis of close contacts. * **Nasopharynx:** The natural reservoir and portal of entry for *N. meningitidis*.

Q: Which is the most common microorganism known to cause tropical pyomyositis?

Staphylococcus aureus. **Explanation:** **Tropical Pyomyositis** is a primary intramuscular abscess involving large skeletal muscles (most commonly the quadriceps or gluteal muscles). It is typically seen in tropical climates and often follows minor local trauma. **Why Staphylococcus aureus is correct:** *Staphylococcus aureus* is the causative agent in **70% to 90%** of all cases. The pathogenesis involves a transient bacteremia (often from an occult site) that seeds into a muscle previously damaged by trauma, vigorous exercise, or nutritional deficiencies. Its potent arsenal of toxins and enzymes (like coagulase and leukocidins) facilitates tissue necrosis and abscess formation within the muscle fascia. **Why other options are incorrect:** * **Streptococcus viridans:** These are commensals of the oral cavity and are primarily associated with subacute bacterial endocarditis, not primary muscle abscesses. * **Staphylococcus albus (S. epidermidis):** This is a coagulase-negative staphylococcus (CoNS). While it is a common cause of prosthetic valve endocarditis and catheter-associated infections, it lacks the virulence factors to cause pyomyositis in healthy muscle. * **Klebsiella pneumoniae:** While Gram-negative bacilli can cause pyomyositis in immunocompromised or diabetic patients, they are rare in the classic "tropical" presentation compared to *S. aureus*. **High-Yield Clinical Pearls for NEET-PG:** * **Most common site:** Quadriceps muscle. * **Clinical Stages:** 1. Invasive stage (cramping/pain), 2. Suppurative stage (abscess formation - most common presentation), 3. Late stage (sepsis). * **Diagnosis:** Ultrasound or MRI is the imaging modality of choice to localize the deep-seated abscess. * **Treatment:** Incision and drainage (I&D) combined with antibiotics (cloxacillin or vancomycin if MRSA is suspected).

Q: Bannwah's syndrome develops secondary to infection with which of the following organisms?

Treponema pallidum. **Explanation:** **Bannwarth’s Syndrome** (also known as Garin-Bujadoux-Bannwarth syndrome) is a clinical triad characterized by lymphocytic pleocytosis in the CSF, cranial neuropathy (most commonly facial nerve palsy), and painful radiculoneuritis. While historically associated with various spirochetes, in modern clinical practice and NEET-PG examinations, it is classically recognized as the hallmark of **Early Disseminated Lyme Disease (Stage 2)**, caused by **Borrelia burgdorferi**. *Note: There appears to be a discrepancy in the provided key. In standard medical literature, Bannwarth’s Syndrome is the neuroborreliosis manifestation of Borrelia burgdorferi, not Treponema pallidum.* **Analysis of Options:** * **A. Borrelia burgdorferi (Correct):** This spirochete causes Lyme disease. Bannwarth’s syndrome represents the neurological involvement during the second stage of the infection, particularly common in European cases. * **B. Treponema pallidum:** Causes Syphilis. While it can cause Neurosyphilis (tabes dorsalis, general paresis), it is not the causative agent of Bannwarth’s syndrome. * **C. Bacillus cereus:** A gram-positive rod associated with food poisoning (emetic and diarrheal forms) and endophthalmitis. * **D. Actinomyces israelii:** An anaerobic bacterium causing "lumpy jaw" (cervicofacial actinomycosis) characterized by sulfur granules. **High-Yield Clinical Pearls for NEET-PG:** * **Lyme Disease Stages:** 1. *Early Localized:* Erythema chronicum migrans (bull’s eye rash). 2. *Early Disseminated:* Bannwarth’s Syndrome, AV block, multiple erythema migrans. 3. *Late:* Chronic arthritis, acrodermatitis chronica atrophicans. * **Vector:** *Ixodes* tick (also transmits Babesia and Anaplasma). * **Treatment:** Doxycycline is the drug of choice; Ceftriaxone is used for neurological or cardiac involvement.

Q: Actinomycosis is sensitive to which of the following antibiotics?

Penicillin. **Explanation:** **Actinomycosis** is a chronic, granulomatous infection caused by *Actinomyces* species (most commonly *A. israelii*). Despite their fungal-sounding name, these organisms are **Gram-positive, anaerobic, non-acid-fast branching bacteria**. 1. **Why Penicillin is Correct:** Because *Actinomyces* are true bacteria (possessing a peptidoglycan cell wall and lacking a nuclear membrane), they are highly susceptible to antibacterial agents. **High-dose Penicillin G** is the drug of choice. Treatment typically requires a prolonged course (weeks to months) due to the dense fibrotic nature of the lesions and the presence of "sulfur granules" which limit drug penetration. 2. **Why Other Options are Incorrect:** * **Streptomycin (A):** This is an aminoglycoside primarily used for aerobic Gram-negative bacteria and *M. tuberculosis*. It is ineffective against the anaerobic environment where *Actinomyces* thrives. * **Nystatin (B):** This is an antifungal medication. While *Actinomyces* was historically confused with fungi due to its branching morphology, it is a bacterium; therefore, antifungals have no effect. * **Iodoxuridine (D):** This is an antiviral agent used primarily for Herpes Simplex Keratitis. It has no role in treating bacterial infections. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Characterized by "Sulfur granules" (yellowish colonies of bacteria) in pus. * **Clinical Presentation:** Most common form is **Cervicofacial actinomycosis** ("Lumpy Jaw"), often following dental procedures or poor oral hygiene. * **Diagnosis:** Crushing a sulfur granule and performing a Gram stain reveals Gram-positive branching filaments. * **Alternative Treatment:** In patients allergic to penicillin, **Tetracyclines** or Erythromycin are the preferred alternatives.

Q: What is the ideal percentage of CO2 required for the growth of Brucella abortus?

5-10%. **Explanation:** *Brucella abortus* is a fastidious, Gram-negative coccobacillus that is **capnophilic**, meaning it requires an enriched carbon dioxide (CO2) atmosphere for primary isolation. **1. Why 5-10% is Correct:** Most strains of *Brucella abortus* (specifically Biotype 1) are strictly dependent on added CO2 for growth, especially during initial culture from clinical specimens. The ideal concentration is **5-10% CO2**. This environment mimics the metabolic conditions required for the organism to trigger its biosynthetic pathways. While *B. melitensis* and *B. suis* generally do not require supplemental CO2, *B. abortus* is the classic example of a capnophilic *Brucella* species. **2. Why Incorrect Options are Wrong:** * **2-5% (Option A):** This concentration is often insufficient to stimulate the growth of fastidious *B. abortus* strains, which require a more robust CO2 tension. * **15-20% and 25-30% (Options C & D):** These concentrations are excessively high. High levels of CO2 can lead to the acidification of the culture medium (forming carbonic acid), which inhibits the growth of *Brucella* species rather than promoting it. **3. NEET-PG High-Yield Pearls:** * **Culture Media:** Use **Castaneda’s medium** (a biphasic medium containing both solid and liquid phases) to reduce the risk of laboratory-acquired infections. * **Incubation:** Cultures should be incubated for at least **3-4 weeks** before being declared negative, although automated systems (like BACTEC) usually detect growth within 5-7 days. * **Clinical Presentation:** Look for **"Undulant Fever"** (Malta fever) and profuse sweating with a "mousy" odor. * **Diagnosis:** The **Standard Agglutination Test (SAT)** is the most common serological test; a titer of 1:160 or more is significant. Note that *Brucella* can show a **prozone phenomenon**, leading to false negatives at low dilutions.

Question 1

How can Mycobacterium TB be differentiated from other mycobacteria?

Accepted Answer

Production of niacin

Answer

Arylsulphatase test

Answer

Coagulase test

Answer

Bile solubility test

Question 2

An oxidase-positive, gram-negative rod which produces a bluish-green pigment has been grown in culture from a swab obtained from an infected burn wound. This organism is susceptible to gentamicin, ticarcillin and tobramicin, but resistant to all the other antibiotics. Which of the following organisms is it likely to be?

Accepted Answer

Pseudomonas aeruginosa

Answer

Escherichia coli

Answer

Klebsiella pneumoniae

Answer

Proteus mirabilis

Question 3

An infant presents with hemolytic uremic syndrome and thrombocytopenia. Stool culture reveals non-lactose fermenting colonies on Sorbitol MacConkey agar that also ferment glucose with gas. Which of the following bacteria is most likely isolated from the stool specimen?

Accepted Answer

E. coli O157/H7

Answer

Shigella

Answer

Salmonella

Answer

Aeromonas

Question 4

What is the drug of choice for meningococcal chemoprophylaxis?

Accepted Answer

Rifampicin

Answer

Penicillin

Answer

Gentamicin

Answer

Chloramphenicol

Question 5

What is the most likely candidate responsible for the production of overwhelming septicemic shock complicating bacteremia with Neisseria meningitidis?

Accepted Answer

Lipopolysaccharide and outer membrane

Answer

Capsular polysaccharide

Answer

Pili

Answer

Low-molecular-weight outer membrane proteins

Question 6

Which is the most common microorganism known to cause tropical pyomyositis?

Accepted Answer

Staphylococcus aureus

Answer

Streptococcus viridans

Answer

Staphylococcus albus

Answer

Klebsiella pneumoniae

Question 7

Which of the following statements regarding Salmonella infection is true?

Accepted Answer

Absence of Vi antibody indicates a poor prognosis.

Answer

Flagellar antigen forms chalky granular clumps.

Answer

Somatic antigen is a protein.

Answer

Serological classification is by Craige's scheme.

Question 8

Bannwah's syndrome develops secondary to infection with which of the following organisms?

Accepted Answer

Treponema pallidum

Answer

Borrelia burgdorferi

Answer

Bacillus cereus

Answer

Actinomyces israelii

Question 9

Actinomycosis is sensitive to which of the following antibiotics?

Accepted Answer

Penicillin

Answer

Streptomycin

Answer

Nystatin

Answer

Iodoxuridine

Question 10

What is the ideal percentage of CO2 required for the growth of Brucella abortus?

Accepted Answer

5-10%

Answer

2-5%

Answer

15-20%

Answer

25-30%

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?