Bacteriology Practice Questions

Q: A patient with conjunctival infection, which led to corneal perforation, was positive for Gram-negative coccoid appearance on Gram stain. Further investigation showed small translucent colonies, which are oxidase positive. What is the most probable causative organism?

Neisseria gonorrhoeae. ### Explanation The clinical presentation of a severe conjunctival infection leading to **corneal perforation** is a hallmark of **Hyperacute Bacterial Conjunctivitis**, most commonly caused by *Neisseria gonorrhoeae*. **Why the correct answer is right:** * **Microscopy:** *Neisseria gonorrhoeae* are Gram-negative, kidney-shaped diplococci (coccoid appearance). * **Biochemical Profile:** They are **Oxidase positive** and Catalase positive. * **Culture:** They produce small, translucent, or grayish-white colonies on enriched media like Chocolate agar or Thayer-Martin medium. * **Pathogenesis:** Unlike most bacteria, *N. gonorrhoeae* can penetrate an **intact corneal epithelium**, leading to rapid ulceration and perforation if not treated aggressively. **Analysis of Incorrect Options:** * **Moraxella catarrhalis:** While it is a Gram-negative coccus and oxidase-positive, it typically causes respiratory infections or otitis media. In the eye, it causes milder angular blepharoconjunctivitis, not rapid corneal perforation. * **Pseudomonas aeruginosa:** A common cause of aggressive corneal ulcers (especially in contact lens users), but it is a **Gram-negative rod**, not a coccus. * **Acinetobacter species:** These are Gram-negative coccobacilli and are notably **Oxidase negative**, which contradicts the findings in the question. **High-Yield NEET-PG Pearls:** * **Organisms that penetrate intact cornea:** *N. gonorrhoeae, N. meningitidis, Corynebacterium diphtheriae,* and *Listeria monocytogenes*. * **Ophthalmia Neonatorum:** *N. gonorrhoeae* presents within 2–5 days of birth (hyperacute); *Chlamydia trachomatis* presents later (5–14 days). * **Drug of Choice:** Systemic Ceftriaxone is mandatory for gonococcal conjunctivitis due to the risk of globe perforation and systemic spread.

Q: Asymptomatic gonorrhoea in females is typically due to infections of which site?

Endocervix. **Explanation:** The **endocervix** is the primary site of infection for *Neisseria gonorrhoeae* in females. This is because the organism has a specific predilection for **columnar epithelium**, which lines the endocervical canal. In approximately 50–70% of cases, endocervical gonorrhea remains **asymptomatic**, acting as a major reservoir for transmission and increasing the risk of unrecognized progression to Pelvic Inflammatory Disease (PID). **Analysis of Options:** * **Vagina (Incorrect):** The adult vagina is lined by stratified squamous epithelium, which is resistant to gonococcal infection. Vaginal involvement (vulvovaginitis) is typically only seen in prepubertal girls where the epithelium is thin. * **Urethra (Incorrect):** While the urethra is frequently co-infected (causing dysuria), it is not the primary site of asymptomatic colonization in females. * **Fundus (Incorrect):** The fundus refers to the upper part of the uterus. While infection can ascend to the endometrium, the primary and most common site of initial infection is the cervix. **Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Culture on **Thayer-Martin medium** (a selective Chocolate agar containing Vancomycin, Colistin, and Nystatin). * **Most Sensitive Test:** Nucleic Acid Amplification Test (NAAT). * **Microscopy:** Gram-negative kidney-shaped diplococci found within polymorphonuclear leukocytes (intracellular). * **Fitz-Hugh-Curtis Syndrome:** A complication of gonococcal PID involving peri-hepatitis ("violin-string" adhesions). * **Treatment:** Current CDC/WHO guidelines recommend Ceftriaxone (IM) as the mainstay of therapy.

Q: What is true about Mycoplasma?

Requires cholesterol for growth. **Explanation:** **Mycoplasma** are the smallest free-living organisms. The defining characteristic of Mycoplasma is the **complete absence of a cell wall**. 1. **Why Option C is Correct:** Since Mycoplasma lack a cell wall, their cell membrane is their only protective barrier. To provide structural integrity and stability to this membrane, they incorporate **sterols (cholesterol)**. Unlike most bacteria, Mycoplasma cannot synthesize sterols and must acquire them from the external environment (e.g., PPLO agar supplemented with horse serum). 2. **Why Other Options are Incorrect:** * **Option A:** Mycoplasma are **extracellular** pathogens (except *M. penetrans*). They typically adhere to the respiratory or urogenital epithelium. * **Option B:** Penicillin acts by inhibiting cell wall synthesis (peptidoglycan). Because Mycoplasma **lack a cell wall**, they are **innately resistant** to all beta-lactam antibiotics (Penicillins, Cephalosporins). * **Option D:** As mentioned, they have **no cell wall**, making them highly pleomorphic (able to take many shapes) and resistant to Gram staining. **High-Yield Clinical Pearls for NEET-PG:** * **Culture:** They produce characteristic **"Fried Egg" colonies** on agar. * **Diagnosis:** *M. pneumoniae* causes **Atypical Pneumonia** (Walking Pneumonia). It is associated with **Cold Agglutinins** (anti-I antibodies) which can cause autoimmune hemolytic anemia. * **Treatment:** Drug of choice is **Macrolides** (Azithromycin) or Tetracyclines (Doxycycline). * **Staining:** They do not Gram stain; **Giemsa or Diene’s stain** is used instead.

Q: Which of the following is NOT true about Shigella dysenteriae?

It produces the mildest form of dysentery.. **Explanation:** The correct answer is **D**. *Shigella dysenteriae* (specifically Serotype 1) is the most virulent species of the genus and causes the **most severe** form of bacillary dysentery. It is associated with extensive mucosal ulceration, high fever, and life-threatening complications. In contrast, *Shigella sonnei* is responsible for the mildest form of the disease, often presenting as self-limiting watery diarrhea. **Analysis of other options:** * **A. Invades the colonic mucosa:** This is a hallmark of *Shigella* pathogenesis. The bacteria enter M cells in Peyer’s patches and spread laterally between colonic epithelial cells using actin tails (similar to *Listeria*), leading to inflammation and abscess formation. * **B. Can cause Hemolytic Uremic Syndrome (HUS):** *S. dysenteriae* Type 1 produces Shiga toxin, which can enter the bloodstream and damage glomerular endothelial cells. This leads to the triad of HUS: Microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. * **C. Produces Shiga toxin (verocytotoxin):** *S. dysenteriae* produces the potent Shiga toxin (Stx), which has one A subunit and five B subunits. It inhibits protein synthesis by inactivating the 60S ribosomal subunit. **High-Yield Clinical Pearls for NEET-PG:** * **Infective Dose:** Very low (10–100 organisms), making it highly communicable. * **Motility:** Non-motile (distinguishes it from *Salmonella*). * **Biochemicals:** Catalase positive (except *S. dysenteriae* Type 1), Oxidase negative, and Non-lactose fermenters (except *S. sonnei*, which is a late lactose fermenter). * **Species Severity:** *S. dysenteriae* (Most severe) > *S. flexneri* (Most common in India) > *S. boydii* > *S. sonnei* (Mildest/Most common in developed countries).

Q: Which of the following is NOT a typical outcome of diphtheria infection?

Endotoxemia. **Explanation:** The correct answer is **Endotoxemia** because *Corynebacterium diphtheriae* is a Gram-positive bacterium. Endotoxins (Lipopolysaccharides) are structural components found exclusively in the outer membrane of **Gram-negative** bacteria. Diphtheria causes pathology primarily through the release of a potent **Exotoxin**, which inhibits protein synthesis via ADP-ribosylation of Elongation Factor-2 (EF-2). **Analysis of Options:** * **Pseudomembrane formation:** This is the hallmark of respiratory diphtheria. The exotoxin causes local tissue necrosis, leading to an inflammatory exudate composed of fibrin, leukocytes, and dead epithelial cells that forms a tough, greyish-white membrane over the tonsils and pharynx. * **Cardiomyopathy:** The diphtheria toxin enters the bloodstream (toxemia) and has a high affinity for cardiac tissue. It can cause acute myocarditis, leading to heart failure or arrhythmias, typically occurring 1–2 weeks after the onset of local symptoms. * **Polyneuropathy:** The toxin also targets the nervous system by causing demyelination. This often manifests as soft palate paralysis, followed by oculomotor disturbances and peripheral polyneuropathy. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Diphtheria toxin is encoded by the **tox gene** introduced by a lysogenic bacteriophage (**Beta-phage**). * **Diagnosis:** Use **Albert’s stain** to visualize metachromatic granules (Volutin/Babes-Ernst granules). * **Culture:** Grows on **Löffler's serum slope** (rapid growth) and **Potassium Tellurite agar** (black colonies). * **Virulence Test:** The **Elek’s gel precipitation test** is used to detect toxin production. * **Treatment:** Immediate administration of **Diphtheria Antitoxin (ADS)** is the priority, as it only neutralizes unbound toxin.

Q: A 28-year-old sexually active man presents with a new, painless genital ulcer on his penis that he noticed 1 week ago. The ulcer is 1 cm in size, with an eroded base and an indurated margin. Dark-field examination of the ulcer fluid confirmed the diagnosis. What is the most likely infecting organism?

Syphilis. ### Explanation The clinical presentation describes a classic **Primary Syphilis** infection caused by the spirochete ***Treponema pallidum*** [1]. **Why Syphilis is correct:** The hallmark of primary syphilis is the **chancre**. It typically presents as a single, **painless**, well-circumscribed ulcer with a clean base and **indurated (firm) margins**. The incubation period is usually 3 weeks (range 10–90 days). **Dark-field microscopy** is the gold standard for immediate diagnosis of primary syphilis, as it allows for the direct visualization of motile spirochetes from the ulcer exudate before serological tests (like VDRL/RPR) become positive [1]. **Why the other options are incorrect:** * **Toxoplasmosis:** Caused by *Toxoplasma gondii*, it typically presents as a systemic febrile illness with lymphadenopathy or CNS involvement in immunocompromised patients, not genital ulcers. * **Tetanus:** Caused by *Clostridium tetani*, it presents with neuromuscular symptoms like lockjaw (trismus) and muscle spasms following a contaminated wound; it does not cause genital lesions. * **Smallpox:** Caused by the Variola virus, it presents with a systemic centrifugal rash (pustules) and has been eradicated globally. **NEET-PG High-Yield Pearls:** * **Painful vs. Painless:** Syphilis (Chancre) is **painless**, whereas Chancroid (*Haemophilus ducreyi*) is **painful** ("You *do cry* with *ducreyi*"). * **Diagnosis:** VDRL/RPR are screening tests (non-specific); FTA-ABS/TPPA are confirmatory (specific). * **Treatment:** The drug of choice for all stages of syphilis is **Benzathine Penicillin G**. * **Jarisch-Herxheimer Reaction:** An acute febrile reaction occurring within 24 hours of starting penicillin treatment for syphilis due to the release of endotoxins from dying spirochetes.

Q: What is the cause of skin lesions in meningococcal meningitis?

Endotoxin. **Explanation:** The characteristic skin lesions in meningococcal meningitis (petechiae and purpura) are primarily caused by the **Endotoxin** of *Neisseria meningitidis*. 1. **Why Endotoxin is correct:** Unlike most Gram-negative bacteria that possess Lipopolysaccharide (LPS), *N. meningitidis* contains **Lipooligosaccharide (LOS)** in its outer membrane. During rapid bacterial proliferation and autolysis, large amounts of LOS are released into the bloodstream (endotoxemia). This triggers a massive inflammatory cascade, activating the complement system and coagulation pathways. This leads to **Disseminated Intravascular Coagulation (DIC)**, vascular endothelial damage, and capillary leakage, manifesting clinically as petechial rashes and purpura fulminans. 2. **Why other options are incorrect:** * **Exotoxin:** *Neisseria meningitidis* does not produce any exotoxins. Its virulence is derived from its capsule and LOS. * **Allergic reaction:** The lesions are a result of acute inflammatory and thrombotic processes, not a Type I or Type IV hypersensitivity reaction. * **Direct vascular damage:** While vascular damage occurs, it is a *secondary* effect mediated by the endotoxin-induced inflammatory surge and microthrombi, rather than direct mechanical invasion of the vessel walls by the bacteria alone. **High-Yield Clinical Pearls for NEET-PG:** * **Waterhouse-Friderichsen Syndrome:** Severe meningococcemia leading to bilateral adrenal hemorrhage and circulatory collapse. * **Virulence Factor:** The **Polysaccharide capsule** is responsible for grouping and resisting phagocytosis, but the **LOS (Endotoxin)** correlates with the severity of the disease and skin manifestations. * **Drug of Choice:** Ceftriaxone is the preferred treatment; Chemoprophylaxis for contacts is Rifampicin (or Ciprofloxacin/Ceftriaxone).

Q: Which of the following statements about Listeria is true?

It is motile by peritrichous flagella.. **Explanation:** *Listeria monocytogenes* is a unique Gram-positive, non-spore-forming bacillus with several high-yield characteristics relevant to clinical practice and exams. **Why Option B is Correct:** Listeria exhibits a characteristic temperature-dependent motility. At room temperature (**20–25°C**), it develops **peritrichous flagella**, resulting in a distinctive **"tumbling motility"** seen on hanging drop preparations. At 37°C, flagella production is downregulated, and it moves intracellularly via actin polymerization ("actin rockets"). **Why Other Options are Incorrect:** * **Option A:** Listeria is a **Gram-positive** bacillus (often described as coccobacillary). It can sometimes be confused with *Corynebacterium* (diphtheroids) or *S. pneumoniae*. * **Option C:** While it is a significant cause of meningitis in specific populations (neonates, elderly, and immunocompromised), the **most common** cause of community-acquired meningitis in the general adult population is *Streptococcus pneumoniae*. * **Option D:** There are **13 known serotypes** of *L. monocytogenes* based on somatic (O) and flagellar (H) antigens. Serotypes 1/2a, 1/2b, and 4b are responsible for the majority of human infections. **High-Yield Clinical Pearls for NEET-PG:** * **Cold Enrichment:** It can grow at temperatures as low as 4°C, a property used for selective isolation from contaminated food (e.g., unpasteurized cheese, cold cuts). * **CAMP Test Positive:** It produces a rectangular (not arrowhead) zone of hemolysis when streaked with *Staphylococcus aureus*. * **Catalase Positive:** This helps differentiate it from *Group B Streptococcus* (GBS), which is catalase-negative but also causes neonatal meningitis. * **Treatment:** **Ampicillin** is the drug of choice. It is inherently resistant to all cephalosporins.

Q: Which of the following findings is true regarding Cerebrospinal Fluid (CSF) in pyogenic meningitis?

Protein elevated, sugar decreased. In pyogenic (bacterial) meningitis, the CSF profile undergoes characteristic changes due to the inflammatory response and bacterial metabolism. ### **Explanation of the Correct Answer** **Option A (Protein elevated, sugar decreased)** is correct because: 1. **Elevated Protein:** Inflammation causes increased permeability of the blood-brain barrier (BBB) and the presence of bacterial proteins and inflammatory exudate, leading to a significant rise in CSF protein levels (typically >100 mg/dL). 2. **Decreased Sugar (Hypoglycorrhachia):** Bacteria and infiltrating polymorphonuclear leukocytes (neutrophils) consume glucose for metabolism. Additionally, there is impaired glucose transport across the BBB. CSF glucose is usually <40 mg/dL or less than 40% of simultaneous blood glucose. ### **Analysis of Incorrect Options** * **Option B:** Incorrect because sugar is consumed, not elevated, and proteins always rise during active infection. * **Options C & D:** Chloride levels in the CSF typically **decrease** in meningitis (especially in Tubercular Meningitis), but they are not the primary diagnostic markers compared to protein and glucose. Protein is never decreased in pyogenic infections. ### **NEET-PG High-Yield Pearls** * **Cell Count:** Pyogenic meningitis shows marked **Neutrophilic pleocytosis** (often >1000 cells/mm³). * **Tubercular Meningitis (TBM):** Characterized by **Lymphocytic** predominance, very high protein, and the most significant drop in **Chloride** levels. * **Viral (Aseptic) Meningitis:** Normal sugar, slightly elevated protein, and lymphocytic predominance. * **Normal CSF Glucose:** Usually 2/3rd (60-70%) of the plasma glucose level.

Question 1

A patient with conjunctival infection, which led to corneal perforation, was positive for Gram-negative coccoid appearance on Gram stain. Further investigation showed small translucent colonies, which are oxidase positive. What is the most probable causative organism?

Accepted Answer

Neisseria gonorrhoeae

Answer

Moraxella catarrhalis

Answer

Pseudomonas aeruginosa

Answer

Acinetobacter actinatus

Question 2

Donovan bodies are best described as:

Accepted Answer

Monocytes that contain the organism, Calymmatobacterium granulomatis

Answer

Leukocytes that contain the organism, Calymmatobacterium granulomatis

Answer

Eosinophils that contain the organism, Calymmatobacterium granulomatis

Answer

Monocytes that contain the organism, Haemophilus influenza

Question 3

Asymptomatic gonorrhoea in females is typically due to infections of which site?

Accepted Answer

Endocervix

Answer

Vagina

Answer

Urethra

Answer

Fundus

Question 4

What is true about Mycoplasma?

Accepted Answer

Requires cholesterol for growth

Answer

Obligate intracellular organism

Answer

Penicillin is an effective treatment

Answer

Has a thick cell wall

Question 5

Which of the following is NOT true about Shigella dysenteriae?

Accepted Answer

It produces the mildest form of dysentery.

Answer

It invades the colonic mucosa.

Answer

It can cause Hemolytic Uremic Syndrome (HUS).

Answer

It produces Shiga toxin (verocytotoxin).

Question 6

Which of the following is NOT a typical outcome of diphtheria infection?

Accepted Answer

Endotoxemia

Answer

Pseudomembrane formation

Answer

Cardiomyopathy

Answer

Polyneuropathy

Question 7

A 28-year-old sexually active man presents with a new, painless genital ulcer on his penis that he noticed 1 week ago. The ulcer is 1 cm in size, with an eroded base and an indurated margin. Dark-field examination of the ulcer fluid confirmed the diagnosis. What is the most likely infecting organism?

Accepted Answer

Syphilis

Answer

Toxoplasmosis

Answer

Tetanus

Answer

Smallpox

Question 8

What is the cause of skin lesions in meningococcal meningitis?

Accepted Answer

Endotoxin

Answer

Exotoxin

Answer

Allergic reaction

Answer

Direct vascular damage

Question 9

Which of the following statements about Listeria is true?

Accepted Answer

It is motile by peritrichous flagella.

Answer

It is a Gram-negative bacillus.

Answer

It is the commonest cause of community-acquired meningitis.

Answer

Only one serovar of Listeria is known.

Question 10

Which of the following findings is true regarding Cerebrospinal Fluid (CSF) in pyogenic meningitis?

Accepted Answer

Protein elevated, sugar decreased

Answer

Protein decreased, sugar elevated

Answer

Chloride elevated, protein decreased

Answer

Protein decreased, chloride decreased

Bacteriology — MCQs

Bacteriology — MCQs

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