Bacteriology Practice Questions

Q: A 19-year-old girl presents with a painless ulcer in the labia majora with everted margins. What is the most likely causative organism?

Treponema pallidum. ### Explanation The clinical presentation described—a **painless ulcer with everted margins** on the genitalia—is the classic description of a **Hard Chancre**, the hallmark of **Primary Syphilis**. **1. Why Treponema pallidum is correct:** *Treponema pallidum* is the causative spirochete of syphilis. The primary stage manifests as a solitary, indurated (firm), and painless ulcer. A key diagnostic feature is the **everted (rolled-out) margins** and a clean base. Because the lesion is painless and often associated with non-tender regional lymphadenopathy, patients may delay seeking treatment. **2. Why the other options are incorrect:** * **Chlamydia:** Specifically, the L1-L3 serovars cause *Lymphogranuloma Venereum (LGV)*. This typically presents with a transient, small, painless papule or shallow erosion that heals quickly, followed by painful, suppurative inguinal lymphadenopathy (the "Bubo" and "Groove sign"). * **Neisseria gonorrhoeae:** This organism primarily causes urethritis or cervicitis characterized by purulent discharge, not a discrete genital ulcer. * **Herpes simplex virus (HSV):** Genital herpes presents as **painful**, multiple, superficial vesicles on an erythematous base that rupture to form "punched-out" erosions. They are never indurated or everted. **High-Yield Clinical Pearls for NEET-PG:** * **Painful vs. Painless:** Remember the mnemonic **"S is for Soft and Sore"**—*Haemophilus ducreyi* (Chancroid) causes a painful, soft chancre. Syphilis is a **Hard, painless** chancre. * **Dark-field Microscopy:** This is the gold standard for visualizing motile spirochetes from the chancre exudate. * **Treatment:** Intramuscular **Benzathine Penicillin G** (2.4 million units) remains the drug of choice for primary syphilis. * **Differential:** If the ulcer was painless but "beefy red" and bled on touch, consider *Donovanosis* (Granuloma inguinale).

Q: Which Salmonella species is anaerogenic?

S. typhi. ### Explanation The classification of *Salmonella* species in the laboratory often relies on their biochemical properties, specifically their ability to ferment sugars and produce gas. **1. Why S. typhi is the Correct Answer:** *Salmonella typhi* is unique among the common enteric pathogens because it is **anaerogenic**, meaning it ferments glucose and other sugars with the production of **acid only**, but **no gas**. In a Triple Sugar Iron (TSI) agar or a glucose fermentation tube, *S. typhi* will show an alkaline slant/acid butt (K/A) without any bubbles or cracks in the medium, indicating the absence of gas production. **2. Analysis of Incorrect Options:** * **S. paratyphi A & B:** Unlike *S. typhi*, most other members of the *Salmonella* genus (including the paratyphoid bacilli) are **aerogenic**. They ferment glucose with the production of both **acid and gas**. Therefore, in a TSI medium, these species will show gas bubbles or displacement of the agar. * **Note on S. paratyphi A:** It is important to remember that *S. paratyphi A* is a frequent "exception" in other biochemical tests (e.g., it does not produce $H_2S$), but it remains aerogenic. **3. High-Yield Clinical Pearls for NEET-PG:** * **TSI Reaction for S. typhi:** K/A (Alkaline slant/Acid butt), Gas negative, $H_2S$ positive (seen as a small mustache-like black ring at the junction of the slant and butt). * **H2S Production:** *S. typhi* produces a small amount of $H_2S$, whereas *S. paratyphi A* is $H_2S$ negative. * **Motility:** All *Salmonella* species are motile (peritrichous flagella) except for *S. Gallinarum* and *S. Pullorum*. * **Biochemical Rule of Thumb:** If a question mentions a *Salmonella* species that is "Gas Negative," always think of *S. typhi*.

Q: A recent immigrant from rural Brazil presents with a swollen face and extremely poor dental hygiene, including loss of an adult tooth, which appears to be the focus of the current infection. There are two open ulcers on the outside of the swollen cheek. Small yellow "grains" are seen in one of the ulcers. Gram stain shows purple-staining fine filaments. What is the most likely disease?

Actinomycotic mycetoma. **Explanation:** The clinical presentation points toward **Actinomycotic mycetoma** (specifically cervicofacial actinomycosis). The key diagnostic clues are the location (swollen face/cheek), association with poor dental hygiene or dental trauma (tooth loss), and the presence of **"sulfur granules"** (small yellow grains) discharging from sinus tracts. 1. **Why it is correct:** Actinomycetes are Gram-positive, anaerobic, or microaerophilic bacteria that form **fine, branching filaments** (resembling fungi, hence the name). On Gram stain, these appear as **purple-staining fine filaments**. The "sulfur granules" are actually microcolonies of the bacteria surrounded by inflammatory cells. 2. **Why other options are wrong:** * **Eumycotic mycetoma:** Caused by true fungi (e.g., *Madurella*). While it also presents with grains and swelling, the filaments would be much thicker (>2–4 μm), septate, and would not stain typically Gram-positive; they are better visualized with KOH or silver stains. * **Chromomycosis:** Characterized by "cauliflower-like" skin lesions and the presence of **Medlar bodies** (copper-penny bodies) on histology, not fine filaments or sulfur granules. * **Sporotrichosis:** Typically follows a "lymphocutaneous" spread (nodules along lymphatics) after a rose-gardener’s injury. It is a dimorphic fungus appearing as cigar-shaped yeast cells, not filaments in tissue. **High-Yield NEET-PG Pearls:** * **Actinomyces israelii** is the most common causative agent. * It is **not** acid-fast (unlike *Nocardia*, which is weakly acid-fast). * **Molar tooth appearance:** The characteristic colony morphology on anaerobic culture (Agar). * **Treatment:** High-dose Penicillin G is the drug of choice (DOC). Surgery may be required for abscess drainage.

Q: Mycoplasma infection simulates which of the following conditions?

Viral pneumonia. **Explanation:** **Mycoplasma pneumoniae** is the leading cause of **Atypical Pneumonia** (Walking Pneumonia). It simulates **Viral Pneumonia** because both conditions present with a clinical-radiological dissociation: the patient appears relatively well despite significant findings on a chest X-ray. 1. **Why Viral Pneumonia is correct:** Both Mycoplasma and viruses (like Influenza or RSV) cause interstitial inflammation rather than alveolar consolidation. They share a gradual onset, non-productive (dry) cough, low-grade fever, and diffuse patchy/interstitial infiltrates on imaging. 2. **Why other options are incorrect:** * **Pneumococcal Pneumonia:** Caused by *S. pneumoniae*, this is a "typical" pneumonia characterized by sudden onset, high fever, productive cough (rusty sputum), and lobar consolidation. * **Hypersensitivity Pneumonia:** This is an immune-mediated (Type III/IV) reaction to inhaled organic dusts (e.g., Farmer’s lung), not an infectious process. * **Aspiration Pneumonia:** Usually caused by anaerobes or gastric acid; it typically involves the dependent segments of the lungs (e.g., right lower lobe) and often leads to abscess formation. **High-Yield Clinical Pearls for NEET-PG:** * **Cell Wall:** Mycoplasma lacks a cell wall; therefore, it is **intrinsically resistant to Beta-lactams** (Penicillins/Cephalosporins). Treatment of choice: **Macrolides** (Azithromycin) or Tetracyclines. * **Diagnosis:** The **Cold Agglutinin Test** (IgM antibodies against I-antigen of RBCs) is a classic bedside test, though non-specific. * **Culture:** Requires specialized media (PPLO broth) containing sterols; colonies show a characteristic **"Fried Egg" appearance**. * **Complications:** Can cause Stevens-Johnson Syndrome, Bullous Myringitis, and Autoimmune Hemolytic Anemia.

Q: All of the following are true about Listeria except?

Gram negative bacteria. **Explanation:** The correct answer is **B (Gram-negative bacteria)** because *Listeria monocytogenes* is actually a **Gram-positive, non-spore-forming coccobacillus**. On Gram stain, it often appears as short chains or pairs and can sometimes be mistaken for *Streptococcus pneumoniae* or diphtheroids. **Analysis of Options:** * **Option A (Transmitted by contaminated milk):** This is a true statement. *Listeria* is a foodborne pathogen commonly transmitted via unpasteurized dairy products (milk, soft cheeses), processed deli meats, and raw vegetables. It is unique because it can grow at refrigeration temperatures (4°C), a process known as **cold enrichment**. * **Option C (Causes abortion in pregnancy):** This is true. Pregnant women are particularly susceptible due to impaired cell-mediated immunity. Infection can lead to amnionitis, premature labor, stillbirth, or spontaneous abortion. * **Option D (Causes meningitis in neonates):** This is true. *Listeria* is the **third most common cause of neonatal meningitis** (after *E. coli* and Group B Streptococcus). It can also cause "Granulomatosis infantiseptica," a severe systemic form of the disease in newborns. **High-Yield Clinical Pearls for NEET-PG:** * **Motility:** Exhibits characteristic **"Tumbling motility"** at 25°C (due to peritrichous flagella) but is non-motile at 37°C. On agar, it shows **"Umbrella-shaped"** growth. * **Intracellular Pathogen:** It is a facultative intracellular organism that uses **"Actin rockets"** (ActA protein) to move from cell to cell, bypassing humoral immunity. * **Hemolysis:** Shows narrow zones of **Beta-hemolysis** on blood agar and is **CAMP test positive** (similar to *S. agalactiae*). * **Treatment:** The drug of choice is **Ampicillin**. It is inherently resistant to all cephalosporins.

Q: An 8-month-old infant presents with a 1-day history of poor feeding and generalized weakness. The mother reports frequently giving the baby honey. What is the mechanism of action of the toxin responsible for this presentation?

Blocks the release of acetylcholine from the nerve terminal. **Explanation:** The clinical presentation describes **Infant Botulism**, caused by *Clostridium botulinum*. In infants, ingestion of honey containing **spores** leads to colonization of the gut and subsequent production of the botulinum toxin (unlike adults, who typically ingest the preformed toxin in canned food). **Correct Option (A):** The botulinum toxin is a neurotoxin that acts at the **neuromuscular junction**. It cleaves **SNARE proteins**, which are essential for the docking and fusion of synaptic vesicles with the presynaptic membrane. This effectively **blocks the release of Acetylcholine**, leading to flaccid paralysis, classically described as "Floppy Baby Syndrome." **Incorrect Options:** * **Option B:** This describes the mechanism of **Tetanospasmin** (*Clostridium tetani*). It blocks inhibitory neurotransmitters (GABA/Glycine) in the spinal cord, leading to spastic paralysis. * **Option C:** This describes the mechanism of **Diphtheria toxin** (*Corynebacterium diphtheriae*) and **Exotoxin A** (*Pseudomonas aeruginosa*), which inhibit protein synthesis via ADP-ribosylation of EF-2. * **Option D:** This refers to the **Alpha toxin** of *Clostridium perfringens*, which acts as a phospholipase (lecithinase) to degrade cell membranes, causing gas gangrene. **NEET-PG High-Yield Pearls:** * **Infant Botulism:** Spores in honey; "Floppy Baby"; constipation is often the first symptom. * **Foodborne Botulism:** Preformed toxin in home-canned alkaline vegetables/fish. * **Wound Botulism:** Associated with black tar heroin use. * **Diagnosis:** Demonstration of toxin/organism in feces (infants) or serum (adults). * **Treatment:** Equine antitoxin (adults) or Human Botulism Immune Globulin (BIG-IV) for infants.

Question 1

A 19-year-old girl presents with a painless ulcer in the labia majora with everted margins. What is the most likely causative organism?

Accepted Answer

Treponema pallidum

Answer

Chlamydia

Answer

Neisseria gonorrhoeae

Answer

Herpes simplex virus

Question 2

Which Salmonella species is anaerogenic?

Accepted Answer

S. typhi

Answer

S. paratyphi A

Answer

S. paratyphi B

Answer

All of the above

Question 3

A recent immigrant from rural Brazil presents with a swollen face and extremely poor dental hygiene, including loss of an adult tooth, which appears to be the focus of the current infection. There are two open ulcers on the outside of the swollen cheek. Small yellow "grains" are seen in one of the ulcers. Gram stain shows purple-staining fine filaments. What is the most likely disease?

Accepted Answer

Actinomycotic mycetoma

Answer

Chromomycosis

Answer

Eumycotic mycetoma

Answer

Sporotrichosis

Question 4

Mycoplasma infection simulates which of the following conditions?

Accepted Answer

Viral pneumonia

Answer

Pneumococcal pneumonia

Answer

Hypersensitivity pneumonia

Answer

Aspiration pneumonia

Question 5

All of the following are true about Listeria except?

Accepted Answer

Gram negative bacteria

Answer

Transmitted by contaminated milk

Answer

Causes abortion in pregnancy

Answer

Causes meningitis in neonates

Question 6

An elderly diabetic patient consults a physician due to severe earache. Examination reveals foul-smelling purulent otorrhea and a red mass lesion in the external ear canal. Biopsy of the mass demonstrates granulation tissue. What is the most likely causative organism?

Accepted Answer

Pseudomonas aeruginosa

Answer

Escherichia coli

Answer

Haemophilus influenzae

Answer

Proteus vulgaris

Question 7

What is the most common cause of acute bacterial endocarditis?

Accepted Answer

Staphylococcus aureus

Answer

Streptococcus viridans

Answer

Streptococcus intermedius

Answer

Candida albicans

Question 8

A young man presents to the emergency department with a maculopapular rash 2 weeks after healing of a painless genital ulcer. What is the most likely etiological agent?

Accepted Answer

Treponema pallidum

Answer

Treponema pennae

Answer

Chlamydia trachomatis

Answer

Calymmatobacterium granulomatis

Question 9

An 8-month-old infant presents with a 1-day history of poor feeding and generalized weakness. The mother reports frequently giving the baby honey. What is the mechanism of action of the toxin responsible for this presentation?

Accepted Answer

Blocks the release of acetylcholine from the nerve terminal

Answer

Blocks the release of inhibitory neurotransmitters such as glycine and GABA

Answer

Has a subunit that inactivates an elongation factor by ADP-ribosylation

Answer

Is a lecithinase

Question 10

Which of the following statements is NOT true about Mycobacterium tuberculosis infection?

Accepted Answer

Lymphocytes are the primary cells infected by M. tuberculosis.

Answer

M. tuberculosis leads to the development of delayed hypersensitivity.

Answer

A positive tuberculin test signifies cell-mediated hypersensitivity.

Answer

The tuberculin test does not differentiate between infection and disease.

Bacteriology — MCQs

Bacteriology — MCQs

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