Bacteriology Practice Questions

Q: A patient presented with vomiting after consuming fried rice. What is the most likely causative organism?

Bacillus cereus. **Explanation:** The clinical presentation of vomiting shortly after consuming **fried rice** is a classic "textbook" scenario for **Bacillus cereus** food poisoning. **Why Bacillus cereus is correct:** * *B. cereus* produces two types of toxins. The **emetic (vomiting) type** is associated with starchy foods like rice. * When rice is cooked and left at room temperature, heat-resistant spores germinate. If the rice is briefly "flash-fried" before serving, the **preformed, heat-stable toxin (cereulide)** is not destroyed. * Because the toxin is preformed, the incubation period is very short (**1–6 hours**), primarily manifesting as nausea and vomiting. **Why the other options are incorrect:** * **Staphylococcus aureus:** Also causes rapid-onset vomiting (1–6 hours) via a preformed enterotoxin, but it is classically associated with **creamy foods** (custards, mayonnaise, potato salad) or processed meats, not fried rice. * **Vibrio cholerae:** Causes "rice-water stools" (profuse watery diarrhea), not primary vomiting. It has a longer incubation period (1–3 days) and is transmitted via contaminated water or seafood. * **Salmonella:** Typically presents with inflammatory diarrhea, fever, and abdominal cramps. The incubation period is longer (12–72 hours) as it requires bacterial multiplication in the gut. **High-Yield Clinical Pearls for NEET-PG:** 1. **B. cereus (Emetic type):** Associated with rice; incubation <6 hours; heat-stable toxin. 2. **B. cereus (Diarrheal type):** Associated with meat/vegetables; incubation 8–16 hours; heat-labile toxin (produced in vivo). 3. **Nagler Reaction:** Used to identify *B. cereus* and *C. perfringens* (detects lecithinase activity). 4. **Culture:** On **MYP (Mannitol Egg Yolk Polymyxin) agar**, *B. cereus* produces large, pinkish colonies with a zone of lecithin hydrolysis.

Q: A 65-year-old male presents with a rapid onset fever, chest pain, and cough with rusty-yellow sputum. A Gram stain of a sputum sample contained many polymorphonuclear leukocytes and extracellular gram-positive diplococci. Biochemical reactions revealed the organism was bile soluble, optochin sensitive, and fermented inulin. Which of the following is the most likely pathogen?

Streptococcus pneumoniae. ### Explanation **1. Why Streptococcus pneumoniae is correct:** The clinical presentation of rapid-onset fever, chest pain, and **rusty-colored sputum** is a classic hallmark of lobar pneumonia caused by *Streptococcus pneumoniae*. The Gram stain findings of **Gram-positive diplococci** (lancet-shaped) and PMNs further support this. The definitive identification relies on three key biochemical markers mentioned in the question: * **Bile solubility:** *S. pneumoniae* possesses autolytic enzymes (amidases) that are activated by bile salts, leading to colony lysis. * **Optochin sensitivity:** It is inhibited by optochin (ethylhydrocupreine hydrochloride), unlike other alpha-hemolytic streptococci. * **Inulin fermentation:** It is the only streptococcus that ferments inulin. **2. Why the other options are incorrect:** * **Enterococcus faecium & E. faecalis:** These are Gram-positive cocci in pairs/short chains but are **bile-esculin positive** and grow in **6.5% NaCl**. They are bile-insoluble and optochin-resistant. Clinically, they usually cause UTIs or endocarditis, not primary lobar pneumonia. * **Streptococcus pyogenes (Group A Strep):** These appear as Gram-positive cocci in **chains** (not diplococci) and are **beta-hemolytic**. They are **Bacitracin sensitive** and PYR positive. They typically cause pharyngitis or skin infections. **3. NEET-PG High-Yield Pearls:** * **Quellung Reaction:** Swelling of the capsule when treated with specific antiserum; the gold standard for serotyping *S. pneumoniae*. * **Virulence Factor:** The **polysaccharide capsule** is the most important virulence factor (anti-phagocytic). * **Culture:** Shows "draughtsman" or "checkerboard" appearance due to central autolysis of older colonies on blood agar. * **Vaccines:** PPV-23 (T-cell independent) and PCV-13 (T-cell dependent/Conjugate).

Q: Which of the following species of Shigella does NOT ferment mannitol?

Shigella dysenteriae. **Explanation:** The classification of *Shigella* species is based on both biochemical properties and antigenic structure (O-antigen). A fundamental biochemical marker used to differentiate these species is the ability to ferment **mannitol**. **1. Why Shigella dysenteriae is the correct answer:** *Shigella dysenteriae* (Serogroup A) is the only species among the four that is **mannitol-non-fermenting**. This is a high-yield diagnostic feature used in laboratory identification to distinguish it from other groups. Additionally, it is the most virulent species, producing the potent Shiga toxin, and is typically catalase-negative (unlike other *Shigella* species). **2. Why the other options are incorrect:** * **Shigella flexneri (Group B), Shigella boydii (Group C), and Shigella sonnei (Group D)** are all **mannitol fermenters**. * **Shigella sonnei** is further distinguished by being a **late lactose fermenter** and being indole negative, whereas *S. flexneri* and *S. boydii* are non-lactose fermenters. **Clinical Pearls for NEET-PG:** * **Mnemonic for Groups:** **D**irty **F**ingers **B**ring **S**higgella (**D**ysenteriae-A, **F**lexneri-B, **B**oydii-C, **S**onnei-D). * **Most common in India:** *Shigella flexneri*. * **Most common in developed countries:** *Shigella sonnei*. * **Most severe disease:** *Shigella dysenteriae* Type 1 (Shiga bacillus), which can lead to Hemolytic Uremic Syndrome (HUS). * **Key Lab Finding:** All *Shigella* are non-motile (lack H-antigen) and do not produce $H_2S$, distinguishing them from *Salmonella*.

Q: All of the following statements about Vibrio cholera are true, except?

It is oxidase test negative.. ### Explanation **Vibrio cholerae** is a Gram-negative, comma-shaped bacterium responsible for cholera. The correct answer is **Option D** because *Vibrio cholerae* is actually **oxidase positive**, not negative. #### Why Option D is the Correct Answer (The Exception): The **Oxidase Test** is a key biochemical reaction used to differentiate Gram-negative rods. *Vibrio* species are characteristically oxidase positive. This distinguishes them from members of the *Enterobacteriaceae* family (like *E. coli* or *Salmonella*), which are oxidase negative. #### Analysis of Other Options: * **Option A (Alkaline pH):** *Vibrio* is halophilic and highly acid-sensitive but acid-tolerant. It thrives in alkaline environments (pH 8.5–9.2). Transport media like **Venkataraman-Ramakrishnan (VR) medium** and **Alkaline Peptone Water** utilize this high pH to inhibit other commensal flora. * **Option B (Aerobic):** *Vibrio cholerae* is a **facultative anaerobe**, meaning it grows best under aerobic conditions but can survive without oxygen. In laboratory settings, it grows rapidly on standard aerobic media. * **Option C (Flagellated):** It possesses a **single polar flagellum**, which provides it with characteristic "darting motility" (often described as "shooting star" motility) when viewed under dark-ground microscopy. #### NEET-PG High-Yield Pearls: * **Culture Media:** **TCBS** (Thiosulfate Citrate Bile Salts Sucrose) agar is the selective medium of choice; *V. cholerae* produces **yellow colonies** due to sucrose fermentation. * **String Test:** Positive (colonies lose turbidity and form a "string" when mixed with 0.5% sodium deoxycholate). * **Cholera Toxin:** An enterotoxin (AB5 type) that increases **cAMP**, leading to the secretion of water and electrolytes, resulting in "rice-water stools." * **Hanging Drop Preparation:** Used to demonstrate the characteristic darting motility.

Q: Treponema pallidum can be grown in which of the following hosts?

Rabbit. **Explanation:** *Treponema pallidum* subsp. *pallidum*, the causative agent of syphilis, is an **obligate intracellular pathogen** that cannot be cultured on artificial (cell-free) media. To maintain the organism for research or diagnostic purposes (like the TPI test), it must be grown in living systems. **1. Why Rabbit is Correct:** The **Rabbit** is the primary laboratory animal used for the propagation of *T. pallidum*. Specifically, the bacteria are inoculated into the **testes of rabbits** (intratesticular inoculation), where they multiply. This method is used to maintain the **Nichols strain**, which is the standard virulent strain used in laboratory research and serological testing. **2. Why Other Options are Incorrect:** * **Armadillo:** This is the specific animal model used for the cultivation of ***Mycobacterium leprae*** (Leprosy), as armadillos have a lower body temperature suitable for its growth. * **Mice/Rodents:** While mice are used in many immunological studies, they do not develop the characteristic syphilitic lesions required for the effective propagation and study of *T. pallidum* in the same way rabbits do. **High-Yield Clinical Pearls for NEET-PG:** * **Cultivation:** *T. pallidum* is "non-cultivable" on agar. It is maintained by serial passage in rabbit testes. * **Microscopy:** Since it cannot be stained by Gram stain (too thin), **Dark-ground microscopy (DGM)** is the gold standard for visualizing motile spirochetes from primary lesions. * **Silver Staining:** In tissue sections, they can be visualized using **Levaditi** or **Warthin-Starry** silver stains. * **Sensitivity:** The organism is extremely sensitive to heat, drying, and penicillin. Penicillin remains the drug of choice.

Q: Salmonella species are characterized by all of the following EXCEPT:

Citrate synthesis. **Explanation:** The genus *Salmonella* belongs to the family *Enterobacteriaceae*. Understanding its biochemical profile is crucial for differentiating it from other enteric pathogens like *Shigella* and *E. coli*. **1. Why "Citrate synthesis" is the correct (EXCEPT) answer:** While most *Salmonella* species (like *S. Typhimurium*) are **Citrate positive** (meaning they can use citrate as a sole carbon source), they do not "synthesize" citrate. In biochemical testing, we look for **Citrate Utilization**, not synthesis. More importantly, the medically significant **Salmonella Typhi is Citrate negative**. Therefore, claiming citrate synthesis/utilization is a universal characteristic of all *Salmonella* species is incorrect. **2. Analysis of Incorrect Options:** * **Option A (Acid/Gas from Glucose):** Most *Salmonella* are aerogenic (produce acid and gas). *Note: S. Typhi is a notable exception as it is anaerogenic (acid only).* * **Option C (H2S production):** This is a hallmark of *Salmonella*. They produce hydrogen sulfide, visualized as black colonies on SSA (Salmonella-Shigella Agar) or Wilson-Blair medium. * **Option D (Indole negative):** *Salmonella* species do not produce tryptophanase; hence, they are Indole negative. **High-Yield NEET-PG Pearls:** * **IMViC Profile:** For *S. Typhi*, it is **(– + – +)** (Indole –, MR +, VP –, Citrate –). * **H2S Exception:** *S. Paratyphi A* is a rare *Salmonella* that does **not** produce H2S. * **Culture Media:** **Wilson-Blair Bismuth Sulfite Agar** is the most sensitive medium for *S. Typhi* (jet black colonies with a metallic sheen). * **Motility:** They are motile via **peritrichous flagella** (except *S. Gallinarum-Pullorum*).

Q: Which of the following is responsible for crepitations in wounds?

Clostridium. **Explanation:** The presence of **crepitations** (a crackling sensation on palpation) in a wound indicates **subcutaneous emphysema**, which is caused by gas production within the tissues. **1. Why Clostridium is correct:** *Clostridium perfringens* is the primary causative agent of **Gas Gangrene** (Clostridial Myonecrosis). As an obligate anaerobe, it utilizes carbohydrates (fermentation) in devitalized, ischemic tissue to produce gas (hydrogen and carbon dioxide). This gas tracks along muscle planes and subcutaneous tissue, manifesting clinically as crepitus. It is a surgical emergency characterized by rapid tissue destruction, "dishwater" discharge, and systemic toxemia. **2. Why the other options are incorrect:** * **Staphylococcus:** Typically causes localized pyogenic infections (abscesses) characterized by thick, creamy pus. While some strains can cause necrotizing fasciitis, they are not primary gas-producers. * **Mycobacterium:** *M. tuberculosis* causes "cold abscesses" and chronic granulomatous inflammation. These are indolent infections without gas formation. * **Streptococcus:** *Streptococcus pyogenes* is known for spreading infections like cellulitis and erysipelas via enzymes (hyaluronidase). While it can cause "Meleney’s gangrene" or necrotizing fasciitis, pure streptococcal infections are typically non-gas-forming unless a polymicrobial (Type I) infection is present. **Clinical Pearls for NEET-PG:** * **Incubation Period:** Gas gangrene has a very short incubation period (usually <24 hours). * **X-ray Finding:** "Feathering" pattern of gas in muscle fibers. * **Management:** Radical surgical debridement is the gold standard; Hyperbaric Oxygen (HBO) and high-dose Penicillin G are adjuncts. * **Non-clostridial Crepitus:** Can be seen in infections caused by *E. coli*, *Klebsiella*, or *Bacteroides* (often in diabetic foot infections).

Q: Which of the following pneumonias is caused by contaminated air conditioners?

Legionella. **Explanation:** **Legionella pneumophila** is the correct answer because it is an aerobic, Gram-negative rod that thrives in aquatic environments. It colonizes man-made water systems such as **air conditioning cooling towers**, humidifiers, whirlpool spas, and showerheads [1]. Transmission occurs via the **inhalation of contaminated aerosols** (mists) produced by these systems. It does not spread person-to-person . **Analysis of Incorrect Options:** * **A. Pneumococci (*S. pneumoniae*):** The most common cause of community-acquired pneumonia (CAP). It is part of the normal nasopharyngeal flora and spreads via respiratory droplets, not environmental water systems. * **B. Staphylococci (*S. aureus*):** Typically causes post-viral pneumonia (e.g., after Influenza) or hematogenous spread (e.g., IV drug users) [1]. It is associated with skin flora and hospital environments. * **C. E. coli:** An enteric Gram-negative organism. It primarily causes pneumonia in neonates or via aspiration in debilitated/ICU patients; it is not associated with aerosolized water from AC units. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Look for "atypical pneumonia" features—high fever, **diarrhea** (GI symptoms), and **hyponatremia** (low sodium). * **Diagnosis:** The **Urinary Antigen Test** is the rapid test of choice. Culture requires **Buffered Charcoal Yeast Extract (BCYE) agar** supplemented with L-cysteine and iron. * **Treatment:** Macrolides (Azithromycin) or Fluoroquinolones (Levofloxacin). It is inherently resistant to Beta-lactams because it is an intracellular pathogen. * **Pontiac Fever:** A milder, flu-like illness caused by *Legionella* without pneumonia.

Question 1

A very ill neonate has widespread granulomas. In utero infection with which of the following organisms is suggested by this finding?

Accepted Answer

Listeria monocytogenes

Answer

Clostridium botulinum

Answer

Escherichia coli

Answer

Haemophilus influenzae

Question 2

A patient presented with vomiting after consuming fried rice. What is the most likely causative organism?

Accepted Answer

Bacillus cereus

Answer

Staphylococcus aureus

Answer

Vibrio cholerae

Answer

Salmonella

Question 3

A 65-year-old male presents with a rapid onset fever, chest pain, and cough with rusty-yellow sputum. A Gram stain of a sputum sample contained many polymorphonuclear leukocytes and extracellular gram-positive diplococci. Biochemical reactions revealed the organism was bile soluble, optochin sensitive, and fermented inulin. Which of the following is the most likely pathogen?

Accepted Answer

Streptococcus pneumoniae

Answer

Enterococcus faecium

Answer

Streptococcus pyogenes

Answer

Enterococcus faecalis

Question 4

Which of the following is NOT true about cholerae?

Accepted Answer

Antibodies to O1 cholera provide protection against infection.

Answer

Incubation period ranges from 1-5 days.

Answer

Produces isotonic diarrhea.

Answer

Cholera toxin plays the principal role.

Question 5

Which of the following species of Shigella does NOT ferment mannitol?

Accepted Answer

Shigella dysenteriae

Answer

Shigella sonnei

Answer

Shigella flexneri

Answer

Shigella boydii

Question 6

All of the following statements about Vibrio cholera are true, except?

Accepted Answer

It is oxidase test negative.

Answer

It is transported in alkaline pH.

Answer

It is an aerobic bacterium.

Answer

It is flagellated.

Question 7

Treponema pallidum can be grown in which of the following hosts?

Accepted Answer

Rabbit

Answer

Mice

Answer

Armadillo

Answer

Rodent

Question 8

Salmonella species are characterized by all of the following EXCEPT:

Accepted Answer

Citrate synthesis

Answer

Acid and gas production from fermenting glucose

Answer

H2S production

Answer

Indole negative

Question 9

Which of the following is responsible for crepitations in wounds?

Accepted Answer

Clostridium

Answer

Staphylococcus

Answer

Mycobacterium

Answer

Streptococcus

Question 10

Which of the following pneumonias is caused by contaminated air conditioners?

Accepted Answer

Legionella

Answer

Pneumococci

Answer

Staphylococci

Answer

E. coli

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?