Bacteriology Practice Questions

Q: Proteus mirabilis is associated with which type of renal calculi?

Struvite. **Explanation:** The correct answer is **Struvite (Option B)**. **Mechanism:** *Proteus mirabilis* is a Gram-negative bacillus characterized by its production of the enzyme **urease**. This enzyme hydrolyzes urea into ammonia and carbon dioxide ($NH_2CONH_2 + H_2O \rightarrow 2NH_3 + CO_2$). The resulting ammonia increases the urine pH (alkalinization). In this alkaline environment, phosphate, magnesium, and ammonium ions precipitate to form **Struvite stones** (Magnesium Ammonium Phosphate). These stones often grow rapidly to fill the renal pelvis and calyces, forming the characteristic **"Staghorn calculus."** **Analysis of Incorrect Options:** * **A. Uric acid:** These stones form in **acidic urine** (low pH). They are radiolucent and associated with conditions like gout or high purine intake, not bacterial infections. * **C. Cysteine:** These are rare genetic stones caused by a defect in the transport of dibasic amino acids (COAL: Cysteine, Ornithine, Arginine, Lysine). They also favor an acidic environment. * **D. Calcium Oxalate:** This is the **most common** type of renal stone overall. While their formation is multifactorial (hypercalciuria, dehydration), they are not specifically caused by urease-producing bacteria. **High-Yield Clinical Pearls for NEET-PG:** * **Swarming Growth:** *Proteus* exhibits "swarming" motility on blood agar due to its peritrichous flagella. * **Dienes Phenomenon:** Used to distinguish different strains of *Proteus*. * **Culture Media:** MacConkey agar shows Non-Lactose Fermenting (NLF) colonies; Phenylalanine Deaminase (PPA) test is positive. * **Staghorn Calculus:** While *Proteus* is the most common cause, other urease-positive organisms like *Klebsiella*, *Staphylococcus saprophyticus*, and *Ureaplasma* can also lead to struvite stones.

Q: The gene that encodes the toxin for cholera is transmitted by which of the following?

Bacteriophage. **Explanation:** The correct answer is **Bacteriophage**. The pathogenicity of *Vibrio cholerae* is primarily due to the production of the **Cholera Toxin (CT)**. The genes encoding this toxin (*ctxA* and *ctxB*) are not part of the native bacterial chromosome; instead, they are carried by a **lysogenic filamentous bacteriophage** known as **CTXφ (CTX phage)**. Through a process called **lysogenic conversion**, the CTX phage infects non-toxigenic strains of *Vibrio cholerae*, integrating its genetic material into the bacterial genome. This transforms a harmless bacterium into a virulent pathogen capable of causing epidemic cholera. **Why other options are incorrect:** * **Protozoa:** While *V. cholerae* can survive in aquatic environments by associating with zooplankton or copepods, protozoa do not transmit the toxin-encoding genes. * **Bacteria:** While bacteria can exchange genetic material via conjugation (plasmids), the specific cholera toxin gene is classically associated with phage transduction. * **Fungus:** Fungi have no role in the genetic transmission or pathogenesis of *Vibrio cholerae*. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Cholera toxin causes permanent activation of **adenylate cyclase**, leading to increased **cAMP** levels. This results in the hypersecretion of water and electrolytes into the intestinal lumen (Rice-water stools). * **Receptor:** The B-subunit of the toxin binds to the **GM1 ganglioside receptor** on enterocytes. * **Other Phage-Encoded Toxins (Mnemonic: ABCD):** * **A:** Group A *Streptococcus* (Pyrogenic exotoxin/Erythrogenic toxin) * **B:** *Botulinum* toxin (certain types) * **C:** *Cholera* toxin * **D:** *Diphtheria* toxin (via the Beta-phage) * **S:** *Shiga* toxin (in EHEC)

Q: Out of 20 children that went to a party, 11 developed abdominal pain, diarrhea with nausea and vomiting around 6 hours after food intake. Most likely causative agent?

Staphylococcus. **Explanation:** The clinical presentation points toward **Staphylococcal Food Poisoning**, a classic "intoxication" rather than an infection. **Why Staphylococcus is correct:** The hallmark of *Staphylococcus aureus* food poisoning is the **short incubation period (1–6 hours)**. It is caused by the ingestion of pre-formed, heat-stable **Enterotoxin (Type A-E)**. Because the toxin is already present in the food (often creamy items, salads, or processed meats), symptoms like nausea, projectile vomiting, and abdominal cramps appear rapidly. Diarrhea may occur but is usually secondary to vomiting. **Why other options are incorrect:** * **Rotavirus:** This is a common cause of pediatric diarrhea, but it typically has a longer incubation period (1–3 days) and is characterized by watery diarrhea and fever rather than rapid-onset vomiting after a specific meal. * **Streptococcus:** While some strains can cause pharyngitis or skin infections, they are not standard causes of acute food poisoning outbreaks. * **Clostridium perfringens:** This also causes food poisoning, but the incubation period is longer (**8–16 hours**). It is caused by toxin production *in vivo* (inside the gut) and is characterized primarily by watery diarrhea and cramps, with vomiting being rare. **High-Yield Clinical Pearls for NEET-PG:** * **Shortest Incubation (1–6 hrs):** *S. aureus* and *Bacillus cereus* (Emetic type). * **Intermediate Incubation (8–16 hrs):** *Clostridium perfringens* and *Bacillus cereus* (Diarrheal type). * **Long Incubation (>16 hrs):** *Vibrio cholerae*, *Salmonella*, and *Shigella*. * **Key Trigger:** *S. aureus* toxins are **heat-stable** (resist boiling for 30 mins), meaning reheating contaminated food does not prevent the illness.

Q: A person develops vomiting 6 hours after consuming rice pudding in a restaurant. Which of the following statements regarding this food poisoning is true?

Caused by a preformed toxin. ### Explanation The clinical presentation of rapid-onset vomiting (incubation period < 6 hours) after consuming **rice pudding** is a classic description of food poisoning caused by **_Bacillus cereus_ (Emetic type)**. **1. Why Option B is Correct:** The emetic syndrome of *B. cereus* is caused by the ingestion of a **preformed toxin** (cereulide) present in the food. This toxin is produced during the germination of spores when cooked rice is left at room temperature. Because the toxin is already present in the food, the symptoms manifest rapidly (1–6 hours), as the body does not need to wait for bacterial colonization or in-vivo toxin production. **2. Why the Other Options are Incorrect:** * **Option A:** While *Staphylococcus aureus* also causes rapid vomiting via a preformed toxin, it is typically associated with dairy products, processed meats, or creamy salads rather than reheated rice. * **Option C:** *Vibrio parahaemolyticus* is associated with contaminated seafood (shellfish) and typically presents with watery diarrhea and abdominal cramps after a longer incubation period (12–24 hours). * **Option D:** The emetic toxin of *B. cereus* is **heat-stable** (resists 126°C for 90 minutes), allowing it to survive reheating. In contrast, the *diarrheal* type of *B. cereus* is caused by a heat-labile toxin produced in the intestines. **Clinical Pearls for NEET-PG:** * **B. cereus Emetic Type:** Short incubation (1–6 hrs), heat-stable toxin, associated with **reheated rice**. * **B. cereus Diarrheal Type:** Long incubation (8–16 hrs), heat-labile toxin, associated with meat and vegetables. * **Mechanism:** The emetic toxin (Cereulide) acts as a 5-HT3 receptor agonist, stimulating the vagus nerve to induce vomiting. * **Diagnosis:** Primarily clinical; confirmed by isolating organisms from the suspected food.

Q: Which Gram-positive bacilli can cause meningitis?

Listeria. **Explanation:** The core of this question lies in identifying the **morphology** and **Gram-stain characteristics** of common meningitis-causing pathogens. **1. Why Listeria is Correct:** *Listeria monocytogenes* is a **Gram-positive bacillus** (rod). It is a significant cause of meningitis in specific populations: neonates, the elderly, and immunocompromised individuals (e.g., transplant recipients or those with malignancies). It is unique among Gram-positive rods for its "tumbling motility" at 25°C and its ability to grow at refrigeration temperatures (cold enrichment). **2. Analysis of Incorrect Options:** * **A. Pneumococci (*Streptococcus pneumoniae*):** While it is the most common cause of bacterial meningitis in adults, it is a **Gram-positive coccus** (typically arranged in lancet-shaped pairs), not a bacillus. * **C. E. coli:** This is a major cause of neonatal meningitis, but it is a **Gram-negative bacillus**. * **D. Meningococci (*Neisseria meningitidis*):** This is a leading cause of epidemic meningitis, but it is a **Gram-negative diplococcus**. **3. NEET-PG High-Yield Clinical Pearls:** * **Morphology:** *Listeria* are small, coccobacillary Gram-positive rods that can sometimes be mistaken for *S. pneumoniae* (if coccoid) or *Diphtheroids* (contaminants). * **Treatment:** Listeria is inherently resistant to cephalosporins (the standard empiric treatment for meningitis). Therefore, **Ampicillin** must be added to the regimen if *Listeria* is suspected. * **Source:** Often associated with the consumption of unpasteurized dairy products or processed deli meats. * **Key Lab Finding:** Look for "Tumbling motility" on wet mount or a "Christmas tree" pattern in semi-solid agar.

Q: A 28-year-old lady presented with headache and a positive Kernig's sign. Cerebrospinal fluid culture showed Gram-positive bacilli. What is the most probable organism?

Listeria monocytogenes. ### Explanation The clinical presentation of headache and a positive Kernig’s sign indicates **meningitis**. The definitive clue in this question lies in the Gram stain morphology of the organism isolated from the Cerebrospinal Fluid (CSF). **Why Listeria monocytogenes is correct:** * **Morphology:** *Listeria monocytogenes* is a classic **Gram-positive bacillus** (rod). * **Clinical Context:** While more common in neonates and the elderly, it is a significant cause of meningitis in immunocompromised individuals and can occur in healthy adults. It is the only organism among the options that fits the "Gram-positive bacilli" description. **Why the other options are incorrect:** * **Haemophilus influenzae:** This is a **Gram-negative coccobacillus**. * **Neisseria meningitidis:** This is a **Gram-negative diplococcus** (kidney-bean shaped). * **Streptococcus pneumoniae:** This is a **Gram-positive coccus** (typically lancet-shaped diplococci), not a bacillus. **High-Yield Clinical Pearls for NEET-PG:** * **Tumbling Motility:** *Listeria* exhibits characteristic "tumbling motility" at 25°C (but is non-motile at 37°C). * **Cold Enrichment:** It can grow at low temperatures (4°C), a property used for selective isolation. * **Umbrella Motility:** Seen in semi-solid agar (Manitol Motility Medium). * **CAMP Test Positive:** It shows a rectangular (not arrowhead) zone of hemolysis when streaked with *Staphylococcus aureus*. * **Treatment:** The drug of choice for *Listeria* meningitis is **Ampicillin** (as cephalosporins are inherently ineffective against *Listeria*).

Q: A cook prepares sandwiches for 10 people going for a picnic. Eight of them develop severe gastroenteritis within 4-6 hours of consumption. Investigations are likely to reveal that the cook is the carrier of which organism?

Staphylococcus aureus. ### Explanation The correct answer is **Staphylococcus aureus**. **1. Why Staphylococcus aureus is correct:** The key to this question lies in the **short incubation period (1–6 hours)**. *Staphylococcus aureus* food poisoning is caused by the ingestion of **pre-formed enterotoxins** (Type A–E) produced in food contaminated by a carrier (often a food handler with a skin lesion or asymptomatic nasal colonization). Because the toxin is already present in the food, the onset of symptoms (nausea, projectile vomiting, and abdominal cramps) is rapid. Common vehicles include protein-rich or creamy foods like sandwiches, salads (mayonnaise), and pastries. **2. Why the other options are incorrect:** * **Salmonella typhi:** Causes Enteric Fever, which has a much longer incubation period (7–14 days). It presents with high-grade fever and "pea-soup" diarrhea, not rapid-onset vomiting. * **Vibrio cholerae:** Causes "rice-water" stools. The incubation period is typically 1–3 days. It is transmitted via contaminated water or seafood, not usually through sandwiches handled by a carrier. * **Entamoeba histolytica:** Causes amoebic dysentery. The incubation period is long (2–4 weeks), and it presents with bloody mucoid stools and chronic progression. **3. Clinical Pearls for NEET-PG:** * **Shortest Incubation Periods:** *S. aureus* (1–6 hours) and *Bacillus cereus* (emetic type: 1–5 hours). * **Heat Stability:** Staphylococcal enterotoxins are **heat-stable** (resist boiling for 30 minutes); reheating food does not prevent illness. * **Mechanism:** The toxin acts as a **Superantigen**, stimulating the vagus nerve and the vomiting center in the brain. * **Diagnosis:** Primarily clinical; culture of the food or the carrier’s nasal/skin swab can confirm the source.

Q: You are examining a specimen taken from a child with Otitis. On microscopy, you see capsulated lanceolate diplococci, and the culture of the specimen shows a 'draughtsman appearance'. What is the diagnosis?

Pneumococcus. **Explanation:** The clinical presentation and morphological features described are classic for **Streptococcus pneumoniae (Pneumococcus)**. **1. Why Pneumococcus is correct:** * **Morphology:** Under microscopy, *S. pneumoniae* typically appears as Gram-positive, **lanceolate (flame-shaped)** diplococci. It is characteristically **capsulated**, which is its primary virulence factor. * **Culture Characteristics:** On Blood Agar, it produces small, translucent colonies. As the culture ages (after 24–48 hours), the central part of the colony undergoes **autolysis**, leading to a collapsed center with a raised periphery. This specific morphology is known as the **'draughtsman' or 'checkerboard' appearance**. * **Clinical Context:** It is the most common cause of Otitis Media in children. **2. Why other options are incorrect:** * **Meningococcus (*N. meningitidis*):** These are Gram-negative, **lenticular (kidney-bean shaped)** diplococci. They do not show autolysis or draughtsman colonies. * **Gonococcus (*N. gonorrhoeae*):** Also Gram-negative kidney-shaped diplococci; primarily associated with STIs and ophthalmia neonatorum, not typically otitis media. * **Enterococcus:** These appear as oval cocci in pairs or short chains. They are relatively resistant to autolysis and do not exhibit the draughtsman appearance. **High-Yield NEET-PG Pearls:** * **Quellung Reaction:** Swelling of the capsule when treated with specific antiserum (Gold standard for identification). * **Bile Solubility Test:** Pneumococci are **bile soluble**, which differentiates them from *S. viridans* (bile insoluble). * **Optochin Sensitivity:** Pneumococci are sensitive to Optochin, while other alpha-hemolytic streptococci are resistant. * **Hemolysis:** Shows **Alpha-hemolysis** (partial/greenish) on blood agar under aerobic conditions.

Q: Initiation of caries by Streptococcus mutans is by the production of which of the following?

Insoluble dextran and glucosyl transferase. **Explanation:** The initiation of dental caries by *Streptococcus mutans* is a classic example of bacterial biofilm formation. The process relies on the metabolism of dietary sucrose through a specific enzymatic pathway. **Why Option B is Correct:** *Streptococcus mutans* produces an extracellular enzyme called **Glucosyltransferase (GTF)**. This enzyme breaks down sucrose into glucose and fructose. It then polymerizes the glucose units into **Insoluble Dextrans (Glucans)**. * **Role of Insoluble Dextran:** Unlike soluble forms, insoluble dextrans act as a "biological glue." They allow *S. mutans* to adhere tenaciously to the tooth enamel (forming dental plaque) and provide a matrix that traps other bacteria and organic acids. These acids eventually demineralize the enamel, leading to caries. **Why Other Options are Incorrect:** * **Option A:** **Dextranase** is an enzyme that *breaks down* dextran. While some oral bacteria produce it to mobilize energy, it does not initiate caries; it would actually oppose the formation of the sticky plaque matrix. * **Option C:** **Soluble dextrans** are easily washed away by saliva and do not contribute significantly to the stable adherence required for plaque formation. The key to pathogenicity is the *insolubility* of the glucan polymer. **High-Yield Clinical Pearls for NEET-PG:** * **Substrate Specificity:** Sucrose is the only sugar that *S. mutans* can use to synthesize these sticky glucans. This is why sucrose is the most cariogenic sugar. * **Acidogenesis:** *S. mutans* is "acidogenic" (produces lactic acid) and "aciduric" (survives in low pH), both of which are critical for enamel erosion. * **Viridans Group:** *S. mutans* belongs to the Viridans group of Streptococci, which are also the most common cause of **Subacute Bacterial Endocarditis (SBE)** following dental procedures.

Question 1

Cellulitis is caused by which of the following pathogens?

Accepted Answer

Moraxella

Answer

Streptococcus

Answer

Staphylococcus

Answer

Hemophilus influenza

Question 2

Proteus mirabilis is associated with which type of renal calculi?

Accepted Answer

Struvite

Answer

Uric acid

Answer

Cysteine

Answer

Calcium Oxalate

Question 3

The gene that encodes the toxin for cholera is transmitted by which of the following?

Accepted Answer

Bacteriophage

Answer

Protozoa

Answer

Bacteria

Answer

Fungus

Question 4

Out of 20 children that went to a party, 11 developed abdominal pain, diarrhea with nausea and vomiting around 6 hours after food intake. Most likely causative agent?

Accepted Answer

Staphylococcus

Answer

Rotavirus

Answer

Streptococcus

Answer

Clostridium perfringens

Question 5

A person develops vomiting 6 hours after consuming rice pudding in a restaurant. Which of the following statements regarding this food poisoning is true?

Accepted Answer

Caused by a preformed toxin

Answer

Caused by Staphylococcus aureus

Answer

Caused by Vibrio parahaemolyticus

Answer

Caused by a heat-labile toxin

Question 6

Which Gram-positive bacilli can cause meningitis?

Accepted Answer

Listeria

Answer

Pneumococci

Answer

E. coli

Answer

Meningococci

Question 7

A 28-year-old lady presented with headache and a positive Kernig's sign. Cerebrospinal fluid culture showed Gram-positive bacilli. What is the most probable organism?

Accepted Answer

Listeria monocytogenes

Answer

Haemophilus influenzae

Answer

Neisseria meningitidis

Answer

Streptococcus pneumoniae

Question 8

A cook prepares sandwiches for 10 people going for a picnic. Eight of them develop severe gastroenteritis within 4-6 hours of consumption. Investigations are likely to reveal that the cook is the carrier of which organism?

Accepted Answer

Staphylococcus aureus

Answer

Salmonella typhi

Answer

Vibrio cholerae

Answer

Entamoeba histolytica

Question 9

You are examining a specimen taken from a child with Otitis. On microscopy, you see capsulated lanceolate diplococci, and the culture of the specimen shows a 'draughtsman appearance'. What is the diagnosis?

Accepted Answer

Pneumococcus

Answer

Meningococcus

Answer

Gonococcus

Answer

Enterococcus

Question 10

Initiation of caries by Streptococcus mutans is by the production of which of the following?

Accepted Answer

Insoluble dextran and glucosyl transferase

Answer

Dextranase and soluble dextran

Answer

Soluble dextran and glycosyl transferase

Answer

None of the above

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?