Bacteriology Practice Questions

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.

Q: Which of the following mycobacteria are non-pathogenic to humans?

M. phlei. **Explanation:** The genus *Mycobacterium* is broadly classified into the *M. tuberculosis* complex, *M. leprae*, and Non-Tuberculous Mycobacteria (NTM). NTM are further categorized by the **Runyon Classification** based on growth rate and pigment production. **M. phlei** (Option D) is a **saprophytic**, rapidly growing mycobacterium (Runyon Group IV). It is commonly found in soil and dust and is considered **non-pathogenic to humans**. It is frequently used in laboratory settings for biochemical testing (e.g., as a control in the urease test) because it does not cause disease. **Analysis of Incorrect Options:** * **M. bovis (Option A):** A member of the *M. tuberculosis* complex. It causes bovine tuberculosis but is highly pathogenic to humans, typically transmitted via unpasteurized milk, leading to extrapulmonary TB. * **M. kansasii (Option B):** A photochromogen (Runyon Group I). It is a significant human pathogen that causes a chronic pulmonary disease clinically indistinguishable from tuberculosis. * **M. avium-intracellulare (Option C):** Also known as MAC (Runyon Group III). It is a major opportunistic pathogen, especially in HIV/AIDS patients with low CD4 counts, causing disseminated infection. **High-Yield Clinical Pearls for NEET-PG:** * **Runyon Group IV (Rapid Growers):** Includes *M. phlei* (saprophyte), *M. smegmatis* (commensal), and *M. fortuitum/chelonae* (potential pathogens causing skin/soft tissue abscesses). * **M. smegmatis:** Found in smegma; must be differentiated from *M. tuberculosis* in urine samples. * **Buruli Ulcer:** Caused by *M. ulcerans* (produces mycolactone toxin). * **Swimming Pool Granuloma:** Caused by *M. marinum*.

Q: Which one of the following enteric organisms is anaerogenic and non-motile?

Shigella sonnei. ### Explanation The correct answer is **Shigella sonnei**. In microbiology, **anaerogenic** refers to an organism that does not produce gas (CO₂ and H₂) during the fermentation of carbohydrates. **Non-motile** refers to the absence of flagella. **Why Shigella sonnei is correct:** All species of the genus *Shigella* are characteristically **non-motile** and **anaerogenic** (with the rare exception of certain *S. flexneri* serotypes). They are Gram-negative, non-lactose fermenting (NLF) bacilli (though *S. sonnei* is a late-lactose fermenter). Their lack of motility is a key diagnostic feature used to differentiate them from other Enterobacteriaceae. **Analysis of Incorrect Options:** * **Salmonella typhi:** While it is anaerogenic (it produces acid but no gas from glucose), it is **motile** via peritrichous flagella. * **Proteus mirabilis:** This organism is highly **motile** (exhibiting characteristic "swarming" on agar) and is **aerogenic** (produces gas during fermentation). * **Klebsiella pneumoniae:** While it is **non-motile** (due to a thick polysaccharide capsule), it is characteristically **aerogenic**, producing abundant gas during the fermentation of lactose and other sugars. **NEET-PG High-Yield Pearls:** * **The "Non-Motile" Rule:** Remember the mnemonic **"K-S-Y"** for non-motile Enterobacteriaceae: *Klebsiella*, *Shigella*, and *Yersinia* (at 37°C; *Yersinia* is motile at 25°C). * **Gas Production:** *Shigella* and *S. typhi* are the two major enteric pathogens that do **not** produce gas from glucose. * **S. sonnei specific:** It is the only *Shigella* species that is a **late-lactose fermenter** (forming pale pink colonies on MacConkey agar after 48 hours) and is Indole negative.

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

It may cause peritonitis.. **Explanation:** **Bacteroides** (specifically *Bacteroides fragilis*) are the most common anaerobic bacteria found in the human colon. 1. **Why Option C is Correct:** *Bacteroides* species are commensals of the gastrointestinal tract. However, when the mucosal barrier is breached—due to trauma, surgery, or a ruptured appendix—they escape into the sterile peritoneal cavity. Their **capsular polysaccharide** is a key virulence factor that induces abscess formation, leading to **secondary peritonitis**. 2. **Why Other Options are Incorrect:** * **Option A:** *Bacteroides* are **Gram-negative bacilli**. They are often pleomorphic and pale-staining. * **Option B:** They are **obligate (strict) anaerobes**. They lack catalase and superoxide dismutase, making them unable to survive in high-oxygen environments. * **Option D:** Since *Bacteroides* are part of the **normal resident flora** of the colon (outnumbering *E. coli* 1000:1), their presence in a stool culture is a normal finding and does not indicate infection or a need for treatment. **High-Yield Clinical Pearls for NEET-PG:** * **Bile Resistance:** *B. fragilis* is unique among anaerobes because it can grow in **20% bile**, a feature used for its laboratory identification (Bile Esculin Agar). * **Antibiotic Profile:** They are characteristically resistant to penicillin due to **beta-lactamase production**. The drug of choice is typically **Metronidazole**. * **LPS Activity:** Unlike most Gram-negative bacteria, *Bacteroides* LPS has low toxicity and does not typically cause classic endotoxic shock.

Q: A 44-year-old man develops intra-abdominal sepsis after undergoing difficult bowel resection and anastomosis. He is initially given ceftizoxime sodium, which is ineffective because of overgrowth of which of the following?

Pseudomonas. **Explanation:** The correct answer is **Pseudomonas (Option A)**. **Why Pseudomonas is correct:** Ceftizoxime is a **third-generation cephalosporin**. While third-generation cephalosporins have expanded Gram-negative coverage compared to earlier generations, they are not all created equal. Ceftizoxime, specifically, has excellent activity against many Enterobacteriaceae and anaerobes (like *Bacteroides*), but it lacks significant activity against **Pseudomonas aeruginosa**. In a clinical setting of intra-abdominal sepsis, if a patient is treated with a drug that covers most flora but misses *Pseudomonas*, that organism can undergo "selective overgrowth," leading to treatment failure. **Why the other options are incorrect:** * **Staphylococcus aureus (B):** While third-generation cephalosporins have less Gram-positive activity than first-generation ones, they still possess some efficacy. However, the specific "overgrowth" scenario in post-surgical bowel sepsis typically points toward a resistant Gram-negative rod. * **Neisseria gonorrhoeae (C):** This is a primary pathogen of the urogenital tract, not a common cause of post-surgical intra-abdominal sepsis. * **Bacteroides fragilis (D):** Ceftizoxime is unique among many third-generation cephalosporins because it actually has **good anaerobic activity**, including against *Bacteroides*. Therefore, *Bacteroides* would likely be covered, not overgrow. **NEET-PG High-Yield Pearls:** * **Anti-Pseudomonal Cephalosporins:** Only **Ceftazidime** (3rd gen) and **Cefepime** (4th gen) have reliable activity against *Pseudomonas*. * **Ceftizoxime vs. Cefotaxime/Ceftriaxone:** Ceftizoxime is often noted in exams for its better anaerobic profile compared to its peers. * **Intra-abdominal Sepsis:** Usually polymicrobial (Gram-negative rods + Anaerobes). If a drug covers anaerobes but lacks anti-pseudomonal activity, *Pseudomonas* is the most likely culprit for persistent infection.

Question 1

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 2

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 3

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 4

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

Question 5

Which of the following mycobacteria are non-pathogenic to humans?

Accepted Answer

M. phlei

Answer

M. bovis

Answer

M. kansii

Answer

M. avium intracellular

Question 6

Regarding Proteus, which statement is true?

Accepted Answer

PPA test positive

Answer

Musty odor

Answer

Most common infection caused is respiratory tract infection

Answer

Esthiomene is a complication in females

Question 7

Which one of the following enteric organisms is anaerogenic and non-motile?

Accepted Answer

Shigella sonnei

Answer

Salmonella typhi

Answer

Proteus mirabilis

Answer

Klebsiella pneumonia

Question 8

Which of the following statements is true about Bacteroides?

Accepted Answer

It may cause peritonitis.

Answer

It is gram-positive bacilli.

Answer

It is strictly aerobic.

Answer

Presence in stool culture indicates a need for treatment.

Question 9

A 44-year-old man develops intra-abdominal sepsis after undergoing difficult bowel resection and anastomosis. He is initially given ceftizoxime sodium, which is ineffective because of overgrowth of which of the following?

Accepted Answer

Pseudomonas

Answer

Staphylococcus aureus

Answer

Neisseria gonorrhoeae

Answer

Bacteroides fragilis

Question 10

Which of the following is NOT true for Bordetella pertussis?

Accepted Answer

Infection can be prevented by an acellular vaccine.

Answer

It is a strict human pathogen.

Answer

It can be cultured from the patient during the catarrhal stage.

Answer

It leads to invasion of the respiratory mucosa.

Bacteriology — MCQs

Bacteriology — MCQs

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