Bacteriology Practice Questions

Q: A positive Schick test indicates that a person is?

Susceptible to diphtheria. ### Explanation The **Schick test** is a classic skin test used to determine the immune status of an individual against *Corynebacterium diphtheriae*. It works on the principle of **toxin-antitoxin neutralization**. **Why Option C is Correct:** A person is considered **susceptible** if they lack circulating antitoxin (antibodies) against the diphtheria toxin. In the test, 0.1 ml of purified diphtheria toxin is injected intradermally into one forearm (test) and inactivated toxin into the other (control). If the person lacks immunity, the toxin remains active and causes a localized inflammatory reaction (erythema and swelling) at the injection site within 4–7 days. This is a **Positive Result**, indicating susceptibility. **Why Other Options are Incorrect:** * **Option A:** If a person is **immune**, their circulating antitoxin neutralizes the injected toxin, resulting in no skin reaction (Negative Result). * **Option B:** Hypersensitivity is detected by a **Pseudo-reaction**, where both the test and control arms show a reaction that disappears rapidly (within 48 hours). This indicates an allergy to the bacterial proteins, not susceptibility to the toxin. * **Option D:** A carrier state is identified through throat or nasal swabs and culture (e.g., on Löffler's serum slope), not by the Schick test, which only measures humoral immunity. ### NEET-PG High-Yield Pearls: * **Positive Reaction:** Erythema (10–50 mm) reaching peak at 4–7 days. Indicates **Susceptibility**. * **Negative Reaction:** No reaction on either arm. Indicates **Immunity**. * **Combined Reaction:** Both arms show reaction, but the test arm reaction is larger and lasts longer. Indicates both **Susceptibility and Hypersensitivity**. * **Current Status:** The Schick test is largely obsolete in clinical practice due to the widespread use of the DPT/Pentavalent vaccine, but it remains a frequent topic in competitive exams.

Q: Which of the following organisms is most commonly responsible for Sudden Infant Death Syndrome (SIDS), particularly after honey consumption by the infant?

Clostridium botulinum. **Explanation:** **Clostridium botulinum** is the correct answer because it is the causative agent of **Infant Botulism**. In infants under 12 months, the intestinal flora is not fully developed, allowing ingested *C. botulinum* spores to germinate, colonize the gut, and release a potent neurotoxin. **Honey** is a well-known environmental reservoir for these spores. The toxin inhibits acetylcholine release at the neuromuscular junction, leading to "Floppy Baby Syndrome" (descending paralysis). In severe cases, it causes sudden respiratory failure, which is a recognized, preventable cause of **Sudden Infant Death Syndrome (SIDS)**. **Analysis of Incorrect Options:** * **Staphylococcus aureus:** While a common cause of skin infections and food poisoning (via preformed enterotoxin), it is not linked to honey consumption or the specific pathogenesis of SIDS. * **E. coli:** Primarily associated with neonatal sepsis, meningitis, or gastroenteritis, but not with the toxidrome leading to SIDS following honey ingestion. * **Clostridium perfringens:** Causes gas gangrene and food poisoning in adults; it does not colonize the infant gut to cause the paralytic syndrome seen in botulism. **Clinical Pearls for NEET-PG:** * **Age Group:** Most common between **2 to 6 months**. * **Classic Presentation:** Constipation (earliest sign), followed by weak cry, poor suck reflex, loss of head control, and generalized hypotonia (**"Floppy Baby"**). * **Diagnosis:** Identification of spores or toxin in the **stool** (not the serum, as toxin levels are often too low to detect). * **Management:** Treatment is with **Botulism Immune Globulin Intravenous (BIG-IV)**. Avoid honey in all infants <1 year of age.

Q: What is the most likely organism causing epididymitis in a 30-year-old man?

Chlamydia. **Explanation:** The etiology of epididymitis is primarily determined by the patient's age and sexual history. In men **under 35 years of age**, the condition is most commonly caused by sexually transmitted infections (STIs). **1. Why Chlamydia is Correct:** *Chlamydia trachomatis* is the most frequent cause of epididymitis in sexually active men aged <35 years [1]. It typically presents with a gradual onset of scrotal pain, swelling, and may be associated with urethritis [1]. While *Neisseria gonorrhoeae* is also a common cause in this demographic, epidemiological studies consistently show *Chlamydia* as the leading pathogen [1]. **2. Why the other options are incorrect:** * **E. coli (Option A):** This is the most common cause of epididymitis in men **over 35 years** or in children. In these groups, the infection is usually associated with urinary tract infections (UTIs), bladder outlet obstruction (e.g., BPH), or recent urological instrumentation. * **Gonococci (Option B):** *Neisseria gonorrhoeae* is a major cause of STI-related epididymitis in young men, but it ranks second to *Chlamydia* [1]. Gonococcal infection often presents with more acute symptoms and a purulent urethral discharge [1]. * **Ureaplasma ureolyticum (Option D):** While *Ureaplasma* can cause non-gonococcal urethritis, it is a much rarer cause of epididymitis compared to *Chlamydia* [1]. **Clinical Pearls for NEET-PG:** * **Age *Gonorrhea*). Treatment: Ceftriaxone (IM) + Doxycycline (Oral). * **Age >35 years:** Think Enteric organisms (*E. coli*, *Klebsiella*, *Pseudomonas*). Treatment: Fluoroquinolones (e.g., Levofloxacin). * **Prehn’s Sign:** Elevation of the scrotum relieves pain in epididymitis (Positive Prehn's), helping differentiate it from **Testicular Torsion** (Negative Prehn's), which is a surgical emergency.

Q: A patient with Rheumatic Heart Disease (RHD) developed infective endocarditis following a dental extraction. Which organism is the most likely cause?

Streptococcus viridans. **Explanation:** **1. Why Streptococcus viridans is correct:** The clinical scenario describes **Subacute Bacterial Endocarditis (SBE)**. *Streptococcus viridans* (a group including *S. mitis, S. sanguinis, and S. mutans*) is the most common cause of SBE in patients with pre-existing valvular damage, such as **Rheumatic Heart Disease (RHD)**. These organisms are commensals of the oral cavity. Dental procedures (like extraction) cause transient bacteremia, allowing these bacteria to adhere to damaged endocardium or fibrin-platelet vegetations via **dextrans** (extracellular polysaccharides). **2. Why the other options are incorrect:** * **Streptococcus pneumoniae:** Primarily causes lobar pneumonia and meningitis; it is an uncommon cause of endocarditis and typically affects normal valves (acute presentation). * **Streptococcus pyogenes (Group A Strep):** While it is the trigger for **Rheumatic Fever** (via molecular mimicry), it is rarely the cause of subsequent infective endocarditis. * **Staphylococcus aureus:** The leading cause of **Acute Bacterial Endocarditis**. It typically affects healthy/normal valves and is the most common cause in **Intravenous Drug Users (IVDU)**, often involving the tricuspid valve. **3. NEET-PG High-Yield Pearls:** * **Most common cause of SBE:** *Streptococcus viridans*. * **Most common cause of Acute Endocarditis:** *Staphylococcus aureus*. * **Most common cause in IVDU:** *Staphylococcus aureus*. * **Prosthetic Valve Endocarditis:** *Staphylococcus epidermidis* (early onset, <1 year). * **Culture-negative Endocarditis:** Most commonly due to *HACEK* group or *Coxiella burnetii*. * **Association with Colon Cancer:** *Streptococcus gallolyticus* (formerly *S. bovis*).

Q: All of the following are true about Bacillus anthracis except?

The toxin is a complex of two fractions.. **Explanation:** The correct answer is **D** because it is a false statement. The *Bacillus anthracis* toxin is actually a **tripartite complex** consisting of **three** distinct protein fractions, not two: 1. **Protective Antigen (PA):** The binding unit that facilitates entry into host cells. 2. **Edema Factor (EF):** An adenylate cyclase that causes localized edema. 3. **Lethal Factor (LF):** A zinc metalloprotease that causes cell death and tissue necrosis. **Analysis of other options:** * **Option A (True):** Virulence in *B. anthracis* is mediated by two large plasmids: **pXO1** (encodes the toxin complex) and **pXO2** (encodes the capsule). * **Option B (True):** Cutaneous anthrax (Hide porter’s disease) is the most common form. In 80-90% of cases, the characteristic black eschar heals spontaneously, though antibiotics are given to prevent systemic spread. * **Option C (True):** Unlike most bacteria that have polysaccharide capsules, *B. anthracis* has a unique **poly-D-glutamic acid (polypeptide) capsule** which is strongly anti-phagocytic. **NEET-PG High-Yield Pearls:** * **McFadyean’s Reaction:** Used for presumptive diagnosis; uses polychrome methylene blue to visualize the purple capsule. * **Morphology:** Described as "Bamboo stick" appearance; colonies on agar show a "Medusa head" or "Frosted glass" appearance. * **String of Pearls Reaction:** Occurs when grown on agar containing low concentrations of penicillin. * **Inhalation Anthrax:** Also known as Woolsorter’s disease; characterized by hemorrhagic mediastinitis (widened mediastinum on X-ray).

Question 1

What reaction is due to lysis of bacterial cell wall and necrotic cell products?

Accepted Answer

Jarisch-Herxheimer reaction

Answer

Arthus reaction

Answer

Serum sickness

Answer

Infectious mononucleosis-ampicillin reaction

Question 2

A positive Schick test indicates that a person is?

Accepted Answer

Susceptible to diphtheria

Answer

Immune to diphtheria

Answer

Hypersensitive to diphtheria

Answer

Carrier of diphtheria

Question 3

A 57-year-old man with a history of cardiac valvular prosthesis implanted 5 years ago presents with several weeks of weakness, fatigue, and intermittent fever. Blood cultures grew gram-positive cocci. Laboratory tests revealed that the organisms failed to ferment mannitol and their growth was inhibited by novobiocin. What is the most likely infectious agent?

Accepted Answer

Staphylococcus epidermidis

Answer

Staphylococcus aureus

Answer

Staphylococcus saprophyticus

Answer

Streptococcus viridans

Question 4

The causative organism can be best isolated in which of the following conditions?

Accepted Answer

Tampons in toxic shock syndrome

Answer

Rheumatic valvulitis

Answer

CSF in tetanus

Answer

Diptheritic myocarditis

Question 5

Which of the following organisms is most commonly responsible for Sudden Infant Death Syndrome (SIDS), particularly after honey consumption by the infant?

Accepted Answer

Clostridium botulinum

Answer

Staphylococcus aureus

Answer

E. coli

Answer

Clostridium perfringens

Question 6

What is the most likely organism causing epididymitis in a 30-year-old man?

Accepted Answer

Chlamydia

Answer

E. coli

Answer

Gonococci

Answer

Ureaplasma ureolyticum

Question 7

A 43-year-old man suddenly develops odynophagia. Which organism is most likely to be isolated on throat culture?

Accepted Answer

Normal pharyngeal flora

Answer

Mononucleosis

Answer

S. aureus

Answer

Group A streptococci

Question 8

A patient with Rheumatic Heart Disease (RHD) developed infective endocarditis following a dental extraction. Which organism is the most likely cause?

Accepted Answer

Streptococcus viridans

Answer

Streptococcus pneumoniae

Answer

Streptococcus pyogenes

Answer

Staphylococcus aureus

Question 9

A sex worker presents with a discharging ulcer. Gram-negative diplococci are seen on microscopy, and growth is obtained on modified Thayer-Martin media. What is the most likely diagnosis?

Accepted Answer

Neisseria gonorrhoeae

Answer

Syphilis

Answer

Lymphogranuloma venereum

Answer

Chancroid

Question 10

All of the following are true about Bacillus anthracis except?

Accepted Answer

The toxin is a complex of two fractions.

Answer

A plasmid is responsible for toxin production.

Answer

Cutaneous anthrax generally resolves spontaneously.

Answer

The capsular polypeptide aids virulence by inhibiting phagocytosis.

Bacteriology — MCQs

Bacteriology — MCQs

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