Bacteriology Practice Questions

Q: Which of the following bacteria does NOT produce toxin?

C. hofmanni. ### Explanation The genus *Corynebacterium* is broadly classified into **pathogenic** (diphtheria-producing) and **commensal** (diphtheroids) species. The primary virulence factor of pathogenic species is the **Diphtheria Toxin (DT)**, which is encoded by the *tox* gene introduced by a lysogenic bacteriophage (Beta-phage). **Why Option C is Correct:** * **C. hofmanni (C. pseudodiphtheriticum):** This is a common commensal found in the normal flora of the nasopharynx. It is a **non-toxigenic** diphtheroid. It lacks the *tox* gene and does not produce any exotoxin. Clinically, it is usually non-pathogenic but can occasionally cause opportunistic infections like endocarditis in immunocompromised patients. **Why Other Options are Incorrect:** * **A. C. diphtheriae:** The prototype pathogen of this genus. It produces the potent Diphtheria Toxin, which inhibits protein synthesis by ADP-ribosylation of Elongation Factor-2 (EF-2). * **B. C. ulcerans:** This is a zoonotic bacterium that can acquire the *tox* gene via lysogenic conversion. It is known to produce a toxin nearly identical to that of *C. diphtheriae*, causing a diphtheria-like respiratory illness. * **D. C. pseudotuberculosis:** Another zoonotic pathogen (primarily affecting sheep/goats) that can carry the *tox* gene and produce Diphtheria Toxin, leading to granulomatous lymphadenitis in humans. **NEET-PG High-Yield Pearls:** 1. **Elek’s Gel Precipitation Test:** Used to detect the production of Diphtheria Toxin (toxigenicity test). 2. **Mechanism of Toxin:** ADP-ribosylation of **EF-2** is a favorite exam topic (shared by *Pseudomonas* Exotoxin A). 3. **Diphtheroids vs. C. diphtheriae:** Diphtheroids (like *C. hofmanni* and *C. xerosis*) are usually arranged in parallel rows (palisades) rather than the classic "Chinese letter" or cuneiform arrangement seen in *C. diphtheriae*.

Q: Acute osteomyelitis is most commonly caused by which microorganism?

Staphylococcus aureus. **Explanation:** **Staphylococcus aureus (Option A)** is the most common cause of acute osteomyelitis across almost all age groups, including children and adults. The underlying medical concept involves the organism's high affinity for bone tissue due to its surface receptors (adhesins) that bind to bone matrix components like collagen, fibronectin, and laminin. In children, it typically spreads via the **hematogenous route**, seeding the highly vascularized metaphysis of long bones. **Analysis of Incorrect Options:** * **Actinomyces bovis (Option B):** This is an anaerobic, filamentous bacterium primarily responsible for "Lumpy Jaw" in cattle. In humans, *Actinomyces israelii* is the more common species, causing chronic granulomatous lesions with sulfur granules, not acute pyogenic osteomyelitis. * **Nocardia asteroides (Option C):** This is an opportunistic pathogen that typically causes pulmonary or cutaneous infections in immunocompromised patients. While it can disseminate, it is a rare cause of bone infection. * **Borrelia vincentii (Option D):** Also known as *Treponema vincentii*, this spirochete is associated with Vincent’s angina (trench mouth) and necrotizing ulcerative gingivitis, not bone infections. **High-Yield Clinical Pearls for NEET-PG:** * **Overall Most Common:** *Staphylococcus aureus*. * **Sickle Cell Disease:** *Salmonella* is a high-yield association, though *S. aureus* remains frequent. * **IV Drug Users:** Increased incidence of *Pseudomonas aeruginosa* (often affecting the spine or sacroiliac joints). * **Neonates:** Group B Streptococcus and *E. coli* are significant alongside *S. aureus*. * **Most Common Site:** Metaphysis of long bones (due to hairpin bends in vessels causing sluggish blood flow).

Q: Which of the following is a major intestinal pathogen that is a non-Lactose fermenter?

Salmonella. ### Explanation The differentiation of Enterobacteriaceae based on their ability to ferment lactose is a fundamental concept in clinical microbiology. This is typically visualized using **MacConkey agar**, where lactose fermenters (LF) produce pink colonies and non-lactose fermenters (NLF) produce pale/colorless colonies. **1. Why Salmonella is Correct:** *Salmonella* species are classic **Non-Lactose Fermenters (NLF)**. They are major intestinal pathogens responsible for enteric fever (Typhoid) and gastroenteritis. On MacConkey agar, they appear as pale colonies because they lack the enzymes (β-galactosidase and permease) required to break down lactose into acid. **2. Analysis of Incorrect Options:** * **Escherichia (E. coli):** A primary inhabitant of the intestinal tract and a frequent pathogen (UTI, diarrhea). It is a **Rapid Lactose Fermenter**, producing bright pink colonies with a surrounding bile salt precipitation zone. * **Klebsiella:** Another **Rapid Lactose Fermenter**. It is characterized by large, mucoid pink colonies due to its prominent polysaccharide capsule. * **Paracolons:** This is an older taxonomic term for organisms like *Citrobacter* or *Enterobacter* that ferment lactose slowly (**Late Lactose Fermenters**). While they may appear NLF at 24 hours, they eventually produce acid. **Clinical Pearls for NEET-PG:** * **NLF Pathogens:** Remember the mnemonic **"Shy Sallie"** (*Shigella* and *Salmonella*) for the two major intestinal NLF pathogens. * **H2S Production:** *Salmonella* can be further differentiated from *Shigella* on **Salmonella-Shigella (SS) Agar** or **Hektoen Enteric Agar** because *Salmonella* produces $H_2S$ (black-centered colonies), whereas *Shigella* does not. * **Exceptions:** *Shigella sonnei* is a late lactose fermenter, unlike other *Shigella* species.

Q: A child is presenting with watery stools and gram staining shows comma-shaped bacilli. What type of motility will the isolated organism exhibit?

Darting. ### Explanation **Correct Option: B. Darting** The clinical presentation of watery stools combined with the microscopic finding of **Gram-negative, comma-shaped bacilli** is classic for ***Vibrio cholerae***. This organism is highly motile due to a single polar flagellum (monotrichous). Under hanging drop microscopy, it exhibits a characteristic rapid, jerky, and purposeful movement known as **darting motility**. This motility is often described as "shooting star" motility. **Analysis of Incorrect Options:** * **A. Tumbling Motility:** This is the hallmark of ***Listeria monocytogenes***. It is best observed at room temperature (20-25°C) and is described as an end-over-end movement. * **C. Gliding Motility:** This is seen in organisms that lack flagella but can move across surfaces, such as ***Mycoplasma pneumoniae***, *Capnocytophaga*, and certain Cyanobacteria. * **D. Stately Motility:** This refers to the slow, deliberate movement characteristic of ***Clostridium tetani***. **Clinical Pearls for NEET-PG:** * **Vibrio cholerae:** Grows on **TCBS agar** (Thiosulfate-Citrate-Bile Salts-Sucrose), producing yellow colonies due to sucrose fermentation. * **String Test:** Used to identify *Vibrio* species; the organism lyses in 0.5% sodium deoxycholate, releasing DNA that forms a viscous "string" when lifted with a loop. * **Other Motility Patterns to Remember:** * **Swarming motility:** *Proteus* species, *Clostridium tetani*, and *Bacillus subtilis*. * **Corkscrew motility:** *Treponema pallidum*. * **Lashing motility:** *Bacillus anthracis* (though typically non-motile in clinical samples).

Q: A 2-year-old child presents with a history of fever and vomiting. On examination, the child has neck rigidity. Cerebrospinal fluid (CSF) analysis shows 2000 cells/ml and protein of 100 mg/dl. Gram stain reveals the presence of Gram-negative bacilli. Culture shows growth of bacteria only on chocolate agar and not on blood agar. What is the causative agent?

Haemophilus influenzae. ### Explanation The clinical presentation of fever, vomiting, and neck rigidity in a 2-year-old child is diagnostic of **Acute Bacterial Meningitis**. The CSF analysis (pleocytosis and elevated protein) confirms this, and the Gram stain identifying **Gram-negative bacilli (GNB)** is the key diagnostic clue. **Why Haemophilus influenzae is correct:** * **Microbiology:** *H. influenzae* is a fastidious Gram-negative coccobacillus. It requires two specific growth factors: **Factor V (NAD)** and **Factor X (Hematin)**. * **Culture Characteristics:** It does **not** grow on sheep blood agar because the NAD-destroying enzymes (V-factorases) are present in the blood. However, it grows on **Chocolate Agar**, where RBCs have been lysed by heat, releasing both Factors X and V and inactivating the V-factorases. This specific growth pattern is a classic "high-yield" differentiator for *H. influenzae*. **Why other options are incorrect:** * **Neisseria meningitidis:** While it causes meningitis and grows on chocolate agar, it is a **Gram-negative diplococcus**, not a bacillus. * **Bordetella pertussis:** This causes whooping cough, not meningitis. It requires specialized media like **Regan-Lowe** or **Bordet-Gengou** agar. * **Streptococcus pyogenes:** This is a **Gram-positive coccus** in chains and typically causes pharyngitis or skin infections, not meningitis. It shows beta-hemolysis on blood agar. **High-Yield Clinical Pearls for NEET-PG:** * **Satellitism:** *H. influenzae* can grow on blood agar near colonies of *Staphylococcus aureus*, which provide the necessary Factor V (NAD). * **Age Factor:** Before the Hib vaccine, *H. influenzae* type b was the most common cause of pediatric meningitis. * **Drug of Choice:** Third-generation cephalosporins (e.g., Ceftriaxone) are the mainstay of treatment. Dexamethasone is often co-administered to reduce the risk of sensorineural hearing loss.

Q: Griffith typing is performed for which bacterium?

Streptococcus. **Explanation:** **Griffith typing** is a serological method used for the classification of **Streptococcus pyogenes (Group A Streptococcus)**. It is based on the presence of the **M protein**, which is the primary virulence factor of the organism. This typing method uses a slide agglutination technique to categorize strains into different "Griffith types" based on surface protein antigens. * **Why Streptococcus is correct:** While Lancefield grouping (based on C-carbohydrate antigen) classifies Streptococci into groups A-V, Griffith typing provides a more granular sub-typing of Group A Streptococci by targeting the M protein. This is clinically significant because certain M types are associated with specific post-streptococcal sequelae (e.g., M types 1, 3, 12 with Glomerulonephritis). * **Why other options are incorrect:** * **Staphylococcus:** Classified primarily by Coagulase production and Phage typing. * **Meningococcus (N. meningitidis):** Classified into serogroups based on capsular polysaccharides (A, B, C, W-135, Y). * **Gonococcus (N. gonorrhoeae):** Classified via auxotyping or serotyping based on Protein I (Por protein), but Griffith typing is not used. **High-Yield Clinical Pearls for NEET-PG:** * **M Protein:** The most important virulence factor of *S. pyogenes*; it is anti-phagocytic and acts by inhibiting the alternate complement pathway. * **Lancefield vs. Griffith:** Remember: **L**ancefield = **C**-carbohydrate (Group); **G**riffith = **M**-protein (Type). * **Molecular Gold Standard:** Today, Griffith typing has been largely replaced by **emm typing**, which involves sequencing the gene (*emm* gene) that codes for the M protein.

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).

Question 1

Which of the following bacteria does NOT produce toxin?

Accepted Answer

C. hofmanni

Answer

C. diphtheriae

Answer

C. ulcerans

Answer

C. pseudotuberculosis

Question 2

Acute osteomyelitis is most commonly caused by which microorganism?

Accepted Answer

Staphylococcus aureus

Answer

Actinomyces bovis

Answer

Nocardia asteroids

Answer

Borrelia vincentii

Question 3

Which of the following is a major intestinal pathogen that is a non-Lactose fermenter?

Accepted Answer

Salmonella

Answer

Klebsiella

Answer

Escherichia

Answer

Paracolons

Question 4

A child is presenting with watery stools and gram staining shows comma-shaped bacilli. What type of motility will the isolated organism exhibit?

Accepted Answer

Darting

Answer

Tumbling

Answer

Gliding

Answer

Stately

Question 5

A 2-year-old child presents with a history of fever and vomiting. On examination, the child has neck rigidity. Cerebrospinal fluid (CSF) analysis shows 2000 cells/ml and protein of 100 mg/dl. Gram stain reveals the presence of Gram-negative bacilli. Culture shows growth of bacteria only on chocolate agar and not on blood agar. What is the causative agent?

Accepted Answer

Haemophilus influenzae

Answer

Neisseria meningitidis

Answer

Bordetella pertussis

Answer

Streptococcus pyogenes

Question 6

Griffith typing is performed for which bacterium?

Accepted Answer

Streptococcus

Answer

Staphylococcus

Answer

Meningococcus

Answer

Gonococcus

Question 7

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 8

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 9

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 10

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.

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?