Bacteriology Practice Questions

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).

Q: What is the likely cause of this manifestation?

Pseudomonas. ***Pseudomonas*** - **Ecthyma gangrenosum** is the classic skin manifestation of **Pseudomonas aeruginosa** infection, particularly in **immunocompromised patients**. - Presents as **necrotic ulcerative lesions** with a characteristic **black eschar** and surrounding erythematous halo, often in the gluteal or perineal region. *Candida* - Typically causes **mucocutaneous infections** like oral thrush, vulvovaginitis, or **intertrigo** in warm, moist areas. - Skin lesions appear as **erythematous patches** with satellite pustules, not necrotic ulcers with black eschars. *Staphylococcus aureus* - Causes **pyogenic skin infections** like **cellulitis**, **impetigo**, or **folliculitis** with purulent discharge. - Lesions are typically **pustular** or **honey-crusted** rather than the necrotic, gangrenous appearance seen in ecthyma gangrenosum. *Fusarium* - A **filamentous fungus** that can cause **necrotizing skin infections** in immunocompromised patients, but less commonly than Pseudomonas. - Skin lesions may be **nodular** or **ulcerative** but lack the characteristic appearance of ecthyma gangrenosum.

Q: Which of the following is a tripartite toxin?

Anthrax toxin. **Explanation:** The correct answer is **Anthrax toxin (Option B)**. **Why Anthrax Toxin is Tripartite:** *Bacillus anthracis* produces a unique **tripartite (three-component) toxin** encoded on the pXO1 plasmid. It consists of three distinct proteins that are non-toxic individually but lethal when combined: 1. **Protective Antigen (PA):** The "B" (binding) component. It binds to host cell receptors and forms a pore, facilitating the entry of the other two components. 2. **Edema Factor (EF):** An adenylate cyclase that increases intracellular cAMP, leading to massive edema. 3. **Lethal Factor (LF):** A zinc metalloprotease that cleaves MAP kinase, leading to cell death and tissue necrosis. * **Combinations:** PA + EF = Edema Toxin; PA + LF = Lethal Toxin. **Why other options are incorrect:** * **A. Diphtheria toxin:** This is a classic **A-B toxin** (dimeric). The B-subunit binds to the receptor (HB-EGF), and the A-subunit inhibits protein synthesis via ADP-ribosylation of EF-2. * **C. Botulinum toxin:** Also an **A-B toxin**. The heavy chain (B) binds to neurons, and the light chain (A) acts as a protease to cleave SNARE proteins, preventing acetylcholine release. * **D. Cholera toxin:** This is a **multimeric A-B5 toxin**. It consists of one A-subunit and five B-subunits. It works by ADP-ribosylation of the Gs protein, leading to permanent activation of adenylate cyclase. **High-Yield NEET-PG Pearls:** * **Anthrax Capsule:** Unlike most bacteria, the capsule of *B. anthracis* is made of **poly-D-glutamic acid** (polypeptide, not polysaccharide), encoded on the **pXO2 plasmid**. * **McFadyean’s Reaction:** Used for presumptive identification of *B. anthracis* (polychrome methylene blue staining shows purple capsules). * **Medusa Head Appearance:** Characteristic morphology of *B. anthracis* colonies on blood agar.

Q: Which of the following is true about Clostridium tetani?

It has a drumstick appearance. **Explanation:** *Clostridium tetani* is a Gram-positive, motile, spore-forming anaerobic bacillus. The correct answer is **Option B** because the organism produces terminal, spherical spores that are wider than the vegetative body, giving the bacterium a characteristic **"drumstick" appearance** (or "tennis racket" appearance) under the microscope. **Analysis of Options:** * **Option A is incorrect:** *C. tetani* is an **obligate anaerobe**. It is extremely sensitive to oxygen and cannot grow if the oxygen tension exceeds **2 mmHg**. It requires a low oxidation-reduction potential for germination and growth. * **Option C is incorrect:** On blood agar, *C. tetani* typically produces a zone of **alpha-hemolysis**, which later develops into **beta-hemolysis** due to the production of the hemolysin known as **Tetanolysin**. * **Option D is incorrect:** Tetanus is more commonly seen in **warm, humid climates** and fertile soil (rich in manure). It is not specifically associated with winter or dry atmospheres; rather, it is a major health issue in agricultural areas and developing nations with poor immunization coverage. **High-Yield Clinical Pearls for NEET-PG:** * **Toxins:** Produces two toxins—**Tetanospasmin** (a potent neurotoxin responsible for clinical tetanus) and **Tetanolysin** (hemolysin). * **Mechanism:** Tetanospasmin acts by blocking the release of inhibitory neurotransmitters (**GABA and Glycine**) from Renshaw cells in the spinal cord, leading to spastic paralysis. * **Culture:** Shows a characteristic **"swarming growth"** on agar due to its peritrichous flagella. * **Clinical Signs:** Look for keywords like *Risus sardonicus* (grimace), *Trismus* (lockjaw), and *Opisthotonus* (backward arching).

Q: What is true about Protein A of Staphylococcus aureus?

Binds to the Fc portion of IgG. **Explanation:** **Protein A** is a major virulence factor located in the cell wall of *Staphylococcus aureus*. 1. **Why Option B is correct:** Protein A has a high affinity for the **Fc portion of IgG molecules** (specifically IgG1, IgG2, and IgG4). By binding the "tail" (Fc) of the antibody rather than the "head" (Fab), it ensures the antibody is oriented upside down. This prevents the host's phagocytes from recognizing the Fc region, effectively acting as an **anti-phagocytic** shield. 2. **Why other options are incorrect:** * **Options A & C:** Protein A **inhibits** opsonization and phagocytosis. Normally, IgG binds to bacteria via the Fab fragment, leaving the Fc portion free to trigger opsonization. Protein A reverses this, preventing the complement system (classical pathway) from being activated and stopping neutrophils from engulfing the bacteria. * **Option D:** Protein A is a **B-cell mitogen**, not a T-cell mitogen. It can cause non-specific polyclonal B-cell proliferation. (Note: *Staphylococcal Enterotoxins* and *TSST-1* are the ones that act as Superantigens/T-cell mitogens). **High-Yield Clinical Pearls for NEET-PG:** * **Cowan 1 Strain:** This specific strain of *S. aureus* is exceptionally rich in Protein A and is used in the **Co-agglutination test** for rapid antigen detection. * **Diagnostic Use:** Because it binds IgG, Protein A is used in various laboratory immunoassays to separate IgG from other immunoglobulins. * **Genetic Marker:** The gene encoding Protein A is the **spa gene**, often used for molecular typing (spa typing) of MRSA strains.

Q: Which bacteria commonly causes vomiting and diarrhea within 6 hours of food intake?

Staphylococci aureus. **Explanation:** The key to answering this question lies in the **incubation period**. Food poisoning occurring within 1–6 hours of ingestion is characteristically caused by **pre-formed toxins** (intoxication) rather than an active infection. **1. Why Staphylococcus aureus is correct:** *S. aureus* produces heat-stable **Enterotoxins (A-E)**. Because the toxin is already present in the food (commonly dairy, processed meats, or creamy salads) at the time of consumption, the onset of symptoms is rapid. The toxin acts directly on the vagus nerve and the vomiting center in the brain, leading to projectile vomiting and abdominal cramps, often followed by diarrhea. **2. Why the other options are incorrect:** * **Clostridium perfringens (Option A):** Has a longer incubation period (8–16 hours). It requires the ingestion of spores that germinate and produce enterotoxin within the gut. It typically causes watery diarrhea and cramps, but vomiting is rare. * **Salmonella spp (Option B):** This is an invasive infection. It requires an incubation period of 12–72 hours for the bacteria to multiply and cause mucosal inflammation. It presents with fever, bloody/mucoid diarrhea, and abdominal pain. * **Vibrio cholerae (Option C):** Has an incubation period of 1–3 days. It causes profuse "rice-water" stools due to the action of the Cholera toxin on adenylate cyclase, leading to massive fluid loss. **NEET-PG High-Yield Pearls:** * **Short Incubation ( 16 hours):** Think *Salmonella*, *Shigella*, or *Vibrio*. * **S. aureus toxin** is heat-stable (resists boiling for 30 mins), meaning reheating food does not prevent the illness.

Q: Non-gonococcal urethritis is most commonly caused by which of the following pathogens?

Chlamydia and Ureaplasma. **Explanation:** **Non-gonococcal urethritis (NGU)** refers to an inflammation of the urethra not caused by *Neisseria gonorrhoeae*. It is the most common sexually transmitted syndrome in men. **Why Option B is Correct:** The most common etiology of NGU is **_Chlamydia trachomatis_ (Serotypes D-K)**, accounting for 30–50% of cases. The second most common cause is **_Ureaplasma urealyticum_**. These organisms are often grouped together in clinical practice because they both lack a traditional peptidoglycan cell wall (making them resistant to beta-lactams) and require similar diagnostic and treatment approaches (e.g., Azithromycin or Doxycycline). **Analysis of Incorrect Options:** * **Option A:** *Streptococcus* species are part of the normal skin flora or cause respiratory/systemic infections; they are not primary pathogens for urethritis. * **Option C:** *Treponema pallidum* is the causative agent of Syphilis. While it is an STI, it typically presents with a painless chancre (primary syphilis) rather than urethral discharge. * **Option D:** While *Mycoplasma genitalium* is a recognized cause of NGU, *Ureaplasma urealyticum* is statistically more frequently associated with the condition in standard medical examinations and classic textbooks. **High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period:** NGU has a longer incubation period (7–14 days) compared to Gonococcal urethritis (2–5 days). * **Clinical Presentation:** Discharge in NGU is typically mucoid or clear, whereas Gonococcal discharge is characteristically thick and purulent. * **Diagnosis:** The "Gold Standard" for *Chlamydia* is **NAAT (Nucleic Acid Amplification Test)**. * **Treatment:** The syndromic management of urethral discharge usually covers both Gonococcus and NGU (e.g., Ceftriaxone + Azithromycin/Doxycycline).

Question 1

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 2

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 3

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 4

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

Question 5

What is the likely cause of this manifestation?

Accepted Answer

Pseudomonas

Answer

Candida

Answer

Staphylococcus aureus

Answer

Fusarium

Question 6

Which of the following is a tripartite toxin?

Accepted Answer

Anthrax toxin

Answer

Diphtheria toxin

Answer

Botulinum toxin

Answer

Cholera toxin

Question 7

Which of the following is true about Clostridium tetani?

Accepted Answer

It has a drumstick appearance

Answer

It is an aerobe that forms surface growth where the oxygen tension is more than 2mmHg

Answer

It does not show any hemolysis

Answer

It is prominently seen in winter and dry atmosphere

Question 8

What is true about Protein A of Staphylococcus aureus?

Accepted Answer

Binds to the Fc portion of IgG

Answer

Causes opsonization

Answer

Stimulates phagocytosis

Answer

Is a T-cell mitogen

Question 9

Which bacteria commonly causes vomiting and diarrhea within 6 hours of food intake?

Accepted Answer

Staphylococci aureus

Answer

Clostridium perfringens

Answer

Salmonella spp

Answer

Vibrio cholerae

Question 10

Non-gonococcal urethritis is most commonly caused by which of the following pathogens?

Accepted Answer

Chlamydia and Ureaplasma

Answer

Chlamydia and Streptococcus

Answer

Chlamydia and Treponema

Answer

Chlamydia and Mycoplasma

Bacteriology — MCQs

Bacteriology — MCQs

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