Bacteriology Practice Questions

Q: An increased frequency of infections due to which of the following organisms has been reported among patients with achlorhydria?

Salmonella. **Explanation:** The correct answer is **Salmonella (Option A)**. **Underlying Medical Concept:** Gastric acid (HCl) serves as a primary innate immune barrier against ingested pathogens. **Achlorhydria** (absence of gastric acid) or the use of proton pump inhibitors (PPIs) significantly lowers the infectious dose required for certain bacteria to cause disease. *Salmonella* species are particularly **acid-labile**, meaning they are easily destroyed by a normal gastric pH of 1.5–3.5. In patients with achlorhydria, these bacteria survive the passage through the stomach and reach the small intestine, leading to an increased frequency of infections like Enteric fever and Salmonellosis. **Analysis of Incorrect Options:** * **B. Shigella:** Unlike Salmonella, *Shigella* is highly **acid-resistant**. It can survive the gastric barrier even at normal acidity levels, requiring an extremely low infectious dose (as few as 10–100 organisms). Thus, its frequency is not significantly altered by achlorhydria. * **C. Entamoeba:** *Entamoeba histolytica* is transmitted via **cysts**, which are naturally resistant to gastric acid. The acid actually helps in the excystation process in the lower GI tract. * **D. Vibrio cholerae:** While *V. cholerae* is also acid-labile, the question specifically targets the organism most classically associated with increased frequency in clinical reports of achlorhydria. While achlorhydria *is* a risk factor for Cholera, *Salmonella* is the more frequent and high-yield association in medical literature and exams regarding this specific physiological deficit. **High-Yield Clinical Pearls for NEET-PG:** * **Acid-Labile Organisms:** *Salmonella*, *Vibrio cholerae*, and *Campylobacter*. * **Acid-Resistant Organisms:** *Shigella*, *Helicobacter pylori*, and *Hepatitis A virus*. * Conditions predisposing to these infections include long-term PPI use, gastrectomy, and pernicious anemia.

Q: True about Campylobacter?

Polar flagella. **Explanation:** *Campylobacter jejuni* is a leading cause of bacterial gastroenteritis worldwide. Understanding its unique physiological and morphological characteristics is high-yield for NEET-PG. **1. Why Option A is Correct:** *Campylobacter* species are characterized by their distinct **comma or S-shaped** morphology (often described as "seagull-wing" appearance). They possess a **single polar flagellum** at one or both ends, which provides them with a characteristic **"darting motility."** This motility is a key diagnostic feature in stool microscopy. **2. Why the Other Options are Incorrect:** * **Option B & D:** *Campylobacter* is **thermophilic**, not psychrophilic. It grows optimally at **42°C**, which is an adaptation to the body temperature of its primary reservoir, poultry. It typically does not grow at 25°C (room temperature). * **Option C:** It is a **microaerophilic** organism, meaning it requires reduced oxygen levels (5–10%) and increased carbon dioxide (10%) for growth. It cannot survive as a strict aerobe. **Clinical Pearls for NEET-PG:** * **Culture Media:** Requires selective media like **Skirrow’s medium** or **Butzler’s medium**. * **Clinical Association:** It is the most common antecedent infection associated with **Guillain-Barré Syndrome (GBS)** due to molecular mimicry between bacterial lipooligosaccharides and human gangliosides. * **Complications:** Can cause pseudo-appendicitis (mesenteric adenitis) and reactive arthritis (HLA-B27 associated). * **Treatment:** Erythromycin or Azithromycin (Macrolides) are the drugs of choice.

Q: Bile solubility is a characteristic feature of which bacterium?

Streptococcus pneumoniae. **Explanation:** **Streptococcus pneumoniae** is the correct answer because it is uniquely characterized by its sensitivity to bile salts (such as sodium deoxycholate). The underlying mechanism involves the activation of **autolytic enzymes** (L-alanine-muramyl amidase). When bile salts are added to a culture, they lower the surface tension, triggering these autolysins to digest the cell wall, resulting in the lysis of the bacteria and the clearing of a turbid suspension. This "Bile Solubility Test" is the gold standard for differentiating *S. pneumoniae* from other alpha-hemolytic streptococci (like Viridans group). **Analysis of Incorrect Options:** * **Staphylococcus aureus:** These are Gram-positive cocci in clusters that are catalase and coagulase positive. They do not possess the specific autolytic enzymes triggered by bile. * **Streptococcus agalactiae (Group B Strep):** These are beta-hemolytic and are identified by a positive **CAMP test** and hippurate hydrolysis. They are bile insoluble. * **Streptococcus pyogenes (Group A Strep):** These are beta-hemolytic and are identified by their sensitivity to **Bacitracin**. They do not undergo lysis in the presence of bile. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *S. pneumoniae* are Gram-positive, lancet-shaped diplococci. * **Quellung Reaction:** Swelling of the capsule when exposed to specific antiserum (positive in *S. pneumoniae*). * **Optochin Sensitivity:** *S. pneumoniae* is sensitive to Optochin (P-disk), while Viridans streptococci are resistant. * **Culture:** Shows "draughtsman" or "checkerboard" appearance due to central autolysis on blood agar.

Q: Suppurating buboes are seen in infection with:

Haemophilus ducreyi. **Explanation:** **Haemophilus ducreyi** is the causative agent of **Chancroid**, a sexually transmitted infection characterized by painful genital ulcers and painful, inflammatory inguinal lymphadenopathy. These enlarged lymph nodes, known as **buboes**, frequently undergo central necrosis and become fluctuant. If left untreated, they often rupture spontaneously, leading to **suppuration** (pus discharge) and the formation of chronic draining sinuses. This "suppurating bubo" is a hallmark clinical feature of Chancroid. **Analysis of Incorrect Options:** * **Chlamydia trachomatis:** Serotypes L1, L2, and L3 cause Lymphogranuloma Venereum (LGV). While LGV presents with buboes (often showing the "Groove sign"), the primary lesion is usually painless and transient, unlike the painful ulcer of *H. ducreyi*. * **Gonococcus (*Neisseria gonorrhoeae*):** Primarily causes urethritis, cervicitis, and pelvic inflammatory disease. While it can cause local inflammation, it does not typically present with suppurating inguinal buboes. * **Treponema pallidum:** Causes Syphilis. The primary chancre is characteristically **painless**, and the associated inguinal lymphadenopathy is typically bilateral, firm, non-tender, and **non-suppurative** ("shotty" nodes). **High-Yield Clinical Pearls for NEET-PG:** * **Chancroid Rule of "P":** **P**ainful ulcer, **P**urulent base, **P**ainful inguinal lymphadenopathy (**P**us-forming buboes). * **School of Fish Appearance:** The characteristic microscopic arrangement of *H. ducreyi* on Gram stain (also described as "railroad track" appearance). * **Culture Medium:** Requires enriched media like **GC agar** with bovine hemoglobin and fetal calf serum. * **Treatment:** Azithromycin (1g single dose) or Ceftriaxone.

Q: A person ate some milk products at a party and after 6 hours started vomiting. Which organism is most likely the cause?

Staphylococcus. This question tests your knowledge of **bacterial food poisoning** and the significance of **incubation periods**. ### **Why Staphylococcus aureus is the correct answer:** *Staphylococcus aureus* food poisoning is caused by the ingestion of **pre-formed enterotoxins** (Type A-E) in contaminated food, particularly milk products, creamy pastries, or salty meats. * **Rapid Onset:** Because the toxin is already present in the food, the incubation period is very short, typically **1 to 6 hours**. * **Clinical Presentation:** The hallmark is **projectile vomiting** and abdominal cramps. Fever is usually absent because it is an intoxication, not an infection. ### **Analysis of Incorrect Options:** * **Bacillus cereus:** While it also has an emetic form with a short incubation (1–6 hours), it is classically associated with **reheated fried rice**, not milk products. * **Clostridium perfringens:** This causes "diarrheal" food poisoning with a longer incubation period of **8 to 16 hours**. It is typically associated with reheated meat dishes. * **Clostridium botulinum:** This causes botulism, characterized by descending paralysis and cranial nerve palsies. The incubation is usually **12 to 36 hours**, and vomiting is not the primary symptom. ### **NEET-PG High-Yield Pearls:** 1. **Shortest Incubation Period:** *S. aureus* and *B. cereus* (emetic type) are the fastest (1–6 hours). 2. **Heat Stability:** Staphylococcal enterotoxin is **heat-stable** (resists boiling for 30 minutes); thus, cooking food does not prevent the illness if the toxin is already formed. 3. **Source:** The source is often a food handler with a skin lesion (staphylococcal carrier). 4. **Mechanism:** The toxin acts as a **Superantigen**, stimulating the vagus nerve and the vomiting center in the brain.

Q: Which of the toxins produced by Clostridium botulinum is non-neurotoxic?

Toxin C2. ### Explanation **Correct Option: D (Toxin C2)** *Clostridium botulinum* produces eight distinct types of toxins, labeled **A through H**. While most are potent neurotoxins that cause flaccid paralysis by inhibiting acetylcholine release, **Toxin C2** is a significant exception. **Why Toxin C2 is the correct answer:** Unlike the neurotropic toxins (A-G), **Toxin C2 is an enterotoxin and a cytotoxin**. It possesses ADP-ribosylating activity that targets cellular actin, leading to increased vascular permeability and tissue damage rather than neurological interference. It does not act on the neuromuscular junction. **Analysis of Incorrect Options:** * **Options A & B (Toxins A and B):** These are the most common causes of human botulism. They are highly potent neurotoxins that proteolytically cleave SNARE proteins (like SNAP-25 or Synaptobrevin), preventing the fusion of acetylcholine vesicles at the synapse. * **Option C (Toxin C1):** While produced by the same strains that produce C2, **C1 is a neurotoxin**. It specifically targets the syntaxin protein at the nerve terminal. --- ### High-Yield NEET-PG Pearls: * **Mechanism of Action:** Botulinum toxins are zinc-dependent endopeptidases that cleave **SNARE proteins**, leading to **flaccid paralysis**. (Contrast with Tetanospasmin, which causes spastic paralysis by inhibiting GABA/Glycine). * **Therapeutic Uses:** Toxin A (Botox) is used for achalasia cardia, strabismus, blepharospasm, and cosmetic wrinkle reduction. * **Infant Botulism:** Associated with **honey** consumption (ingestion of spores), leading to "Floppy Baby Syndrome." * **Food-borne Botulism:** Usually due to ingestion of pre-formed toxins in **canned foods**. * **Detection:** The gold standard for diagnosis is the **Mouse Bioassay** (Mouse Neutralization Test).

Q: What is the most frequent causative agent for acute epiglottitis in children under 5 years of age?

Haemophilus influenzae type B. **Explanation:** **Acute Epiglottitis** is a life-threatening medical emergency characterized by rapid inflammation and edema of the epiglottis and supraglottic structures. **1. Why Option A is Correct:** Historically and clinically, **_Haemophilus influenzae_ type B (Hib)** is the most frequent causative agent in children. Although the incidence has significantly decreased in regions with high Hib vaccination coverage, it remains the leading bacterial cause globally. The pathogenesis involves bacterial invasion of the epiglottic mucosa, leading to the classic "cherry-red" swollen epiglottis that can cause sudden airway obstruction. **2. Why Other Options are Incorrect:** * **Option B (Influenza virus type A):** While it can cause respiratory distress, it is not a primary cause of epiglottitis; it more commonly leads to viral pneumonia or tracheitis. * **Option C (Parainfluenza virus type 1):** This is the most common cause of **Croup (Laryngotracheobronchitis)**, not epiglottitis. Croup presents with a barking cough and subglottic narrowing (Steeple sign). * **Option D (Staphylococcus aureus):** This is a common cause of **Bacterial Tracheitis**. While it can occasionally cause epiglottitis in older children or adults, it is less frequent than Hib in the under-5 age group. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad (The 3 D’s):** Drooling, Dysphagia, and Distress (Respiratory). * **Positioning:** Children often assume the **"Tripod position"** (leaning forward on hands) to maintain the airway. * **X-ray Finding:** Lateral neck X-ray shows the **"Thumbprint sign"** (swollen epiglottis). * **Management Gold Standard:** Secure the airway (intubation) in a controlled environment (OR/ICU). **Never** use a tongue depressor for examination in a suspected case, as it may trigger fatal laryngospasm.

Q: Pseudomonas is resistant to which of the following antimicrobial agents?

Vancomycin. ### Explanation **1. Why Vancomycin is the Correct Answer:** *Pseudomonas aeruginosa* is a Gram-negative rod. **Vancomycin** is a glycopeptide antibiotic that acts by inhibiting cell wall synthesis (binding to the D-Ala-D-Ala terminus of nascent peptidoglycan). Due to its **large molecular size**, Vancomycin cannot penetrate the outer membrane of Gram-negative bacteria to reach its target. Therefore, *Pseudomonas* (and almost all Gram-negative organisms) are **intrinsically resistant** to Vancomycin. **2. Why the Other Options are Incorrect:** * **B. Aztreonam:** This is a Monobactam. It is unique because it has **no activity against Gram-positives or anaerobes** but is highly effective against aerobic Gram-negative bacteria, including *Pseudomonas*. * **C. Ciprofloxacin:** This is a second-generation Fluoroquinolone. It is one of the few **orally available** drugs with potent activity against *Pseudomonas*. * **D. Polymyxin B:** These are "detergent-like" antibiotics that disrupt the outer membrane. They are often used as **last-resort drugs** for multi-drug resistant (MDR) *Pseudomonas* infections. **3. High-Yield Clinical Pearls for NEET-PG:** * **Anti-Pseudomonal Penicillins:** Piperacillin and Ticarcillin. * **Anti-Pseudomonal Cephalosporins:** Ceftazidime (3rd gen) and Cefepime (4th gen). * **Carbapenems:** Imipenem and Meropenem are effective, but **Ertapenem** has NO activity against *Pseudomonas* (a common "catch" in exams). * **Aminoglycosides:** Amikacin and Tobramycin are frequently used in combination therapy. * **Intrinsic Resistance:** Remember that *Pseudomonas* is naturally resistant to many drugs due to its efflux pumps and low-permeability outer membrane.

Question 1

An increased frequency of infections due to which of the following organisms has been reported among patients with achlorhydria?

Accepted Answer

Salmonella

Answer

Shigella

Answer

Entamoeba

Answer

Vibrio cholera

Question 2

True about Campylobacter?

Accepted Answer

Polar flagella

Answer

Grows at 25°C

Answer

Strict aerobe

Answer

Psychrophilic

Question 3

Bile solubility is a characteristic feature of which bacterium?

Accepted Answer

Streptococcus pneumoniae

Answer

Staphylococcus aureus

Answer

Streptococcus agalactactiae

Answer

Streptococcus pyogenes

Question 4

Suppurating buboes are seen in infection with:

Accepted Answer

Haemophilus ducreyi

Answer

Chlamydia trachomatis

Answer

Gonococcus

Answer

Treponema

Question 5

A person ate some milk products at a party and after 6 hours started vomiting. Which organism is most likely the cause?

Accepted Answer

Staphylococcus

Answer

Bacillus cereus

Answer

Clostridium perfringens

Answer

Clostridium botulinum

Question 6

A 37-year-old man is admitted to the hospital with shortness of breath, cyanosis, and fever. Chest X-ray films reveal consolidation of the right lower lobe with relative sparing of the remaining lobes. A clinical diagnosis of lobar pneumonia is made and supported by the results of sputum cultures. Which of the following is the genus of the bacterium most likely to be isolated from this patient's sputum?

Accepted Answer

Streptococcus, alpha-hemolytic

Answer

Haemophilus

Answer

Klebsiella

Answer

Streptococcus, beta-hemolytic

Question 7

Which of the toxins produced by Clostridium botulinum is non-neurotoxic?

Accepted Answer

Toxin C2

Answer

Toxin A

Answer

Toxin B

Answer

Toxin C1

Question 8

What is the most frequent causative agent for acute epiglottitis in children under 5 years of age?

Accepted Answer

Haemophilus influenzae type B

Answer

Influenza virus type A

Answer

Parainfluenza virus type 1

Answer

Staphylococcus aureus

Question 9

Pseudomonas is resistant to which of the following antimicrobial agents?

Accepted Answer

Vancomycin

Answer

Aztreonam

Answer

Ciprofloxacin

Answer

Polymyxin B

Question 10

A child presented with otitis media. The suspected organism is Hemophilus influenzae. All are true about H. influenzae, EXCEPT:

Accepted Answer

Outer membrane protein plays an important role in pathogenesis in type 'b' H. influenzae

Answer

Requires Factors X and V for growth

Answer

In CNS, mostly causes meningitis

Answer

Not a common cause for meningitis in less than 2 months of life

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?