Bacteriology Practice Questions

Q: Red leg disease in amphibians is caused by which organism?

Aeromonas. **Explanation:** **Aeromonas hydrophila** is the primary causative agent of **"Red Leg Disease"** in amphibians (frogs, toads, and salamanders). This condition is a form of bacterial septicemia characterized by subcutaneous hemorrhages, particularly on the ventral surface of the legs and abdomen, giving them a distinct reddish appearance. * **Why Aeromonas is correct:** *Aeromonas* species are Gram-negative, facultatively anaerobic bacilli commonly found in freshwater environments. They are opportunistic pathogens; in amphibians, stress or poor water quality allows the bacteria to invade the bloodstream, leading to capillary fragility and systemic bleeding. In humans, *Aeromonas* is high-yield for causing gastroenteritis (diarrheal disease) and cellulitis following exposure to contaminated water. **Analysis of Incorrect Options:** * **Pseudomonas:** While *Pseudomonas* can cause various opportunistic infections in aquatic animals, it is not the classic etiologic agent associated with the specific "Red Leg" syndrome. * **Histoplasmosis:** This is a fungal infection (*Histoplasma capsulatum*) primarily affecting the pulmonary system in humans via inhalation of bird or bat droppings; it does not cause septicemic skin changes in amphibians. * **Shigella hemolyticus:** This is a distractor. *Shigella* species cause bacillary dysentery in humans. There is no recognized species named *Shigella hemolyticus* associated with amphibian pathology. **High-Yield Clinical Pearls for NEET-PG:** * **Aeromonas hydrophila** is oxidase-positive (distinguishing it from Enterobacteriaceae) and resistant to the vibriostatic agent O/129 (distinguishing it from *Vibrio*). * In humans, it is a rare but serious cause of **necrotizing fasciitis** following freshwater trauma. * Always associate **freshwater exposure + cellulitis/diarrhea** with *Aeromonas*.

Q: Which drug is used in the treatment of resistant gonorrhoea?

Spectinomycin. **Explanation:** **Why Spectinomycin is correct:** Spectinomycin is an aminocyclitol antibiotic that inhibits protein synthesis by binding to the 30S ribosomal subunit. It is specifically indicated for the treatment of acute gonorrheal urethritis and proctitis, particularly in cases caused by **PPNG (Penicillinase-Producing Neisseria gonorrhoeae)** or in patients allergic to penicillins/cephalosporins. While Ceftriaxone is currently the first-line treatment for uncomplicated gonorrhea, Spectinomycin remains a high-yield answer for "resistant" cases in the context of classical microbiology questions. **Analysis of Incorrect Options:** * **A. Penicillin:** Historically the drug of choice, it is no longer used due to the widespread emergence of PPNG strains and plasmid-mediated resistance. * **B. Cotrimoxazole:** This combination (Sulfamethoxazole + Trimethoprim) is not effective against *N. gonorrhoeae* and is primarily used for UTIs or respiratory infections. * **C. Erythromycin:** While used for neonatal prophylaxis (ophthalmia neonatorum) or in pregnancy when other drugs are contraindicated, it is not the standard treatment for resistant adult gonorrhea due to poor efficacy and resistance. **NEET-PG High-Yield Pearls:** * **Current CDC/WHO Guidelines:** The current gold standard for uncomplicated gonorrhea is a single IM dose of **Ceftriaxone** (often combined with Azithromycin to cover potential Chlamydia co-infection). * **Spectinomycin Limitation:** It is **ineffective against pharyngeal gonorrhea** and does not treat syphilis (unlike Penicillin). * **Culture Media:** *N. gonorrhoeae* is fastidious and grows on **Thayer-Martin Medium** (Selective VCN medium: Vancomycin, Colistin, Nystatin). * **Gram Stain:** Look for Gram-negative kidney-shaped diplococci within polymorphonuclear leukocytes (intracellular).

Q: All are true about Gonococci except?

Vaginal swab is done for diagnosis in females. **Explanation:** The correct answer is **B**, as vaginal swabs are **not** the preferred specimen for diagnosing Gonorrhoea in females. 1. **Why Option B is the Correct Answer (The Exception):** In adult females, *Neisseria gonorrhoeae* primarily infects the **endocervical canal** because the organism has a predilection for columnar epithelium. The adult vagina is lined by stratified squamous epithelium, which is resistant to infection. Therefore, an **endocervical swab** is the specimen of choice. A vaginal swab is only used in cases of prepubertal vulvovaginitis (where the vaginal mucosa is thin) or when using NAAT-based screening. 2. **Analysis of Other Options:** * **Option A:** The incubation period for Gonorrhoea is typically short, ranging from **2 to 7 days**, making this statement true. * **Option C:** In males, the primary site of infection is the urethra. A **purulent urethral discharge** or a urethral swab is the standard specimen for microscopy and culture. * **Option D:** *N. gonorrhoeae* is a fastidious organism. **Thayer-Martin (TM) medium** is a selective medium (Chocolate agar base + Vancomycin, Colistin, Nystatin) specifically designed to inhibit normal flora and allow the growth of Gonococci. **High-Yield Clinical Pearls for NEET-PG:** * **Microscopy:** Gram-negative, kidney-shaped **intracellular diplococci** (found within polymorphonuclear leukocytes). * **Transport Media:** If processing is delayed, use **Stuart’s or Amies medium**. * **Biochemical Test:** Gonococci are **Oxidase positive** and ferment only **Glucose** (G for Gonococci, G for Glucose). * **Treatment:** Due to rising resistance, the current CDC/WHO recommendation is usually a single dose of **Ceftriaxone** (IM). * **Complications:** In females, untreated infection can lead to Pelvic Inflammatory Disease (PID) and Fitz-Hugh-Curtis Syndrome.

Q: A traveler drinks a glass of fruit juice with ice cubes made from tap water. E. coli contaminating the water supply grow in the traveler's intestine and synthesize a protein that causes his intestinal epithelium to overproduce cyclic AMP, resulting in a watery diarrhea. Which pathogenic strain of E. coli is typically responsible for this syndrome?

Enterotoxigenic. **Explanation:** The clinical presentation describes **Traveler’s Diarrhea**, which is most commonly caused by **Enterotoxigenic *E. coli* (ETEC)**. **Why the correct answer is right:** ETEC causes watery diarrhea through the production of two types of enterotoxins: 1. **Heat-labile toxin (LT):** This toxin acts similarly to the Cholera toxin. It ADP-ribosylates the Gs protein, leading to the permanent activation of adenylate cyclase. This results in **increased levels of intracellular cyclic AMP (cAMP)**, which triggers the secretion of chloride ions and water into the intestinal lumen. 2. **Heat-stable toxin (ST):** This toxin increases **cyclic GMP (cGMP)** by activating guanylate cyclase. The question specifically mentions the overproduction of cAMP, which is the hallmark mechanism of ETEC's LT toxin. **Why incorrect options are wrong:** * **Enteroaggregative (EAEC):** Associated with persistent diarrhea in children and HIV patients; it uses "stacked-brick" adhesion but does not primarily act via cAMP. * **Enterohemorrhagic (EHEC):** Causes bloody diarrhea (hemorrhagic colitis) and Hemolytic Uremic Syndrome (HUS). It produces **Shiga-like toxin**, which inhibits protein synthesis (60S ribosome). It does not increase cAMP. * **Enteroinvasive (EIEC):** Invades the colonic mucosa, causing a dysentery-like syndrome (fever, cramps, blood/pus in stool) similar to *Shigella*. **High-Yield NEET-PG Pearls:** * **ETEC:** Most common cause of Traveler’s Diarrhea. Mnemonic: **L**abile increases **A**MP (**L**A), **S**table increases **G**MP (**S**G). * **EHEC:** Most common serotype is **O157:H7**. It does *not* ferment sorbitol (distinguishing it from other *E. coli* on Sorbitol-MacConkey agar). * **EPEC:** Causes pediatric diarrhea in developing countries; characterized by "attaching and effacing" lesions (destruction of microvilli).

Q: A patient with HIV presents with diarrhea and acid-fast bacilli (AFB) positive in stool. What is the most likely causative organism?

Mycobacterium avium intracellulare. **Explanation:** The correct answer is **Mycobacterium avium intracellulare (MAC)**. In patients with advanced HIV/AIDS (typically with CD4 counts <50 cells/mm³), **Mycobacterium avium complex (MAC)** is the most common cause of disseminated opportunistic bacterial infection. While MAC primarily affects the reticuloendothelial system, it frequently involves the gastrointestinal tract, leading to chronic diarrhea, abdominal pain, and malabsorption. A key diagnostic feature is the presence of **Acid-Fast Bacilli (AFB)** in the stool or biopsy, representing the organism's acid-fast nature due to mycolic acid in its cell wall. **Why other options are incorrect:** * **Mycobacterium tuberculosis (MTB):** While MTB is common in HIV patients, it usually presents with pulmonary symptoms or ileocecal tuberculosis (causing constipation or obstruction rather than simple chronic diarrhea). It is less likely than MAC to show positive AFB in a stool sample unless there is active intestinal TB. * **Mycobacterium leprae:** This organism causes Leprosy, affecting the skin and peripheral nerves. It does not cause gastrointestinal disease or diarrhea. * **Mycoplasma:** These are the smallest free-living organisms and **lack a cell wall**; therefore, they are not acid-fast and cannot be detected via AFB staining. **High-Yield Clinical Pearls for NEET-PG:** * **Prophylaxis:** Azithromycin or Clarithromycin is indicated for MAC prophylaxis in HIV patients when CD4 <50 cells/mm³. * **Diagnosis:** Gold standard is blood culture (using BACTEC) or bone marrow culture. * **Stain:** MAC appears as short, coccobacillary acid-fast bacilli, often found within macrophages (foamy histiocytes). * **Differential Diagnosis:** If a stool AFB is positive in an HIV patient, also consider **Cryptosporidium parvum**, but these appear as acid-fast **oocysts**, not bacilli.

Q: A patient presents with fever for 3 weeks. On examination, he is observed to have splenomegaly. Ultrasonography reveals a hypoechoic shadow in the spleen near the hilum. Gram-negative bacilli are isolated on blood culture. Which of the following is the most likely causative organism?

Salmonella. ### Explanation The clinical presentation of prolonged fever (3 weeks), splenomegaly, and a hypoechoic shadow on USG (suggestive of a **splenic abscess**) points toward a systemic bacterial infection. The definitive clue is the isolation of **Gram-negative bacilli** from the blood culture. **1. Why Salmonella is correct:** *Salmonella Typhi* and *Paratyphi* are the causative agents of Enteric Fever. In the third week of untreated infection, complications such as intestinal perforation or metastatic abscesses in organs like the gallbladder, bones, or spleen occur. *Salmonella* is a classic **Gram-negative bacillus**. Splenic abscesses are a recognized, albeit rare, complication of enteric fever, often appearing as hypoechoic lesions on ultrasonography. **2. Why the other options are incorrect:** * **Cytomegalovirus (CMV):** While CMV can cause fever and splenomegaly (Infectious Mononucleosis-like syndrome), it is a **virus**, not a Gram-negative bacillus. * **Toxoplasmosis:** Caused by the protozoan *Toxoplasma gondii*, it typically presents with lymphadenopathy. It would not yield bacilli on a standard blood culture. * **Lymphoma virus (e.g., EBV/HTLV):** These are viruses associated with malignancies. While lymphoma can cause splenic lesions, the presence of Gram-negative bacilli in the blood culture rules this out as the primary cause of the acute febrile illness. **Clinical Pearls for NEET-PG:** * **Enteric Fever Timeline:** Week 1 (Bacteremia/Fever), Week 2 (Rose spots/Splenomegaly), Week 3 (Complications: Perforation/Abscess). * **Diagnosis:** **B**lood culture (1st week), **A**ntibody/Widal (2nd week), **S**tool culture (3rd week), **U**rine culture (4th week) — Mnemonic: **BASU**. * **Splenic Abscess:** Most common bacterial causes include *Staphylococcus aureus*, *Streptococci*, and *Salmonella* (especially in patients with Sickle Cell Disease).

Q: The appearance of cobweb formation in CSF indicates which of the following?

Tuberculous meningitis. **Explanation:** The appearance of a **"cobweb" or "pellicle" formation** in Cerebrospinal Fluid (CSF) is a classic diagnostic hallmark of **Tuberculous Meningitis (TBM)**. **Why Tuberculous Meningitis is correct:** In TBM, the CSF typically shows a high protein content (often >100 mg/dL) and a high concentration of **fibrinogen**. When the CSF sample is allowed to stand undisturbed for 12–24 hours, the fibrinogen converts into fibrin, forming a delicate, spider-web-like clot known as a cobweb or pellicle. This occurs due to the chronic inflammatory process caused by *Mycobacterium tuberculosis*. **Why other options are incorrect:** * **Pyogenic Meningitis:** The CSF is typically grossly turbid or purulent due to a very high neutrophil count. It does not form a delicate cobweb; instead, it may show a thick, heavy sediment. * **Viral Meningitis:** Also known as aseptic meningitis, the CSF is usually clear, with normal or slightly elevated protein levels. Fibrinogen levels are not high enough to form a pellicle. * **Fungal Meningitis:** Commonly caused by *Cryptococcus neoformans*, the CSF is usually clear or slightly opalescent. Diagnosis is typically made via India Ink preparation (showing capsules) or CrAg testing, not pellicle formation. **NEET-PG High-Yield Pearls for TBM:** * **CSF Biochemistry:** Low glucose (hypoglycorrhachia), high protein, and **lymphocytic pleocytosis** (predominance of lymphocytes). * **Target for Cobweb:** If a pellicle forms, it is the best site to look for Acid-Fast Bacilli (AFB) using Ziehl-Neelsen staining, as the bacilli get trapped in the fibrin mesh. * **Gold Standard Diagnosis:** Automated liquid culture (e.g., MGIT) or Molecular testing (GeneXpert/CBNAAT). * **Common Site:** TBM typically involves the **basal cisterns** (basal exudates).

Question 1

Red leg disease in amphibians is caused by which organism?

Accepted Answer

Aeromonas

Answer

Pseudomonas

Answer

Histoplasmosis

Answer

Shigella hemolyticus

Question 2

A 34-year-old female patient presented with left leg pain. On examination, the area was erythematous, warm, and tender with indistinct borders. The doctor suspected cellulitis and started her on IV oxacillin. The doctor noted that the pain was disproportionate to the level of cellulitis, and hence admitted the patient. During the night, the patient developed hypotension, deranged renal function tests, and a deranged coagulation profile. X-ray and CT of the leg were conducted. Which is the most common organism causing the condition described?

Accepted Answer

Group A Streptococcus

Answer

Staphylococcus aureus

Answer

Pseudomonas aeruginosa

Answer

Group D Streptococcus

Question 3

Which drug is used in the treatment of resistant gonorrhoea?

Accepted Answer

Spectinomycin

Answer

Penicillin

Answer

Cotrimoxazole

Answer

Erythromycin

Question 4

All are true about Gonococci except?

Accepted Answer

Vaginal swab is done for diagnosis in females

Answer

Incubation period is 2-7 days

Answer

Urethral swab for diagnosis in males

Answer

Culture media is Thayer Martin media

Question 5

A traveler drinks a glass of fruit juice with ice cubes made from tap water. E. coli contaminating the water supply grow in the traveler's intestine and synthesize a protein that causes his intestinal epithelium to overproduce cyclic AMP, resulting in a watery diarrhea. Which pathogenic strain of E. coli is typically responsible for this syndrome?

Accepted Answer

Enterotoxigenic

Answer

Enteroaggregative

Answer

Enterohemorrhagic

Answer

Enteroinvasive

Question 6

A patient with HIV presents with diarrhea and acid-fast bacilli (AFB) positive in stool. What is the most likely causative organism?

Accepted Answer

Mycobacterium avium intracellulare

Answer

Mycobacterium tuberculosis

Answer

Mycobacterium leprae

Answer

Mycoplasma

Question 7

Which of the following statements about E. coli are true?

Accepted Answer

The heat-labile toxin in ETEC acts via cAMP.

Answer

In types causing UTI, the organism attaches via pili.

Answer

The heat-stable toxin of ETEC is responsible for causing hemolytic-uremic syndrome (HUS).

Answer

EIEC invasiveness is under plasmid control.

Question 8

A patient presents with fever for 3 weeks. On examination, he is observed to have splenomegaly. Ultrasonography reveals a hypoechoic shadow in the spleen near the hilum. Gram-negative bacilli are isolated on blood culture. Which of the following is the most likely causative organism?

Accepted Answer

Salmonella

Answer

Cytomegalovirus

Answer

Toxoplasmosis

Answer

Lymphoma virus

Question 9

The appearance of cobweb formation in CSF indicates which of the following?

Accepted Answer

Tuberculous meningitis

Answer

Pyogenic meningitis

Answer

Viral meningitis

Answer

Fungal meningitis

Question 10

Mycoplasma organisms may also cause disease in non-pulmonary sites. Which of the following is the most commonly affected non-pulmonary site?

Accepted Answer

Urethritis

Answer

Meningitis

Answer

Prosthetic heart valve

Answer

Septic arthritis

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?