Bacteriology Practice Questions

Q: What is the causative organism of hemolytic uremic syndrome?

Shigella. **Explanation:** **Hemolytic Uremic Syndrome (HUS)** is a clinical triad consisting of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. **Why Shigella is the correct answer:** The primary causative agent among the options is **Shigella dysenteriae type 1**. It produces the potent **Shiga toxin (Stx)**. This toxin binds to the Gb3 receptors on glomerular endothelial cells, leading to endothelial damage, microthrombi formation, and subsequent renal failure. While *Enterohemorrhagic E. coli* (EHEC/O157:H7) is the most common cause of HUS globally, *Shigella dysenteriae* is a classic and frequently tested cause in the context of bacillary dysentery. **Why the other options are incorrect:** * **ETEC (Option A):** Causes traveler’s diarrhea via heat-labile (LT) and heat-stable (ST) toxins. It does not produce Shiga toxin and is not associated with HUS. * **Salmonella (Option B):** Typically causes enteric fever or gastroenteritis. While it can cause systemic illness, it does not produce the specific toxins required to trigger HUS. * **Pseudomonas (Option D):** An opportunistic pathogen causing pneumonia, UTIs, and sepsis (especially in burns or cystic fibrosis), but it is not an etiological agent for HUS. **NEET-PG High-Yield Pearls:** * **Most common cause overall:** Enterohemorrhagic *E. coli* (EHEC), specifically the O157:H7 strain (produces Shiga-like toxin/Verotoxin). * **The Toxin:** Shiga toxin inhibits protein synthesis by damaging the **28S ribosomal RNA** (60S subunit). * **Clinical Tip:** Antibiotics are generally **avoided** in EHEC-related diarrhea as they may increase toxin release and worsen the risk of HUS. * **Schistocytes:** A peripheral smear in HUS will characteristically show fragmented RBCs (schistocytes) due to mechanical destruction in microvasculature.

Q: Which of the following is NOT a metastatic complication of gonococci infection?

Nephritis. **Explanation:** *Neisseria gonorrhoeae* primarily causes localized infections of the urogenital tract. However, in approximately 0.5% to 3% of cases, the bacteria enter the bloodstream, leading to **Disseminated Gonococcal Infection (DGI)**. **1. Why Nephritis is the Correct Answer:** Nephritis is **not** a recognized metastatic complication of gonococcal infection. While *N. gonorrhoeae* can cause ascending infections like pelvic inflammatory disease (PID) or localized urethritis, it does not typically seed the kidneys via the hematogenous route. In contrast, renal involvement is more characteristic of other bacterial infections (like *Staphylococcus aureus* or *Streptococcus pyogenes* via immune-mediated mechanisms). **2. Analysis of Incorrect Options (Metastatic Sites):** When gonococci disseminate (DGI), they tend to localize in specific extra-genital sites: * **Arthritis (Option D):** This is the most common metastatic complication. It typically presents as either a polyarthralgia-tenosynovitis syndrome or a purulent monoarthritis (septic arthritis). * **Endocarditis (Option A):** A rare but severe complication where the bacteria seed the heart valves. * **Meningitis (Option B):** Another rare manifestation where the organism crosses the blood-brain barrier. **High-Yield Clinical Pearls for NEET-PG:** * **Thayer-Martin Medium:** The selective medium of choice for isolating *N. gonorrhoeae*. * **DGI Triad:** Tenosynovitis, dermatitis (painless pustules), and polyarthralgia. * **Virulence Factor:** Pili are essential for initial attachment to mucosal surfaces. * **Deficiency Link:** Patients with **late complement component deficiencies (C5–C9)** are at a significantly higher risk for disseminated gonococcal infections.

Q: Which species of Brucella is acquired from contact with goats and sheep?

B. melitensis. **Explanation:** Brucellosis is a zoonotic infection caused by small, Gram-negative coccobacilli. Humans are accidental hosts, acquiring the infection through direct contact with infected animals or consumption of unpasteurized dairy products. **1. Why B. melitensis is correct:** * **B. melitensis** is the most common and virulent species causing human brucellosis worldwide. Its primary reservoirs are **goats and sheep**. It is highly prevalent in the Mediterranean, Middle East, and parts of India. **2. Why the other options are incorrect:** * **B. suis:** The primary reservoir is **pigs (swine)**. It is known for causing chronic infections and localized suppurative lesions. * **B. abortus:** The primary reservoir is **cattle**. While it causes "contagious abortion" in cows, it generally causes a milder form of the disease in humans compared to *B. melitensis*. * **B. canis:** The primary reservoir is **dogs**. Human infections are rare and usually result in mild symptoms. **3. High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Characterized by **Undulant fever** (fever that rises and falls like waves), profuse malodorous sweating, and splenomegaly. * **Diagnosis:** * **Standard Agglutination Test (SAT):** Detects antibodies; a titer of 1:160 or more is significant. * **Castaneda’s Medium:** A biphasic culture medium used for blood cultures (though automated systems like BACT/ALERT are now preferred). * **Rose Bengal Test:** A rapid screening test. * **Treatment:** The WHO recommends a combination of **Rifampicin and Doxycycline** for 6 weeks. * **Occupational Hazard:** It is a significant risk for veterinarians, slaughterhouse workers, and laboratory personnel (highly infectious via aerosols).

Q: Bile solubility is used for the differentiation of which bacteria?

Pneumococci from other Streptococci. **Explanation:** The **Bile Solubility Test** is the gold-standard biochemical method used to differentiate *Streptococcus pneumoniae* (Pneumococci) from other alpha-hemolytic streptococci (primarily the Viridans group). **1. Why the Correct Answer is Right:** *Streptococcus pneumoniae* possesses an intracellular autolytic enzyme called **L-alanine-amidase**. Surface-active agents like bile salts (e.g., sodium deoxycholate) lower the surface tension on the cell membrane, which accelerates the natural autolysis process. This leads to the rapid lysis of the bacterial cell wall, causing a turbid suspension to become clear. Other alpha-hemolytic streptococci lack this specific autolytic mechanism and remain insoluble in bile. **2. Analysis of Incorrect Options:** * **Option A:** Staphylococcus and Streptococcus are differentiated by the **Catalase test** (Staph is positive; Strep is negative). * **Option B:** Group B Streptococcus (*S. agalactiae*) is differentiated from other streptococci using the **CAMP test** or Hippurate hydrolysis. * **Option D:** Neisseria (Gram-negative cocci) and Streptococci (Gram-positive cocci) are primarily differentiated by **Gram staining** and the **Oxidase test** (Neisseria is positive). **3. NEET-PG High-Yield Pearls:** * **Pneumococcus Identification:** Characterized by being Gram-positive lancet-shaped diplococci, Catalase negative, **Optochin sensitive**, and **Bile soluble**. * **Quellung Reaction:** Used for serotyping *S. pneumoniae* based on its polysaccharide capsule (capsular swelling). * **Culture:** On Blood Agar, Pneumococci produce **alpha-hemolysis** (partial/green) and characteristic **draughtsman (checkerboard) colonies** due to central autolysis. * **Bile Solubility Reagent:** 10% Sodium deoxycholate is typically used at a pH of 7.0.

Q: Most of the clinical cases of Listeria monocytogenes infections are attributed to which serovar?

4b. **Explanation:** *Listeria monocytogenes* is a Gram-positive, facultative intracellular bacillus known for causing foodborne illnesses, neonatal sepsis, and meningitis in immunocompromised individuals. While there are 13 known serovars based on somatic (O) and flagellar (H) antigens, the vast majority of human clinical disease is caused by only three: **1/2a, 1/2b, and 4b.** **Why 4b is the correct answer:** Serovar **4b** is the most clinically significant because it is responsible for the majority of **large-scale foodborne outbreaks** and sporadic cases of listeriosis worldwide. It possesses specific virulence factors that enhance its ability to cross the intestinal, blood-brain, and placental barriers, making it the predominant serovar in invasive disease. **Analysis of Incorrect Options:** * **Option A (1):** While serovars 1/2a and 1/2b are common causes of sporadic human listeriosis, they are less frequently associated with major epidemic outbreaks compared to 4b. * **Option B (4a):** This serovar is primarily associated with environmental sources or animal infections and is rarely isolated from human clinical cases. * **Option D (6):** Serovar 6 (and others like 5 or 7) are extremely rare in human pathology and do not contribute significantly to the clinical burden of the disease. **High-Yield Clinical Pearls for NEET-PG:** * **Tumbling Motility:** Characteristically seen at 25°C (peritrichous flagella), but absent at 37°C. * **Cold Enrichment:** It can grow at temperatures as low as 4°C (important for refrigerated food contamination like soft cheese and deli meats). * **CAMP Test Positive:** Shows "block" hemolysis (unlike the "arrowhead" of Group B Streptococcus). * **Treatment of Choice:** Ampicillin (Listeria is inherently resistant to all cephalosporins).

Q: What is the most common diagnostic test for Lymphogranuloma venereum (LGV)?

Cell culture. **Explanation:** **Lymphogranuloma venereum (LGV)** is a sexually transmitted infection caused by the **L1, L2, and L3 serovars of *Chlamydia trachomatis***. **Why Cell Culture is the Correct Answer:** *Chlamydia* species are obligate intracellular pathogens, meaning they cannot be grown on artificial agar. **Cell culture** (using cell lines like McCoy, HeLa, or BHK-21) remains the **gold standard** and the most specific diagnostic test for LGV. It allows for the isolation of the organism from bubo aspirates or ulcer swabs. Once grown, the inclusions are identified using iodine or Giemsa stain. **Analysis of Incorrect Options:** * **A. Fluorescent antibody testing:** While useful for detecting chlamydial antigens (DFA), it is generally less sensitive than culture or nucleic acid amplification tests (NAAT) for LGV-specific diagnosis. * **B. Complement fixation test (CFT):** This is a serological test. While a titer of >1:64 is suggestive of LGV, it lacks specificity as it often cross-reacts with other chlamydial infections (e.g., *C. pneumoniae*). * **D. Frei’s test:** This is a historical skin test (delayed hypersensitivity). It is **no longer used** because it is non-specific and remains positive long after the infection has cleared. **NEET-PG High-Yield Pearls:** * **Clinical Presentation:** Characterized by a painless primary lesion followed by painful inguinal lymphadenopathy (**Buboes**) and the **"Groove sign"** (enlarged nodes separated by the inguinal ligament). * **NAAT:** In modern clinical practice, Nucleic Acid Amplification Testing (NAAT) is the preferred rapid diagnostic tool, though culture remains the definitive "textbook" gold standard. * **Treatment:** The drug of choice is **Doxycycline** (100 mg BID for 21 days). Erythromycin is an alternative.

Question 1

Which of the following is false regarding H. pylori?

Accepted Answer

With chronic infection, the urease breath test can become negative.

Answer

H. pylori infection remains lifelong if untreated.

Answer

Endoscopy is diagnostic.

Answer

Toxigenic strains usually cause ulcers.

Question 2

What is the causative organism of hemolytic uremic syndrome?

Accepted Answer

Shigella

Answer

Enterotoxigenic Escherichia coli (ETEC)

Answer

Salmonella

Answer

Pseudomonas

Question 3

A 35-year-old patient complains of abdominal cramps along with profuse diarrhea. The treating physician wants to process the stool specimen for isolation of Campylobacter jejuni. Which of the following is the method of choice for the culture of stool?

Accepted Answer

Culture on Skirrow's medium incubated at 42°C under micro-aerophilic conditions.

Answer

Culture on TCBS medium incubated at 37°C under aerobic conditions.

Answer

Culture on MacConkey's medium incubated at 42°C under anaerobic conditions.

Answer

Culture on Wilson and Blair's medium incubated at 37° under microaerophilic conditions.

Question 4

Which of the following drugs is associated with the longest lag period in bacterial growth?

Accepted Answer

Streptomycin

Answer

Rifampicin

Answer

Isoniazid (INH)

Answer

Ethambutol

Question 5

Which of the following is NOT a metastatic complication of gonococci infection?

Accepted Answer

Nephritis

Answer

Endocarditis

Answer

Meningitis

Answer

Arthritis

Question 6

Which species of Brucella is acquired from contact with goats and sheep?

Accepted Answer

B. melitensis

Answer

B. suis

Answer

B. abortus

Answer

B. canis

Question 7

Which one of the following statements is true regarding the pathogenicity of Mycobacteria species?

Accepted Answer

M. kansasii can cause a disease indistinguishable from tuberculosis.

Answer

M. tuberculosis is more pathogenic than M. bovis in humans.

Answer

M. africanum infection is acquired from an environmental source.

Answer

M. marinum is responsible for tubercular lymphadenopathy.

Question 8

Bile solubility is used for the differentiation of which bacteria?

Accepted Answer

Pneumococci from other Streptococci

Answer

Staphylococcus from Streptococcus

Answer

Group B Streptococcus from other Streptococci

Answer

Streptococci from Neisseria

Question 9

Most of the clinical cases of Listeria monocytogenes infections are attributed to which serovar?

Accepted Answer

4b

Answer

1

Answer

4a

Answer

6

Question 10

What is the most common diagnostic test for Lymphogranuloma venereum (LGV)?

Accepted Answer

Cell culture

Answer

Fluorescent antibody testing

Answer

Complement fixation test

Answer

Frei's test

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?