Bacteriology Practice Questions

Q: Which selective medium is used for Vibrio species?

TCBS (Thiosulfate Citrate Bile Salts agar). **Explanation:** **Vibrio species**, particularly *Vibrio cholerae*, are Gram-negative, comma-shaped bacilli that are highly sensitive to acidic environments but thrive in alkaline conditions. **Why TCBS is the Correct Answer:** **Thiosulfate Citrate Bile Salts (TCBS) agar** is the gold standard selective medium for *Vibrio*. Its high pH (8.6) inhibits most commensal intestinal flora. * **Bile salts and Citrate** inhibit Gram-positive bacteria and Enterobacteriaceae. * **Sucrose** acts as the fermentable carbohydrate. *V. cholerae* ferments sucrose, producing acid that turns the bromothymol blue indicator **yellow** (yellow colonies). Non-sucrose fermenters like *V. parahaemolyticus* produce **green** colonies. **Analysis of Incorrect Options:** * **Stuart transport medium:** A non-nutrient redox-stable medium used for transporting clinical swabs (e.g., *Neisseria gonorrhoeae*), not for selective growth. * **Skirrow’s agar:** A selective medium containing antibiotics (Vancomycin, Polymyxin B, Trimethoprim) used specifically for isolating **Campylobacter** species. * **MYPA:** Used for the isolation of *Bacillus cereus*. (Note: MacConkey agar is a general differential medium for Enterobacteriaceae). **High-Yield Clinical Pearls for NEET-PG:** * **Enrichment Media for Vibrio:** Alkaline Peptone Water (APW) and Monsur’s Taurocholate Tellurite Peptone Water. * **Transport Media for Vibrio:** Venkatraman-Ramakrishnan (VR) medium and Cary-Blair medium. * **String Test:** Used to identify *Vibrio* (positive result: a mucoid string forms when a colony is mixed with 0.5% sodium deoxycholate). * **Halophilic nature:** All *Vibrio* species require NaCl for growth except *V. cholerae* and *V. mimicus*.

Q: Which of the following statements is FALSE regarding Enterobacteriaceae?

They are oxidase positive. The **Enterobacteriaceae** family consists of a large group of Gram-negative bacilli that share specific biochemical characteristics. Understanding these "family rules" is essential for identifying pathogens like *E. coli*, *Salmonella*, and *Klebsiella*. ### Why Option B is the Correct Answer (The False Statement) All members of the Enterobacteriaceae family are **Oxidase negative** (except for the genus *Plesiomonas*). The oxidase test detects the presence of cytochrome c oxidase; Enterobacteriaceae lack this enzyme because they utilize different pigments in their electron transport chain. This is the primary biochemical test used to differentiate Enterobacteriaceae from other Gram-negative rods like *Pseudomonas* and *Vibrio*, which are oxidase positive. ### Analysis of Other Options * **A. Non-fastidious growth:** Most members grow readily on ordinary media like Nutrient Agar and MacConkey Agar. They do not require specialized growth factors. * **C. Catalase positive:** All members of this family possess the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. * **D. Reduce nitrates:** They possess the enzyme nitrate reductase, which reduces nitrates to nitrites. This property is the basis for the **Nitrite test** on urine dipsticks used to diagnose Urinary Tract Infections (UTIs). ### High-Yield Clinical Pearls for NEET-PG * **The "Big Three" Rules:** To be classified as Enterobacteriaceae, an organism must be: 1. Oxidase negative 2. Glucose fermenters (ferment glucose with the production of acid or acid and gas) 3. Nitrate reducers * **MacConkey Agar:** Used to differentiate the family into **Lactose Fermenters** (Pink colonies: *E. coli, Klebsiella*) and **Non-Lactose Fermenters** (Pale colonies: *Salmonella, Shigella, Proteus*). * **Exceptions:** *Plesiomonas shigelloides* is the only member that is oxidase positive.

Q: A child is suffering from watery diarrhea. The isolated organism is showing darting motility. What is the most probable diagnosis?

Vibrio cholerae. ### Explanation The correct answer is **Vibrio cholerae**. **1. Why Vibrio cholerae is correct:** The clinical presentation of **watery diarrhea** (often described as "rice-water stools") combined with the classic laboratory finding of **darting motility** is pathognomonic for *Vibrio cholerae*. This rapid, jerky movement is due to a single polar flagellum. In a clinical setting, this motility can be visualized via hanging drop preparation of the stool sample and is inhibited by adding specific antisera (Pfeiffer’s phenomenon). **2. Why the other options are incorrect:** * **Campylobacter:** While *Campylobacter jejuni* also exhibits darting motility (often described as "corkscrew"), it typically causes **bloody diarrhea** (dysentery) and is a thermophilic organism (grows at 42°C). * **Enterotoxigenic E. coli (ETEC):** This is a common cause of traveler’s diarrhea (watery). However, it does not exhibit the characteristic "darting" motility; its motility is generally described as peritrichous/sluggish. * **Shigella:** This organism is **non-motile**. It typically presents with inflammatory diarrhea containing blood and mucus (bacillary dysentery). **3. High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar is the selective medium where *V. cholerae* produces **yellow colonies** (sucrose fermenter). * **Enrichment Media:** Alkaline Peptone Water (APW) and Monsur’s Taurocholate Tellurite Peptone Water. * **Transport Media:** Venkatraman-Ramakrishnan (VR) medium. * **Biochemical Test:** Oxidase positive (differentiates it from Enterobacteriaceae). * **String Test:** Positive (colonies lose turbidity and form a "string" when mixed with 0.5% sodium deoxycholate).

Q: Which of the following bacteria is catalase positive and novobiocin susceptibility negative?

Staph saprophyticus. ### Explanation The question tests the ability to differentiate between Gram-positive cocci using biochemical markers and antibiotic susceptibility. **1. Why the Correct Answer is Right:** * **Catalase Test:** All members of the genus *Staphylococcus* are **catalase positive**, which distinguishes them from *Streptococcus* (catalase negative). * **Novobiocin Susceptibility:** Among the coagulase-negative staphylococci (CoNS), **_Staphylococcus saprophyticus_** is uniquely **resistant (negative susceptibility)** to Novobiocin. This is a key laboratory diagnostic feature used to identify it as a common cause of urinary tract infections (UTIs) in young, sexually active females. **2. Why the Incorrect Options are Wrong:** * **Option A: *Staph epidermidis*:** While it is catalase positive, it is **Novobiocin sensitive**. It is the most common CoNS and is typically associated with prosthetic valve endocarditis and catheter infections. * **Option B: *Streptococcus pyogenes*:** This is a Group A Streptococcus (GAS). It is **catalase negative** and shows a zone of inhibition with Bacitracin (Bacitracin sensitive). * **Option C: *Streptococcus viridans*:** These are **catalase negative** alpha-hemolytic streptococci. They are differentiated from *S. pneumoniae* by being Optochin resistant. **3. Clinical Pearls for NEET-PG:** * **Mnemonic for Novobiocin:** *"On the **Staph** retreat, there's **NO** **S**aprophyticus **E**ntrance"* (**NO**vobiocin: **S**aprophyticus is **R**esistant; **E**pidermidis is **S**ensitive). * **Clinical Association:** *S. saprophyticus* is the second most common cause of community-acquired UTI in young women (after *E. coli*), often referred to as "Honeymoon Cystitis." * **Coagulase Test:** Both *S. saprophyticus* and *S. epidermidis* are Coagulase-negative (CoNS), whereas *S. aureus* is Coagulase-positive.

Q: Which of the following organisms' culture demonstrates the phenomenon of satellitism?

Haemophilus. ### Explanation **1. Why Haemophilus is Correct:** The phenomenon of **satellitism** is a classic laboratory characteristic of *Haemophilus influenzae*. This organism is fastidious and requires two essential growth factors: **Factor X (Hemin)** and **Factor V (NAD)**. While blood agar contains Factor X, the Factor V is sequestered inside red blood cells (RBCs). When *Staphylococcus aureus* is streaked across a blood agar plate inoculated with *H. influenzae*, the Staphylococci lyse the RBCs (releasing Factor V) and also synthesize additional NAD as a metabolic byproduct. Consequently, *H. influenzae* colonies grow only in the immediate vicinity of the *S. aureus* streak, appearing as "satellites." **2. Why Other Options are Incorrect:** * **Streptococcus:** Most species are non-fastidious or require enriched media (like blood agar) for hemolysis patterns, but they do not rely on other bacteria for specific growth factors via satellitism. * **Klebsiella:** This is a robust member of the Enterobacteriaceae family that grows easily on basic media like MacConkey agar without needing external factor supplementation. * **Proteus:** Known for its characteristic **"swarming motility"** on agar due to its peritrichous flagella, not satellitism. **3. NEET-PG High-Yield Pearls:** * **Culture Media:** *H. influenzae* grows best on **Chocolate Agar** because the heating process inactivates V-factor inhibitors and releases factors from RBCs. * **Factor Requirements:** *H. influenzae* requires both X and V; *H. parainfluenzae* requires only V; *H. ducreyi* (causes chancroid) requires only X. * **The "Satellite Test":** Used specifically to identify *Haemophilus* species in labs where chocolate agar is unavailable.

Q: Which of the following is associated with Pseudomonas aeruginosa?

Causes Shanghai fever. **Explanation:** **Pseudomonas aeruginosa** is a non-fermentative, Gram-negative bacillus known for its versatility and multi-drug resistance. 1. **Why Option B is Correct:** **Shanghai Fever** is a rare clinical syndrome caused by *Pseudomonas aeruginosa* septicemia. It typically presents in children with symptoms mimicking typhoid fever (fever, diarrhea, and prostration) but is often associated with characteristic skin lesions like **ecthyma gangrenosum**. 2. **Why Other Options are Incorrect:** * **Option A:** Heat-stable enterotoxins are characteristic of *Escherichia coli* (ST) or *Staphylococcus aureus*. Pseudomonas primarily produces **Exotoxin A**, which inhibits protein synthesis via ADP-ribosylation of EF-2 (similar to Diphtheria toxin). * **Option C:** *Pseudomonas aeruginosa* is not particularly heat-resistant. It is easily killed by moist heat at **55°C for 1 hour**. This distinguishes it from spore-forming bacteria or thermophilic organisms. * **Option D:** Pseudomonas is famous for producing various pigments, including **Pyocyanin** (blue-green, unique to *P. aeruginosa*), **Pyoverdin** (fluorescent yellow-green), and **Pyorubrin** (red). **High-Yield Clinical Pearls for NEET-PG:** * **Culture Characteristics:** Exhibits a characteristic **fruity/grape-like odor** due to aminoacetophenone production. * **Key Virulence Factor:** Alginate production leads to a mucoid phenotype, commonly seen in **Cystic Fibrosis** patients. * **Common Infections:** Ventilator-associated pneumonia (VAP), Otitis externa (Swimmer’s ear), and puncture wound osteomyelitis (especially through sneakers). * **Treatment:** Requires anti-pseudonal drugs like Piperacillin-Tazobactam, Ceftazidime, or Carbapenems.

Question 1

Which selective medium is used for Vibrio species?

Accepted Answer

TCBS (Thiosulfate Citrate Bile Salts agar)

Answer

Stuart transport medium

Answer

Skirrow's agar

Answer

MYPA (MacConkey Yeast Extract Peptone Agar)

Question 2

Which of the following statements is FALSE regarding Enterobacteriaceae?

Accepted Answer

They are oxidase positive

Answer

Non-fastidious growth

Answer

They are catalase positive

Answer

They reduce nitrates

Question 3

A child is suffering from watery diarrhea. The isolated organism is showing darting motility. What is the most probable diagnosis?

Accepted Answer

Vibrio cholerae

Answer

Campylobacter

Answer

Enterotoxigenic E. coli

Answer

Shigella

Question 4

Which of the following bacteria is catalase positive and novobiocin susceptibility negative?

Accepted Answer

Staph saprophyticus

Answer

Staph epidermidis

Answer

Streptococcus pyogenes

Answer

Streptococcus viridans

Question 5

Which of the following organisms' culture demonstrates the phenomenon of satellitism?

Accepted Answer

Haemophilus

Answer

Streptococcus

Answer

Klebsiella

Answer

Proteus

Question 6

Which of the following is associated with Pseudomonas aeruginosa?

Accepted Answer

Causes Shanghai fever

Answer

Produces heat-stable enterotoxin

Answer

Cannot be destroyed at 55 degrees Celsius for 1 hour

Answer

Does not produce any pigment

Question 7

Lung abscess is common following which type of pneumonia?

Accepted Answer

Klebsiella

Answer

Streptococcal

Answer

Staphylococcal

Answer

Viral

Question 8

Lactobacilli are numerous in carious lesions because they:

Accepted Answer

a) can produce insoluble extracellular polysaccharides

Answer

b) can attach to smooth enamel surfaces

Answer

c) are more violently cariogenic for animals than Streptococcus mutans

Answer

d) are secondary invaders

Question 9

Which of the following organisms has been implicated in the pathogenesis of atherosclerosis?

Accepted Answer

Chlamydia

Answer

Pseudomonas

Answer

Streptococcus

Answer

Yersinia

Question 10

Which statement about Streptococcus is FALSE?

Accepted Answer

M protein is responsible for the production of mucoid colonies.

Answer

M protein is the major surface protein of group A streptococci.

Answer

Mucoid colonies are virulent.

Answer

Endotoxin causes the rash of scarlet fever.

Bacteriology — MCQs

Bacteriology — MCQs

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