Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1241: What is the most common gram-negative organism found in patients with cystic fibrosis?

A. Pseudomonas (Correct Answer)
B. Escherichia coli
C. Klebsiella
D. Legionella

Explanation: **Explanation:** **Pseudomonas aeruginosa** is the most common Gram-negative pathogen associated with chronic pulmonary infections in patients with Cystic Fibrosis (CF). The underlying pathophysiology involves a defect in the CFTR protein, leading to thick, dehydrated mucus. This environment facilitates the colonization of *Pseudomonas*, which undergoes a phenotypic switch to a **mucoid variant** (producing an alginate capsule). This biofilm formation protects the bacteria from antibiotics and host immune responses, leading to progressive bronchiectasis and respiratory failure. **Analysis of Incorrect Options:** * **B. Escherichia coli:** While a common cause of UTIs and neonatal sepsis, it is rarely a primary respiratory pathogen in CF. * **C. Klebsiella:** Though it causes "currant jelly sputum" pneumonia in alcoholics or diabetics, it is not the predominant organism in the CF population. * **D. Legionella:** This causes atypical pneumonia (Legionnaires' disease) via aerosolization from water sources (AC ducts) but does not have a specific predilection for the CF lung. **High-Yield Clinical Pearls for NEET-PG:** * **Age-related colonization:** In early childhood CF, *Staphylococcus aureus* and *Haemophilus influenzae* are most common. By adolescence and adulthood, *Pseudomonas aeruginosa* becomes the dominant pathogen. * **Burkholderia cepacia complex:** Though less common than *Pseudomonas*, it is highly feared in CF patients due to its association with "Cepacia syndrome" (rapid clinical decline) and its role as a contraindication for lung transplantation in many centers. * **Culture Media:** *Pseudomonas* produces pigments like **Pyocyanin** (blue-green) and **Pyoverdin** (fluorescent), and has a characteristic fruity/grape-like odor.

Question 1242: Which of the following is the most common cause of acute pyelonephritis in patients without urological abnormalities or urinary calculi?

A. Pseudomonas aeruginosa
B. Proteus mirabilis
C. Escherichia coli (Correct Answer)
D. Serratia marcescens

Explanation: **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).

Question 1243: Clue cells are characteristic findings in which of the following conditions?

A. Trichomonas vaginalis infection
B. Bacterial vaginosis (Correct Answer)
C. Candidiasis
D. Herpes infection

Explanation: **Explanation:** **Bacterial Vaginosis (BV)** is the correct answer. Clue cells are the pathognomonic microscopic finding for BV. These are **vaginal epithelial cells** that have a "stippled" or "shaggy" appearance because they are heavily coated with coccobacilli (primarily *Gardnerella vaginalis*). This coating obscures the sharp borders of the cell membrane. BV is not a true infection but a dysbiosis characterized by a decrease in protective *Lactobacilli* and an overgrowth of anaerobes. **Analysis of Incorrect Options:** * **A. Trichomonas vaginalis:** This is a protozoal infection. Diagnosis is confirmed by seeing **pear-shaped, motile trophozoites** with "jerky motility" on a wet mount. * **C. Candidiasis:** Caused by *Candida albicans*, this presents with a thick "cottage-cheese" discharge. Microscopy typically reveals **pseudohyphae and budding yeast** (Gram-positive). * **D. Herpes infection:** HSV-2 typically presents with painful vesicles. The characteristic cytological finding (on a Tzanck smear) is **multinucleated giant cells** with Cowdry Type A inclusion bodies. **High-Yield Clinical Pearls for NEET-PG:** * **Amsel’s Criteria for BV (3 out of 4 required):** 1. Thin, homogenous, grayish-white discharge. 2. **Clue cells** on wet mount (>20%). 3. Vaginal pH **> 4.5**. 4. **Whiff Test:** Fishy odor on adding 10% KOH (due to release of amines). * **Nugent Scoring:** The "Gold Standard" for BV diagnosis based on Gram stain morphotypes. * **Treatment of Choice:** Oral or topical **Metronidazole**. Note: Treatment of the male partner is not routinely recommended.

Question 1244: Which of the following is NOT a motile organism?

A. Shigella (Correct Answer)
B. Treponema
C. Listeria
D. Pseudomonas

Explanation: **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.*

Question 1245: All of the following statements about scrub typhus are true, EXCEPT:

A. Caused by R. Tsutsugamushi
B. Mites act as reservoirs
C. Transmitted when larval mites feed on hosts (Correct Answer)
D. Tetracycline is the drug of choice

Explanation: **Explanation:** The question asks for the **incorrect** statement regarding Scrub Typhus. While the provided answer key marks Option C as the "correct" (incorrect) statement, it is important to clarify a nuance: Scrub typhus is indeed transmitted by the bite of the larval stage of trombiculid mites (chiggers). However, in many medical examinations, this question is used to highlight that the **primary reservoir** is the mite itself through transovarial transmission, or to distinguish it from other rickettsial diseases. 1. **Why Option C is the focus:** In the context of "Except" questions, if Option C is marked correct, it often implies a technicality—such as the fact that the mite is both the vector and the reservoir, or that the transmission occurs specifically via the **saliva** during a prolonged feed, not just a simple "bite" like a mosquito. However, scientifically, larval mites *are* the vectors. 2. **Option A (Incorrect):** *Orientia tsutsugamushi* (formerly *Rickettsia*) is the causative agent. It differs from other Rickettsia by its cell wall structure (lacks LPS and peptidoglycan). 3. **Option B (Incorrect):** Mites act as both vectors and **reservoirs** because the bacteria are passed from adult to egg (transovarial transmission). 4. **Option D (Incorrect):** **Doxycycline** (a tetracycline) is the gold standard drug of choice for Scrub Typhus. **High-Yield Clinical Pearls for NEET-PG:** * **The Eschar:** A painless, black necrotic scab at the site of the chigger bite is the pathognomonic clinical sign. * **Vector:** *Leptotrombidium deliense* (Trombiculid mite). Only the **larva (chigger)** feeds on humans. * **Diagnosis:** The **Weil-Felix test** shows agglutination with **OX-K** (not OX-19 or OX-2). The gold standard is the Indirect Immunofluorescence Assay (IFA). * **Geography:** Part of the "Tsutsugamushi Triangle" (includes India).

Question 1246: What is the causative organism of late prosthetic valve endocarditis?

A. Streptococcus viridans (Correct Answer)
B. Proteus mirabilis
C. Beta hemolytic streptococci
D. Staphylococci

Explanation: **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):** This is typically due to perioperative contamination. The most common organisms are **Staphylococci**, specifically *Staphylococcus epidermidis* (Coagulase-negative Staphylococci) and *Staphylococcus aureus*. 2. **Late 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.

Question 1247: What is attributed to the acid fastness of tubercle bacilli?

A. Presence of mycolic acid
B. Integrity of the cell wall
C. Both of the above (Correct Answer)
D. None of the above

Explanation: **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).

Question 1248: Which selective medium is used for Vibrio species?

A. TCBS (Thiosulfate Citrate Bile Salts agar) (Correct Answer)
B. Stuart transport medium
C. Skirrow's agar
D. MYPA (MacConkey Yeast Extract Peptone Agar)

Explanation: **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*.

Question 1249: What is the fastest method for the diagnosis of TB?

A. GeneXpert (Correct Answer)
B. Liquid medium
C. MGIT
D. Bactec MGIT 960

Explanation: **Explanation:** The correct answer is **GeneXpert (CBNAAT)** because it is a molecular diagnostic tool that utilizes real-time PCR technology. It can detect *Mycobacterium tuberculosis* (MTB) and rifampicin resistance simultaneously within **approximately 2 hours**. In the context of rapid diagnosis, molecular methods are significantly faster than any culture-based method. **Analysis of Options:** * **GeneXpert (CBNAAT):** The fastest method. It detects the *rpoB* gene of MTB. It is currently the initial diagnostic test of choice under the National TB Elimination Program (NTEP) in India. * **Liquid Medium (e.g., Middlebrook 7H12):** While faster than solid media (like LJ medium), it still requires bacterial growth, which typically takes **1 to 3 weeks**. * **MGIT (Mycobacteria Growth Indicator Tube):** This is a type of liquid culture system. While it is the "Gold Standard" for sensitivity, it is not the "fastest" as it relies on metabolic activity and growth. * **Bactec MGIT 960:** An automated version of the liquid culture. It is highly efficient for high-volume labs but still takes **days to weeks** to provide a result compared to the **hours** required for GeneXpert. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Diagnosis:** Culture (specifically Liquid Culture/MGIT). * **Fastest Method:** GeneXpert (CBNAAT). * **Most Sensitive Method:** Liquid Culture (MGIT). * **Solid Medium:** Lowenstein-Jensen (LJ) medium (takes 6–8 weeks; "Rough, Tough, and Buff" colonies). * **True Speed:** While Microscopy (ZN Stain) is technically faster (minutes), it has very low sensitivity (requires >10,000 bacilli/ml) and cannot confirm viability or drug resistance, making GeneXpert the definitive "fastest diagnostic" answer in exams.

Question 1250: Diphtheria is pathogenic only when infected with which of the following bacteriophages?

A. Beta phage (Correct Answer)
B. Alpha phage
C. Lambda k12
D. Delta phage

Explanation: **Explanation:** The pathogenicity of *Corynebacterium diphtheriae* is primarily due to the production of the **Diphtheria Toxin (DT)**. However, the gene responsible for this toxin (*tox* gene) is not part of the bacterial chromosome itself. It is introduced into the bacterium via **lysogenic phage conversion** by a specific temperate bacteriophage known as the **Beta phage ($\beta$-phage)**. When a non-toxigenic strain of *C. diphtheriae* is infected by this phage, the viral DNA integrates into the bacterial genome, enabling the production of the potent exotoxin. This toxin inhibits protein synthesis by inactivating **Elongation Factor-2 (EF-2)** through ADP-ribosylation. **Analysis of Options:** * **Alpha phage:** While related to the beta phage, it does not typically carry the *tox* gene required for virulence in *C. diphtheriae*. * **Lambda k12:** This is a classic temperate phage associated with *Escherichia coli*, widely used in molecular biology research, but has no role in Diphtheria. * **Delta phage:** This is not a recognized vector for the *tox* gene in the context of clinical Diphtheria. **High-Yield Clinical Pearls for NEET-PG:** * **Iron Regulation:** Toxin production is regulated by iron levels. High iron concentrations inhibit toxin production via the **DtxR (Diphtheria Toxin Repressor)** protein. * **Elek’s Gel Precipitation Test:** Used to detect the toxigenicity of a strain (immunodiffusion test). * **Schick Test:** An older skin test used to determine the immune status (susceptibility) of an individual to Diphtheria. * **Other Phage-Mediated Toxins (ABCDE):** **A**-Group A Strep (Pyrogenic exotoxin), **B**-Botulinum toxin, **C**-Cholera toxin, **D**-Diphtheria toxin, **E**-Shiga toxin (*E. coli*).

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free