Bacteriology Practice Questions

Q: Which of the following statements about Gram-positive cocci is false?

Most Enterococci are resistant to penicillin. ### Explanation The correct answer is **C**. While many clinicians associate Enterococci with high levels of antibiotic resistance, the statement is technically false in a microbiological context because **most *Enterococcus faecalis* (the most common species) remain susceptible to Penicillin/Ampicillin.** Resistance is more frequently seen in *Enterococcus faecium*. Enterococci are "intrinsically resistant" to cephalosporins and aminoglycosides (monotherapy), but penicillin remains a backbone of treatment unless high-level resistance is specifically detected. **Analysis of Other Options:** * **Option A:** *Staphylococcus saprophyticus* is a common cause of "honeymoon cystitis" (UTIs) in young, sexually active females. It is characteristically **Novobiocin resistant**. * **Option B:** Micrococci are often confused with Staphylococci. A key biochemical differentiator is that **Micrococci are oxidase-positive**, while Staphylococci are oxidase-negative. * **Option C:** *Streptococcus pneumoniae* (Pneumococcus) is a classic **encapsulated** organism. Its polysaccharide capsule is the primary virulence factor and the basis for the Quellung reaction and current vaccines. **NEET-PG High-Yield Pearls:** * **Enterococci:** Exhibit "Double Zone of Hemolysis" on blood agar and can grow in **6.5% NaCl** and **40% bile**. * **Treatment:** For serious Enterococcal infections (like endocarditis), a combination of Cell Wall Synthesis Inhibitors (Penicillin/Vancomycin) + Aminoglycosides is used for **synergistic bactericidal activity**. * **Catalase Test:** Used to differentiate Staphylococci (+) from Streptococci (-). * **Bile Solubility:** Pneumococci are bile soluble, whereas Viridans streptococci are not.

Q: Whitmore's bacillus is:

B. pseudomallei. **Explanation:** **Burkholderia pseudomallei** (Option C) is the correct answer. Historically known as *Whitmore’s bacillus*, it is the causative agent of **Melioidosis** (also called Whitmore’s disease). It is a Gram-negative, motile, aerobic bacillus commonly found in soil and surface water in Southeast Asia and Northern Australia. On microscopy, it often exhibits a characteristic **"safety-pin" appearance** (bipolar staining) and produces wrinkled, "earthy" smelling colonies on culture. **Analysis of Incorrect Options:** * **H. influenzae (Option A):** Known as *Pfeiffer’s bacillus*. It is a pleomorphic Gram-negative coccobacillus that requires Factors X (hemin) and V (NAD) for growth. * **B. mallei (Option B):** The causative agent of **Glanders**, a disease primarily affecting horses. Unlike *B. pseudomallei*, it is non-motile. * **B. cepaciae (Option D):** An opportunistic pathogen often associated with nosocomial infections and respiratory tract infections in patients with **Cystic Fibrosis**. **High-Yield Clinical Pearls for NEET-PG:** * **Melioidosis Presentation:** Can range from localized abscesses to fulminant septicemia or chronic pulmonary infection mimicking Tuberculosis. * **Radiology:** Pulmonary melioidosis often shows upper lobe cavitation (similar to TB). * **Culture:** Ashdown’s medium (selective medium) is used, where it produces characteristic wrinkled, purple-colored colonies. * **Drug of Choice:** Initial intensive therapy usually involves **Ceftazidime** or Meropenem, followed by long-term maintenance with Cotrimoxazole.

Q: All of the following bacteria show bipolar staining except?

Haemophilus influenzae. **Explanation:** Bipolar staining, often described as a **"safety-pin appearance,"** is a characteristic feature of certain Gram-negative bacteria where the ends of the bacilli stain more intensely than the center. This occurs due to the accumulation of storage granules or specific capsular material at the poles. **Why Haemophilus influenzae is the correct answer:** * **Haemophilus influenzae** is a small, pleomorphic, Gram-negative coccobacillus. While it can appear in various shapes, it **does not** exhibit bipolar staining. It typically stains uniformly with Safranin or basic fuchsin. **Analysis of Incorrect Options (Bacteria that DO show bipolar staining):** * **Yersinia pestis:** The classic example of bipolar staining (best seen with Wayson, Giemsa, or Methylene blue stains). It is the causative agent of Plague. * **Francisella tularensis:** A tiny, pleomorphic coccobacillus that causes Tularemia; it frequently demonstrates bipolarity. * **Pasteurella multocida:** Commonly associated with animal bites (cats/dogs), this organism characteristically shows bipolar staining, especially in tissue samples. **NEET-PG High-Yield Pearls:** 1. **Mnemonic for Bipolar Staining:** Remember **"Y.P.F.B."** (Your Pets Feel Better) or **"Way to Go"** for **W**hitmore’s bacillus (*Burkholderia pseudomallei*), **Y**ersinia, **P**asteurella, and **F**rancisella. 2. **Burkholderia pseudomallei:** Also shows a prominent safety-pin appearance and causes Melioidosis. 3. **Vibrio cholerae:** Occasionally shows bipolar staining, though its "comma shape" is more diagnostic. 4. **Stains used:** While Gram stain can show it, **Wayson stain** or **Leishman/Giemsa stains** are superior for demonstrating the safety-pin morphology.

Q: A patient presents with fever for 3 weeks. On examination, splenomegaly is observed. Ultrasonography reveals a hypoechoic shadow in the spleen near the hilum. Gram-negative bacilli are isolated on blood culture. Which of the following is the most likely causative organism?

Salmonella. **Explanation:** The clinical presentation of prolonged fever (3 weeks), splenomegaly, and a hypoechoic lesion on ultrasonography (suggestive of a **splenic abscess**) in the context of Gram-negative bacilli points strongly toward **Salmonella typhi** (Enteric fever). [1] 1. **Why Salmonella is correct:** Enteric fever is a systemic infection characterized by prolonged fever and hepatosplenomegaly. [1] Salmonella species are **Gram-negative bacilli**. A known, though infrequent, complication of enteric fever is the formation of a splenic abscess. These appear as hypoechoic areas on ultrasound. While *Staphylococcus aureus* is the most common cause of splenic abscesses globally, *Salmonella* is a significant cause in endemic regions like India and in patients with sickle cell disease. 2. **Why other options are incorrect:** * **Cytomegalovirus (CMV) & Toxoplasmosis:** While both can cause fever and splenomegaly (Infectious Mononucleosis-like syndrome), they are **not bacteria**. They would not grow as "Gram-negative bacilli" on standard blood cultures. * **Lymphoma virus (EBV/HTLV):** Lymphoma can cause fever and splenic lesions, but it is a neoplastic process. Viral etiologies would not yield Gram-negative bacilli on culture. **Clinical Pearls for NEET-PG:** * **Splenic Abscess Triad:** Fever, left upper quadrant pain, and tender splenomegaly. * **Enteric Fever:** Look for "Rose spots," bradycardia (Faget’s sign), and leukopenia. [1] * **Investigation of Choice:** Blood culture is most positive in the 1st week; Widal test becomes positive in the 2nd week. * **Imaging:** USG is the initial screening tool, but **CT scan** is the gold standard for diagnosing splenic abscesses.

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.

Question 1

Which of the following statements about Gram-positive cocci is false?

Accepted Answer

Most Enterococci are resistant to penicillin

Answer

Staphylococcus saprophyticus causes urinary tract infections in sexually active females

Answer

Micrococci are oxidase-positive

Answer

Pneumococci are encapsulated

Question 2

Whitmore's bacillus is:

Accepted Answer

B. pseudomallei

Answer

H. influenzae

Answer

B. mallei

Answer

B. cepaciae

Question 3

All of the following bacteria show bipolar staining except?

Accepted Answer

Haemophilus influenzae

Answer

Yersinia pestis

Answer

Francisella tularensis

Answer

Pasteurella multocida

Question 4

A patient presents with fever for 3 weeks. On examination, splenomegaly is observed. Ultrasonography reveals a hypoechoic shadow in the spleen near the hilum. Gram-negative bacilli are isolated on blood culture. Which of the following is the most likely causative organism?

Accepted Answer

Salmonella

Answer

Cytomegalovirus

Answer

Toxoplasmosis

Answer

Lymphoma virus

Question 5

Which of the following bacteria does NOT produce toxin?

Accepted Answer

C. hofmanni

Answer

C. diphtheriae

Answer

C. ulcerans

Answer

C. pseudotuberculosis

Question 6

Acute osteomyelitis is most commonly caused by which microorganism?

Accepted Answer

Staphylococcus aureus

Answer

Actinomyces bovis

Answer

Nocardia asteroids

Answer

Borrelia vincentii

Question 7

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 8

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 9

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 10

Griffith typing is performed for which bacterium?

Accepted Answer

Streptococcus

Answer

Staphylococcus

Answer

Meningococcus

Answer

Gonococcus

Bacteriology — MCQs

Bacteriology — MCQs

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