Bacteriology Practice Questions

Q: Lymphogranuloma venereum (LGV) is caused by which of the following agents?

Chlamydia trachomatis. **Explanation:** **Lymphogranuloma venereum (LGV)** is a sexually transmitted infection caused by specific invasive serovars of **Chlamydia trachomatis**, namely **L1, L2, and L3**. Unlike the non-invasive serovars (D-K) that cause urethritis, the LGV strains are lymphotropic, meaning they spread from the primary site of infection to the regional lymph nodes, causing significant inflammation and suppuration. **Analysis of Options:** * **A. Chlamydia trachomatis (Correct):** Specifically serotypes L1-L3. The disease typically presents in three stages: a painless primary vesicle, a secondary stage characterized by painful inguinal lymphadenopathy (the "Bubo" stage), and a tertiary stage involving chronic inflammation and proctocolitis. * **B. Leishmania donovani:** This is a protozoan parasite responsible for Visceral Leishmaniasis (Kala-azar), characterized by fever, hepatosplenomegaly, and pancytopenia, not genital lesions. * **C. Madura mycosis:** Also known as Mycetoma, this is a chronic granulomatous fungal or bacterial infection of the subcutaneous tissue, usually affecting the foot (Madura foot). * **D. Herpes genitalis:** Caused by Herpes Simplex Virus (HSV-2), it presents with multiple, painful, superficial vesicles and ulcers, but does not cause the massive lymphatic involvement seen in LGV. **High-Yield Clinical Pearls for NEET-PG:** * **Groove Sign of Greenblatt:** A characteristic clinical sign where the inguinal ligament creates a depression between inflamed superficial and deep inguinal lymph nodes. * **Frei Test:** A historical skin test (delayed hypersensitivity) used for LGV diagnosis, now largely replaced by NAAT (Nucleic Acid Amplification Test). * **Drug of Choice:** Doxycycline (100 mg twice daily for 21 days) is the preferred treatment. * **Esthiomene:** A late-stage complication in females involving chronic hypertrophic ulceration and elephantiasis of the external genitalia.

Q: Dysphagia, diplopia, and dysarthria are characteristic symptoms of food poisoning due to which bacterium?

Clostridium botulinum. ### Explanation The correct answer is **Clostridium botulinum**. **1. Why Clostridium botulinum is correct:** The symptoms described—**Dysphagia** (difficulty swallowing), **Diplopia** (double vision), and **Dysarthria** (difficulty speaking)—are known as the "**3 Ds**," which are hallmark signs of **Botulism**. These result from the action of the botulinum toxin, a potent neurotoxin that irreversibly blocks the release of **acetylcholine** at the neuromuscular junction. This leads to a symmetric, **descending flaccid paralysis**, typically starting with the cranial nerves (causing the 3 Ds and ptosis) and progressing to respiratory failure. **2. Why the other options are incorrect:** * **Staphylococcus aureus:** Causes rapid-onset food poisoning (1–6 hours) characterized by severe vomiting and nausea due to a preformed enterotoxin. It does not cause neurological deficits. * **Salmonella typhimurium:** Primarily causes inflammatory diarrhea (gastroenteritis) with fever, abdominal cramps, and chills. It is an invasive pathogen, not a neurotoxin producer. * **Bacillus cereus:** Associated with "Reheated Rice Syndrome." It presents in two forms: emetic (short incubation, vomiting) or diarrheal (longer incubation, watery diarrhea). It does not cause descending paralysis. **3. NEET-PG High-Yield Pearls:** * **Mechanism:** Botulinum toxin cleaves **SNARE proteins**, preventing vesicle fusion and neurotransmitter release. * **Infant Botulism:** Associated with **honey** consumption; presents as "Floppy Baby Syndrome." * **Wound Botulism:** Associated with black tar heroin use. * **Diagnosis:** Primarily clinical; confirmed by detecting the toxin in serum, stool, or suspected food via the mouse lethality assay (gold standard). * **Therapy:** Prompt administration of equine antitoxin (for adults) or BIG-IV (for infants).

Q: Which of the following Chlamydia species has only one serotype?

Chlamydia pneumoniae. **Explanation:** The genus *Chlamydia* consists of obligate intracellular bacteria characterized by a unique life cycle involving elementary bodies (infectious) and reticulate bodies (replicative). The classification of these species is often based on their antigenic composition and serotyping. **Why Option B is Correct:** *Chlamydia pneumoniae* (TWAR agent) is characterized by having **only one serotype**. Unlike other species in the genus, it shows very little genetic or antigenic variation. It is a common cause of atypical pneumonia, bronchitis, and sinusitis, and is transmitted via respiratory droplets. **Why Other Options are Incorrect:** * **Option A: Chlamydia psittaci:** This species is highly diverse and contains **at least 10-15 serotypes** (genotypes). It is the causative agent of Psittacosis (ornithosis), a zoonotic infection transmitted from birds. * **Option C: Chlamydia trachomatis:** This species is divided into numerous serovars based on the Major Outer Membrane Protein (MOMP). It has **at least 15-19 serotypes**: * **A, B, Ba, C:** Cause endemic Trachoma. * **D–K:** Cause genital infections (NGU, PID) and inclusion conjunctivitis. * **L1, L2, L3:** Cause Lymphogranuloma Venereum (LGV). **High-Yield NEET-PG Pearls:** * **Staining:** Chlamydia are Gram-negative but are better visualized using **Giemsa, Castaneda, or Machiavello stains**. * **Inclusion Bodies:** *C. trachomatis* forms iodine-positive inclusions (contain glycogen), whereas *C. pneumoniae* and *C. psittaci* form **iodine-negative** inclusions. * **Drug of Choice:** Azithromycin (single dose) or Doxycycline (7 days) are the preferred treatments for Chlamydial infections. * **C. pneumoniae Association:** It has been linked in some studies to the pathogenesis of **atherosclerosis** and coronary artery disease.

Q: A 46-year-old male, an IV drug abuser, presents to the OPD with complaints of fatigue, malaise, and rigors. His temperature was 38.5°C, pulse 84/min, BP 130/80 mm Hg. On cardiovascular examination, a pansystolic murmur was heard, loudest at the left sternal border. Three sets of blood cultures were taken consecutively. Transthoracic echocardiography (TTE) revealed vegetation on the tricuspid valve with tricuspid regurgitation. From the given history, which organism is most likely to be positive in blood cultures?

Staphylococcus aureus. ### Explanation **Correct Option: D. Staphylococcus aureus** The patient presents with the classic triad of **IV drug abuse (IVDA)**, fever, and a pansystolic murmur at the left lower sternal border (indicative of **Tricuspid Regurgitation**). This clinical picture is diagnostic of **Right-sided Infective Endocarditis (IE)** [4]. In IV drug users, the skin is the primary source of entry for pathogens. **Staphylococcus aureus** is the most common causative organism (responsible for >60% of cases) in this population [3]. It typically presents acutely with high-grade fever and rapid valvular destruction. The tricuspid valve is involved in over 70% of IVDA-associated IE cases. **Analysis of Incorrect Options:** * **A. Streptococcus sanguis:** A member of the Viridans group streptococci, it is the most common cause of subacute IE following dental procedures, typically affecting previously damaged native valves (left-sided) [3]. * **B. Enterococcus faecium:** Often associated with IE following genitourinary or gastrointestinal manipulations, particularly in elderly patients or those with chronic prosthetic valve issues [3]. * **C. Coxiella burnetii:** The causative agent of Q fever [1]. It is a common cause of **culture-negative endocarditis** and usually presents as a chronic infection rather than an acute febrile illness. **Clinical Pearls for NEET-PG:** * **Most common valve in IVDA:** Tricuspid valve (Right-sided). * **Most common organism in IVDA:** *Staphylococcus aureus* [3]. * **Most common organism overall (Native Valve):** *Staphylococcus aureus* (previously Viridans Streptococci) [3]. * **Most common organism in Subacute IE:** *Streptococcus viridans*. * **Most common organism in Early Prosthetic Valve IE (<1 year):** *Staphylococcus epidermidis*. * **Diagnostic Gold Standard:** Duke’s Criteria (Major: Positive blood cultures and Echo evidence of vegetation) [2].

Q: Shiga toxin is produced by which organism?

Enterohemorrhagic E. coli. **Explanation:** The correct answer is **Enterohemorrhagic E. coli (EHEC)**. EHEC (specifically the O157:H7 serotype) produces **Shiga toxin** (Stx1 and Stx2), also known as Verocytotoxin. This toxin is identical to the toxin produced by *Shigella dysenteriae* type 1. It acts by inhibiting the 60S ribosomal subunit, leading to the cessation of protein synthesis and cell death. Clinically, this results in hemorrhagic colitis and the potentially fatal **Hemolytic Uremic Syndrome (HUS)**, characterized by the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. **Analysis of Incorrect Options:** * **Enteropathogenic E. coli (EPEC):** Does not produce toxins. It causes diarrhea in infants by using **intimin** (an adhesin) to cause "attachment and effacement" (A/E) lesions on intestinal villi. * **Enteroinvasive E. coli (EIEC):** Does not produce Shiga toxin. It acts similarly to *Shigella* by invading the colonic epithelium using actin polymerization, causing inflammatory diarrhea with blood and mucus (dysentery). * **Enterotoxigenic E. coli (ETEC):** Produces two distinct toxins: **Heat-labile (LT)**, which increases cAMP, and **Heat-stable (ST)**, which increases cGMP. It is the leading cause of **Traveler’s diarrhea**. **High-Yield Clinical Pearls for NEET-PG:** * **HUS Warning:** Antibiotics are generally avoided in EHEC infections as they may trigger increased Shiga toxin release, worsening the risk of HUS. * **Sorbitol Agar:** EHEC (O157:H7) is unique because it **cannot ferment sorbitol**, appearing as colorless colonies on Sorbitol-MacConkey (SMAC) agar. * **Mnemonic for ETEC:** "**L**abile **A**mp, **S**table **G**mp" (**L**ook **A**t **S**uper **G**irl).

Q: A 32-year-old woman consults her gynecologist for fever and progressively severe pain in her left lower abdominal quadrant. The pain began several days ago, shortly after the onset of her menses. Pelvic examination demonstrates bilateral adnexal tenderness, much more marked on the left than the right. Which of the following is the most likely pathogen?

Neisseria. **Explanation:** The clinical presentation of fever, lower abdominal pain, and bilateral adnexal tenderness following the onset of menses is classic for **Pelvic Inflammatory Disease (PID)**. **Why Neisseria is the Correct Answer:** *Neisseria gonorrhoeae* and *Chlamydia trachomatis* are the most common causative agents of acute PID. A key high-yield feature in this vignette is the **timing of symptoms**. Gonococcal PID typically manifests **shortly after the onset of menses**. During menstruation, the protective cervical mucus plug is lost, and the alkaline pH of menstrual blood facilitates the upward migration of *N. gonorrhoeae* from the lower genital tract to the endometrium and fallopian tubes. **Analysis of Incorrect Options:** * **Escherichia coli (A):** While *E. coli* can be part of the polymicrobial flora in chronic or severe PID (especially in older women or post-procedural infections), it is not the primary initiator of acute PID in a young, menstruating woman. * **Staphylococcus (C) and Streptococcus (D):** These are common skin or respiratory flora. While *Streptococcus agalactiae* (Group B) can colonize the vagina, these organisms are rarely the primary cause of acute, sexually transmitted PID. **NEET-PG High-Yield Pearls:** 1. **Most common cause of PID:** *Chlamydia trachomatis* (often subacute/asymptomatic). 2. **Most common cause of acute/symptomatic PID:** *Neisseria gonorrhoeae*. 3. **Fitz-Hugh-Curtis Syndrome:** Perihepatitis (violin-string adhesions) associated with PID. 4. **Diagnosis:** Primarily clinical (Cervical Motion Tenderness/Adnexal tenderness). 5. **Treatment:** Must cover both *Neisseria* (Ceftriaxone) and *Chlamydia* (Doxycycline), plus anaerobes (Metronidazole) if a tubo-ovarian abscess is suspected.

Question 1

A patient with a history of discharge from the right ear for the past 1 year presented with severe earache. The discharge was cultured, and the organism was found to be Gram-positive cocci. What is the least likely cause of this condition?

Accepted Answer

Haemophilus influenza

Answer

Pseudomonas

Answer

Streptococcus pneumoniae

Answer

Staphylococcus

Question 2

Lymphogranuloma venereum (LGV) is caused by which of the following agents?

Accepted Answer

Chlamydia trachomatis

Answer

Leishmania donovani

Answer

Madura mycosis

Answer

Herpes genitalis

Question 3

Dysphagia, diplopia, and dysarthria are characteristic symptoms of food poisoning due to which bacterium?

Accepted Answer

Clostridium botulinum

Answer

Staphylococcus aureus

Answer

Salmonella typhimurium

Answer

Bacillus cereus

Question 4

If after 2 months of conventional antituberculous therapy, sputum smear examination is negative but culture is positive, what does this indicate?

Accepted Answer

Transitional resistance

Answer

Treatment failure

Answer

Resistant tuberculosis

Answer

Category-II failure

Question 5

Which of the following statements about Chlamydia trachomatis is true?

Accepted Answer

Use of OCP is associated with increased risk of asymptomatic chlamydial infection

Answer

May be isolated on artificial media by culture of purulent endocervical discharge

Answer

Most genital chlamydial infections are symptomatic

Answer

Penicillin is the drug of choice for treatment

Question 6

Which of the following Chlamydia species has only one serotype?

Accepted Answer

Chlamydia pneumoniae

Answer

Chlamydia psittaci

Answer

Chlamydia trachomatis

Answer

None of the above

Question 7

In a patient with cystic fibrosis presenting with high fever and productive cough, what is the most common causative organism?

Accepted Answer

Pseudomonas aeruginosa

Answer

Haemophilus influenzae

Answer

Staphylococcus aureus

Answer

Streptococcus species

Question 8

A 46-year-old male, an IV drug abuser, presents to the OPD with complaints of fatigue, malaise, and rigors. His temperature was 38.5°C, pulse 84/min, BP 130/80 mm Hg. On cardiovascular examination, a pansystolic murmur was heard, loudest at the left sternal border. Three sets of blood cultures were taken consecutively. Transthoracic echocardiography (TTE) revealed vegetation on the tricuspid valve with tricuspid regurgitation. From the given history, which organism is most likely to be positive in blood cultures?

Accepted Answer

Staphylococcus aureus

Answer

Streptococcus sanguis

Answer

Enterococcus faecium

Answer

Coxiella burnetii

Question 9

Shiga toxin is produced by which organism?

Accepted Answer

Enterohemorrhagic E. coli

Answer

Enteropathogenic E. coli

Answer

Enteroinvasive E. coli

Answer

Enterotoxigenic E. coli

Question 10

A 32-year-old woman consults her gynecologist for fever and progressively severe pain in her left lower abdominal quadrant. The pain began several days ago, shortly after the onset of her menses. Pelvic examination demonstrates bilateral adnexal tenderness, much more marked on the left than the right. Which of the following is the most likely pathogen?

Accepted Answer

Neisseria

Answer

Escherichia

Answer

Staphylococcus

Answer

Streptococcus

Bacteriology — MCQs

Bacteriology — MCQs

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