Bacteriology Practice Questions

Q: Which of the following is the most common cause of acute pyelonephritis in patients without urological abnormalities or urinary calculi?

Escherichia coli. **Explanation:** **1. Why Escherichia coli is correct:** *Escherichia coli* is the most common cause of both uncomplicated and complicated Urinary Tract Infections (UTIs), including acute pyelonephritis. In patients with normal urinary tract anatomy (no calculi or abnormalities), **Uropathogenic *E. coli* (UPEC)** accounts for approximately **75–90%** of cases. Its dominance is due to specific virulence factors, most notably **P-pili (Pyelonephritis-associated pili)**, which allow the bacteria to adhere to the P-blood group antigen on uroepithelial cells, facilitating ascending infection to the kidneys. **2. Why the other options are incorrect:** * **Proteus mirabilis:** While a common cause of UTIs, it is specifically associated with **struvite (staghorn) calculi** due to its potent urease production, which alkalinizes the urine. * **Pseudomonas aeruginosa:** This is typically an **opportunistic or nosocomial** pathogen. It is more common in patients with chronic catheterization, structural abnormalities, or post-urological procedures. * **Serratia marcescens:** This is a rare cause of UTI, usually seen in **hospitalized or immunocompromised** patients, often associated with instrumentation. **3. Clinical Pearls for NEET-PG:** * **Most common route of infection:** Ascending infection (except *S. aureus*, which often spreads hematogenously). * **Virulence Factor:** P-pili are essential for *E. coli* to cause pyelonephritis; Type 1 fimbriae are more associated with cystitis. * **Saprophyticus:** *Staphylococcus saprophyticus* is the second most common cause of UTIs in young, sexually active females ("honeymoon cystitis"). * **Diagnosis:** Significant bacteriuria is defined as $\geq 10^5$ CFU/mL (Kass criteria).

Q: Which of the following is NOT a motile organism?

Shigella. **Explanation:** The correct answer is **Shigella**. In bacteriology, motility is a key diagnostic feature used to differentiate genera within the same family. **1. Why Shigella is the correct answer:** *Shigella* species are classically **non-motile** because they lack flagella. Within the *Enterobacteriaceae* family, *Shigella* and *Klebsiella* are the two primary genera that are non-motile. While *Shigella* can move within host epithelial cells by inducing actin polymerization (actin tails), it does not possess the structural apparatus for independent swimming motility in culture media. **2. Analysis of Incorrect Options:** * **Treponema:** These are spirochetes that exhibit characteristic **corkscrew motility** using periplasmic flagella (axial filaments) located between the inner and outer membranes. * **Listeria monocytogenes:** This organism shows a unique **"tumbling motility"** at 25°C due to peritrichous flagella, though it is non-motile at 37°C. * **Pseudomonas:** These are highly motile via a **single polar flagellum** (monotrichous), often described as having "darting motility." **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Non-Motile Bacteria:** "**S**ky **K**y **P**e **Y**e" (**S**higella, **K**lebsiella, **P**asteurella, **Y**ersinia pestis). Note: *Yersinia enterocolitica* is motile at 25°C but non-motile at 37°C. * **Swarming Motility:** Characteristically seen in *Proteus* species and *Vibrio parahaemolyticus*. * **Falling Leaf Motility:** Characteristic of the parasite *Giardia lamblia*. * **Stately Motility:** Seen in *Bacillus anthracis* (which is non-motile) vs. other *Bacillus* species. *Correction: B. anthracis is a rare non-motile Gram-positive rod.*

Q: What is the causative organism of late prosthetic valve endocarditis?

Streptococcus viridans. **Explanation:** Prosthetic Valve Endocarditis (PVE) is categorized based on the timing of onset after surgery, which dictates the most likely causative pathogen: 1. **Early PVE ( 12 months post-surgery):** The microbiology of late PVE closely resembles that of community-acquired native valve endocarditis. **Streptococcus viridans** (Option A) is the most common cause, as these organisms enter the bloodstream via the oral cavity or gastrointestinal tract and seed the prosthetic valve. **Analysis of Incorrect Options:** * **Option B (Proteus mirabilis):** Gram-negative bacilli are rare causes of endocarditis, usually associated with IV drug use or specific healthcare exposures, but not a primary cause of late PVE. * **Option C (Beta-hemolytic streptococci):** While these can cause acute endocarditis, they are less common than the Viridans group in the subacute/late prosthetic presentation. * **Option D (Staphylococci):** These are the predominant cause of **Early PVE** (especially *S. epidermidis*). While they can cause late PVE, *S. viridans* is more characteristic of the late-onset period. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause of NVE (Native Valve Endocarditis):** *Streptococcus viridans*. * **Most common cause of IVDU Endocarditis:** *Staphylococcus aureus* (often affecting the Tricuspid valve). * **Culture-negative Endocarditis:** Most commonly due to prior antibiotic therapy or HACEK group organisms. * **Duke’s Criteria:** The gold standard for clinical diagnosis of Infective Endocarditis.

Q: What is attributed to the acid fastness of tubercle bacilli?

Both of the above. **Explanation:** The acid-fastness of *Mycobacterium tuberculosis* is a unique staining characteristic defined by the ability of the organism to resist decolorization by weak mineral acids (like 25% sulfuric acid) after being stained with strong carbol fuchsin. 1. **Presence of Mycolic Acid:** The cell wall of tubercle bacilli is exceptionally rich in lipids, specifically **mycolic acids** (long-chain fatty acids). These lipids create a waxy, hydrophobic barrier that traps the primary dye (carbol fuchsin) within the cell. 2. **Integrity of the Cell Wall:** While mycolic acid is the chemical basis, the physical **structural integrity** of the cell wall is equally vital. If the cell wall is mechanically damaged (e.g., by vigorous grinding or autolysis), the bacterium loses its acid-fast property even if mycolic acid is still chemically present. Therefore, acid-fastness is a combined function of chemical composition and physical structure. **Analysis of Options:** * **Option A:** Correct, but incomplete on its own. * **Option B:** Correct, but incomplete on its own. * **Option C:** **Correct**, as both chemical (mycolic acid) and physical (integrity) factors are required. **NEET-PG High-Yield Pearls:** * **Staining Technique:** The **Ziehl-Neelsen (ZN) stain** is the "hot method," while the **Kinyoun stain** is the "cold method." * **Acid-Fast Degree:** *M. tuberculosis* is strongly acid-fast (uses 25% $H_2SO_4$). In contrast, *M. leprae* is less acid-fast (uses 5% $H_2SO_4$), and *Nocardia* is weakly acid-fast (uses 1% $H_2SO_4$). * **Fluorescence:** **Auramine-Rhodamine** stain is used for rapid screening of sputum smears under a fluorescence microscope (appears bright yellow-orange).

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 of the following is the most common cause of acute pyelonephritis in patients without urological abnormalities or urinary calculi?

Accepted Answer

Escherichia coli

Answer

Pseudomonas aeruginosa

Answer

Proteus mirabilis

Answer

Serratia marcescens

Question 2

Which of the following is NOT a motile organism?

Accepted Answer

Shigella

Answer

Treponema

Answer

Listeria

Answer

Pseudomonas

Question 3

What is the causative organism of late prosthetic valve endocarditis?

Accepted Answer

Streptococcus viridans

Answer

Proteus mirabilis

Answer

Beta hemolytic streptococci

Answer

Staphylococci

Question 4

What is attributed to the acid fastness of tubercle bacilli?

Accepted Answer

Both of the above

Answer

Presence of mycolic acid

Answer

Integrity of the cell wall

Answer

None of the above

Question 5

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 6

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 7

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 8

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 9

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

Accepted Answer

Haemophilus

Answer

Streptococcus

Answer

Klebsiella

Answer

Proteus

Question 10

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

Bacteriology — MCQs

Bacteriology — MCQs

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