Bacteriology Practice Questions

Q: What is true about Staphylococcus aureus?

Grows in 10% NaCl. ### Explanation **Correct Option: C. Grows in 10% NaCl** *Staphylococcus aureus* is a **haloduric** (salt-tolerant) organism. It can survive and grow in high salt concentrations, up to 10% NaCl. This physiological property is utilized in the laboratory for selective isolation. **Mannitol Salt Agar (MSA)**, which contains 7.5% NaCl, is the selective medium of choice; *S. aureus* not only grows on it but also ferments mannitol, turning the phenol red indicator yellow. **Analysis of Incorrect Options:** * **A. Microaerophilic:** This is incorrect. *Staphylococci* are **facultative anaerobes**. They grow best under aerobic conditions but can also grow in the absence of oxygen. Microaerophilic organisms (like *Campylobacter*) require reduced levels of oxygen. * **B. Produces lemon yellow colonies:** This is a common distractor. *S. aureus* typically produces **golden yellow** colonies (hence the name *aureus*, meaning gold) due to the pigment **staphyloxanthin**. Lemon yellow colonies are characteristic of *Micrococcus*. * **D. All are true:** Incorrect, as options A and B are false. **NEET-PG High-Yield Pearls:** * **Coagulase Test:** The definitive test to differentiate *S. aureus* (Positive) from Coagulase-negative Staphylococci (CoNS). * **Catalase Test:** Differentiates *Staphylococci* (Positive) from *Streptococci* (Negative). * **Protein A:** A major virulence factor that binds to the Fc portion of IgG, preventing opsonization and phagocytosis. * **Commonest Cause:** *S. aureus* is the leading cause of surgical site infections (SSI), osteomyelitis, and acute bacterial endocarditis in IV drug users.

Q: A 34-year-old pregnant female presented to the ER with fever, backache, headache, and myalgias. Blood cultures were collected and subjected to examination. Microbiological examination revealed positive motility at 25 degrees Celsius, catalase positivity, esculin positivity, and positive growth in 6.5% NaCl. Gram staining revealed the following characteristics of the organism isolated. Which of the following is the most common route of transmission of this organism?

Ingestion of contaminated food. ***Ingestion of contaminated food*** - The microbiological characteristics (motility at 25°C, **catalase positive**, **esculin positive**, growth in 6.5% NaCl) identify this as **Listeria monocytogenes**, which is primarily transmitted through **contaminated food**. - **Ready-to-eat foods** like deli meats, soft cheeses, and unpasteurized dairy products are common sources, making this especially concerning in **pregnancy** due to risk of **listeriosis**. *Aerosol person-to-person transmission* - **Listeria monocytogenes** is not transmitted through respiratory droplets or aerosols between individuals. - This route is typical for organisms like **Mycobacterium tuberculosis** or **influenza virus**, not for Listeria. *Feco-oral route* - While Listeria can be found in feces, **direct feco-oral transmission** is not the primary route of infection. - This route is more characteristic of **enteric pathogens** like **Salmonella** or **Shigella** species. *Sexually transmitted* - **Listeria monocytogenes** is not considered a **sexually transmitted pathogen**. - **Vertical transmission** from mother to fetus can occur, but this is through **hematogenous spread**, not sexual contact.

Q: A 65-year-old diabetic man presents with a severe productive cough producing thick, bloody sputum with a "currant-jelly" appearance. MacConkey agar reveals pink colonies, and routine laboratory media shows large mucoid colonies. Which of the following organisms is most likely responsible for this patient's pneumonia?

Klebsiella pneumoniae. **Explanation:** The clinical presentation and laboratory findings are classic for **Klebsiella pneumoniae**. **1. Why the Correct Answer is Right:** * **Clinical Presentation:** *Klebsiella* typically causes severe, necrotizing lobar pneumonia in patients with underlying conditions like **diabetes mellitus** or chronic alcoholism. The hallmark sign is **"currant-jelly" sputum**, which results from significant tissue necrosis and inflammation leading to blood-tinged, thick mucus. * **Laboratory Findings:** *Klebsiella* is a **Lactose Fermenter (LF)**, appearing as **pink colonies** on MacConkey agar. It possesses a prominent polysaccharide capsule, which manifests as **large, mucoid colonies** on routine media (e.g., Blood agar or Nutrient agar). **2. Why Incorrect Options are Wrong:** * **Enterobacter cloacae:** While it is a lactose fermenter, it is rarely associated with the specific "currant-jelly" sputum or the high degree of mucoid growth seen in *Klebsiella*. * **Escherichia coli:** Although an LF (pink colonies), it is an uncommon cause of primary lobar pneumonia; it more frequently causes UTIs or neonatal meningitis. * **Pseudomonas aeruginosa:** This is a **Non-Lactose Fermenter (NLF)**, producing pale/colorless colonies on MacConkey agar. It typically produces a characteristic fruity odor and blue-green pigment (pyocyanin). **3. High-Yield Clinical Pearls for NEET-PG:** * **Friedlander’s Bacillus:** The historical name for *K. pneumoniae*. * **String Test:** Positive for hypervirulent strains (a loop can pull a mucoid string >5mm from the colony). * **Imaging:** Often shows a "bulging fissure sign" on chest X-ray due to heavy inflammatory exudate. * **Biochemicals:** It is **Indole negative** (unlike *E. coli*), **Urease positive**, and **non-motile** (distinguishing it from other Enterobacteriaceae).

Q: Which of the following does not cause invasive diarrhea?

Bacillus cereus. **Explanation:** The mechanism of diarrhea is broadly classified into **secretory (non-invasive)** and **inflammatory (invasive)**. **Why Bacillus cereus is the correct answer:** *Bacillus cereus* causes diarrhea through the production of enterotoxins rather than mucosal invasion. It exhibits two distinct clinical syndromes: the **emetic type** (associated with fried rice and a preformed toxin, *cereulide*) and the **diarrheal type** (associated with meat/vegetables and an *in-vivo* produced heat-labile enterotoxin). The diarrheal toxin stimulates the adenylate cyclase-cAMP system, leading to intestinal secretion of water and electrolytes without invading the bowel wall. **Analysis of incorrect options:** * **Salmonella:** Non-typhoidal Salmonella species (like *S. enteritidis*) invade the intestinal mucosa of the distal small bowel and colon, leading to an inflammatory response and potential bacteremia. * **Shigella:** This is the prototype for invasive diarrhea. It invades the M cells of Peyer's patches and spreads laterally between epithelial cells using actin tails, causing mucosal ulceration and bloody stools (dysentery). * **Aeromonas:** Though often associated with water-borne transmission, *Aeromonas hydrophila* can cause an invasive, dysentery-like illness by directly invading the intestinal mucosa and producing cytotoxins. **NEET-PG High-Yield Pearls:** * **Invasive Organisms:** Remember the mnemonic **"SSYCE"**—*Shigella, Salmonella, Yersinia, Campylobacter,* and *EIEC (Enteroinvasive E. coli)*. * **B. cereus Emetic Type:** Short incubation (1–6 hours); heat-stable toxin; mimics *S. aureus* food poisoning. * **B. cereus Diarrheal Type:** Long incubation (8–16 hours); heat-labile toxin; mimics *C. perfringens* food poisoning.

Q: Phage typing is widely used for the intraspecies classification of which of the following bacteria?

Staphylococci. **Explanation:** **Phage typing** is a method used for the epidemiological surveillance and intraspecies classification of bacteria based on their susceptibility to specific bacteriophages. **Why Staphylococci is correct:** *Staphylococcus aureus* is the classic organism for which phage typing is the "gold standard" for strain differentiation. It is primarily used to trace the source of hospital-acquired outbreaks (nosocomial infections). The International Reference Center for Staphylococcal Phage Typing has established a standardized set of 22 phages (divided into four groups) to identify specific patterns or "phage types" of *S. aureus*. **Why other options are incorrect:** * **E. coli:** While phage typing exists for certain strains (like O157:H7), it is not the primary method for intraspecies classification. Serotyping (O and H antigens) and molecular methods (MLST) are more commonly used. * **Klebsiella pneumoniae:** Classification is predominantly done via **Capsular (K) typing** and **Bacteriocin typing**. * **Pseudomonas aeruginosa:** While phage typing is possible, **Pyocin typing** (bacteriocin typing) is the more traditional and widely recognized method for intraspecies differentiation of this organism. **High-Yield Clinical Pearls for NEET-PG:** * **Bacteriocin Typing:** Most commonly used for *Pseudomonas aeruginosa* (Pyocin) and *Shigella sonnei* (Colicin). * **Vi-phage typing:** Specifically used for *Salmonella Typhi* to track typhoid fever outbreaks. * **Modern Trend:** Phage typing is being rapidly replaced by molecular methods like **PFGE (Pulsed-field gel electrophoresis)** and **Whole Genome Sequencing (WGS)**, which offer higher discriminatory power.

Q: What is the most common organism causing osteomyelitis?

Staphylococcus Aureus. **Explanation:** **Staphylococcus aureus** is the most common cause of pyogenic osteomyelitis across almost all age groups and clinical scenarios. Its dominance is due to specific virulence factors, such as **surface adhesins** (MSCRAMMs) that allow it to bind to bone matrix components like collagen, and its ability to form **biofilms**, which protect the bacteria from host immune responses and antibiotics. **Analysis of Options:** * **Option A (Correct):** *S. aureus* is the overall leading cause. Even in specific populations like neonates or patients with Sickle Cell Disease (where *Salmonella* is a high-yield association), *S. aureus* remains a primary pathogen. * **Option B:** *Streptococcus pneumoniae* can cause osteomyelitis but is much rarer, usually occurring secondary to respiratory infections or in immunocompromised states. * **Option C:** *Haemophilus influenzae* was previously a common cause in children under age 5; however, its incidence has plummeted due to the widespread adoption of the **HiB vaccine**. * **Option D:** *Escherichia coli* and other Gram-negative bacilli are common causes of osteomyelitis in elderly patients, IV drug users, or following urinary tract infections, but they do not surpass *S. aureus* in frequency. **High-Yield Clinical Pearls for NEET-PG:** * **Sickle Cell Disease:** *Salmonella* is the most characteristic organism, but *S. aureus* is still statistically the most common. * **IV Drug Users:** High incidence of *Pseudomonas aeruginosa* (often affecting the S-joints: Sternoclavicular and Sacroiliac). * **Puncture wound through footwear:** Highly associated with *Pseudomonas aeruginosa*. * **Neonates:** Group B Streptococcus and *E. coli* are significant alongside *S. aureus*. * **Chronic Osteomyelitis:** Characterized by the **Involucrum** (new bone) and **Sequestrum** (dead bone).

Q: Draughtsman colonies are characteristic of which organism?

Streptococcus pneumoniae. **Explanation:** **Streptococcus pneumoniae** (Pneumococcus) is the correct answer. The characteristic **"Draughtsman" or "Checkerboard" appearance** of its colonies occurs due to a process called **autolysis**. As the culture ages (usually after 24 hours of incubation), the central part of the colony undergoes spontaneous autolysis mediated by the enzyme *L-alanine-muramyl amidase*. This causes the center to collapse or sink, leaving a raised peripheral rim, resembling the pieces used in a game of draughts (checkers). **Analysis of Incorrect Options:** * **Bacillus anthracis:** Produces **"Medusa head" colonies** on blood agar. These are large, greyish, non-hemolytic colonies with wavy outgrowths resembling locks of hair. * **Bordetella pertussis:** Produces **"Mercury drops" or "Bisected pearls"** appearance on Bordet-Gengou medium. The colonies are small, shiny, and dome-shaped. * **Yersinia pestis:** Characterized by **"Stalactite growth"** in liquid media (ghee broth) and **"Hammered copper"** appearance on agar. **High-Yield Clinical Pearls for NEET-PG:** * **Capsule:** The most important virulence factor of *S. pneumoniae* is its polysaccharide capsule (detected by the **Quellung Reaction**). * **Bile Solubility Test:** *S. pneumoniae* is bile soluble (autolysis is enhanced by bile salts), which differentiates it from *Streptococcus viridans* (bile insoluble). * **Optochin Sensitivity:** *S. pneumoniae* is sensitive to Optochin, whereas *S. viridans* is resistant. * **Morphology:** They are Gram-positive, lancet-shaped diplococci.

Q: Enterotoxins are produced by all except?

Streptococcus pyogenes. **Explanation:** The correct answer is **Streptococcus pyogenes** because it primarily produces **Pyrogenic Exotoxins** (SpeA, SpeB, SpeC), which act as superantigens causing Scarlet Fever and Streptococcal Toxic Shock Syndrome, rather than enterotoxins that target the gastrointestinal tract. **Analysis of Options:** * **Clostridium perfringens (Option A):** Produces *C. perfringens enterotoxin (CPE)*, typically associated with Type A strains. It is a common cause of food poisoning, where the toxin is released in the intestine during sporulation, leading to watery diarrhea. * **Staphylococcus aureus (Option B):** Produces heat-stable enterotoxins (A-E). These are pre-formed in contaminated food (like creamy salads or meats). They act as superantigens and trigger the vomiting center via the vagus nerve, causing rapid-onset food poisoning. * **Bacillus cereus (Option D):** Produces two types of enterotoxins: a heat-stable toxin (causing the emetic/vomiting form) and a heat-labile toxin (causing the diarrheal form). **Clinical Pearls for NEET-PG:** 1. **Staph. aureus vs. B. cereus:** If symptoms start within 1–6 hours of eating, suspect pre-formed toxins (Staph or Emetic B. cereus). 2. **Superantigens:** Both Staphylococcal enterotoxins and Streptococcal pyrogenic exotoxins are superantigens, but only the former is classified as an enterotoxin. 3. **C. perfringens:** It is unique because the toxin is produced *in vivo* (inside the gut) during the transition from vegetative cell to spore. 4. **Vibrio cholerae:** Produces the classic "Choleragen" enterotoxin, which increases cAMP, leading to rice-water stools.

Q: Which of the following statements regarding Shigella dysenteriae type I is true?

It can lead to hemolytic uremic syndrome.. **Explanation:** **Shigella dysenteriae type I** (also known as Shiga bacillus) is the most virulent species of the Shigella genus. 1. **Why Option A is Correct:** *S. dysenteriae* type I produces the potent **Shiga toxin (Stx)**. This toxin enters the bloodstream and targets the glomerular endothelial cells. It leads to microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure—the triad defining **Hemolytic Uremic Syndrome (HUS)**. This is a classic high-yield association for NEET-PG. 2. **Why Other Options are Incorrect:** * **Option B:** While Shigella is highly invasive, the Shiga toxin is an **exotoxin** (specifically a cytotoxin), not an "invasive enterotoxin." The invasion of the colonic mucosa is mediated by large virulence plasmids, not the toxin itself. * **Option C:** Shigella species are **facultative anaerobes**, not facultative aerobes. They can grow with or without oxygen but prefer anaerobic conditions in the gut. * **Option D:** Shigella is **Methyl Red (MR) positive**. In biochemical testing, it ferments glucose with acid production, maintaining a low pH that keeps the MR indicator red. **Clinical Pearls for NEET-PG:** * **Low Infectious Dose:** Only 10–100 organisms are required to cause infection (due to resistance to gastric acid), unlike *Vibrio cholerae* or *Salmonella*. * **Non-Motile:** Shigella lacks H-antigen (non-flagellated). * **Culture:** On DCA (Deoxycholate Citrate Agar) or MacConkey agar, it produces **pale, non-lactose fermenting (NLF)** colonies. * **Catalase Test:** *S. dysenteriae* type I is unique as it is **catalase negative**, whereas all other Shigella species are catalase positive.

Q: Exudative plaques in the form of pseudomembranes are due to:

Clostridium difficile. **Explanation:** The correct answer is **Clostridium difficile**. This organism is the primary causative agent of **Pseudomembranous Colitis (PMC)**, typically occurring after broad-spectrum antibiotic use (e.g., Clindamycin, Fluoroquinolones). **Why Clostridium difficile is correct:** The pathogenesis involves the release of two potent exotoxins: **Toxin A (Enterotoxin)** and **Toxin B (Cytotoxin)**. These toxins cause mucosal inflammation, epithelial cell necrosis, and the recruitment of inflammatory cells. This results in the formation of characteristic **"volcano lesions"**—exudative plaques composed of fibrin, mucus, and necrotic inflammatory cells that coalesce to form a yellowish-white **pseudomembrane** over the colonic mucosa. **Why the other options are incorrect:** * **Escherichia coli:** Most strains cause watery diarrhea (ETEC) or hemorrhagic colitis (EHEC) without pseudomembrane formation. * **Salmonella:** Typically causes enteric fever or gastroenteritis characterized by mucosal inflammation and potential ulceration (especially in Peyer’s patches), but not pseudomembranes. * **Campylobacter jejuni:** A common cause of inflammatory diarrhea/dysentery; while it causes mucosal ulceration and bloody stools, it does not produce the classic exudative pseudomembranes seen in PMC. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Oral **Vancomycin** or Fidaxomicin (Metronidazole is now a second-line agent). * **Diagnosis:** Detection of Toxin A and B in stool via ELISA or PCR (GDH assay is used for screening). * **Morphology:** On colonoscopy, look for "yellowish-white raised plaques." * **Risk Factor:** Most common antibiotic associated is Clindamycin, but most frequent causes currently are Cephalosporins and Fluoroquinolones.

Question 1

What is true about Staphylococcus aureus?

Accepted Answer

Grows in 10% NaCl

Answer

Microaerophilic

Answer

Produces lemon yellow colonies

Answer

All are true

Question 2

A 34-year-old pregnant female presented to the ER with fever, backache, headache, and myalgias. Blood cultures were collected and subjected to examination. Microbiological examination revealed positive motility at 25 degrees Celsius, catalase positivity, esculin positivity, and positive growth in 6.5% NaCl. Gram staining revealed the following characteristics of the organism isolated. Which of the following is the most common route of transmission of this organism?

Accepted Answer

Ingestion of contaminated food

Answer

Aerosol person-to-person transmission

Answer

Feco-oral route

Answer

Sexually transmitted

Question 3

A 65-year-old diabetic man presents with a severe productive cough producing thick, bloody sputum with a "currant-jelly" appearance. MacConkey agar reveals pink colonies, and routine laboratory media shows large mucoid colonies. Which of the following organisms is most likely responsible for this patient's pneumonia?

Accepted Answer

Klebsiella pneumoniae

Answer

Enterobacter cloacae

Answer

Escherichia coli

Answer

Pseudomonas aeruginosa

Question 4

Which of the following does not cause invasive diarrhea?

Accepted Answer

Bacillus cereus

Answer

Salmonella

Answer

Aeromonas

Answer

Shigella

Question 5

Phage typing is widely used for the intraspecies classification of which of the following bacteria?

Accepted Answer

Staphylococci

Answer

E. coli

Answer

Klebsiella pneumoniae

Answer

Pseudomonas aeruginosa

Question 6

What is the most common organism causing osteomyelitis?

Accepted Answer

Staphylococcus Aureus

Answer

Streptococcus Pneumoniae

Answer

Haemophilus Influenzae

Answer

Escherichia Coli

Question 7

Draughtsman colonies are characteristic of which organism?

Accepted Answer

Streptococcus pneumoniae

Answer

Bacillus anthracis

Answer

Bordetella pertussis

Answer

Yersinia pestis

Question 8

Enterotoxins are produced by all except?

Accepted Answer

Streptococcus pyogenes

Answer

Clostridium perfringens

Answer

Staphylococcus aureus

Answer

Bacillus cereus

Question 9

Which of the following statements regarding Shigella dysenteriae type I is true?

Accepted Answer

It can lead to hemolytic uremic syndrome.

Answer

It produces an invasive enterotoxin.

Answer

It is a facultative aerobe.

Answer

It is MR negative.

Question 10

Exudative plaques in the form of pseudomembranes are due to:

Accepted Answer

Clostridium difficile

Answer

Escherichia coli

Answer

Salmonella

Answer

Campylobacter jejuni

Bacteriology — MCQs

Bacteriology — MCQs

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