Bacteriology Practice Questions

Q: Toxin production by Corynebacterium diphtheriae is due to which mechanism?

Lysogenic conversion. **Explanation:** The pathogenicity of *Corynebacterium diphtheriae* is primarily due to the production of the **Diphtheria Toxin (DT)**. This toxin is not encoded on the bacterial chromosome itself, but rather by the **tox gene**, which is introduced into the bacterium by a temperate bacteriophage (specifically the **Beta-phage**). This process, where a bacteriophage integrates its DNA into the bacterial genome and confers new phenotypic properties (like toxin production), is called **Lysogenic conversion**. Only strains of *C. diphtheriae* that are "lysogenized" by this phage can cause clinical diphtheria. **Analysis of Options:** * **A. Chromosomal mutation:** While mutations can lead to antibiotic resistance, they do not account for the acquisition of the diphtheria toxin gene. * **B. Presence of a plasmid:** Plasmids often carry genes for antibiotic resistance (R-factors), but the *tox* gene in *C. diphtheriae* is specifically phage-mediated, not plasmid-mediated. * **D. Transformation:** This involves the uptake of naked DNA from the environment. While a mechanism of horizontal gene transfer, it is not the natural mechanism for toxigenicity in Diphtheria. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Diphtheria toxin inhibits protein synthesis by **ADP-ribosylation of Elongation Factor-2 (EF-2)**. * **Regulation:** Toxin production is regulated by iron levels. High iron inhibits toxin production via the **DtxR** (Diphtheria toxin receptor) protein. * **Diagnosis:** The **Elek’s Gel Precipitation Test** is the gold standard for detecting toxigenicity (immunodiffusion). * **Other Lysogenic Conversion Examples:** Remember the mnemonic **ABCDES**: **A**dherence (Group A Strep), **B**otulinum toxin, **C**holera toxin, **D**iphtheria toxin, **E**rythrogenic toxin (Strep pyogenes), and **S**higa toxin.

Q: Mycobacterium leprae can be grown on:

Foot pad of mouse or Armadillo. ### Explanation **1. Why the Correct Answer is Right:** *Mycobacterium leprae* is an **obligate intracellular bacterium** that has never been grown on artificial (in vitro) culture media. This is because it lacks several genes required for independent metabolism. It has a unique preference for lower body temperatures (around 30–33°C). * **Foot pad of mouse:** Introduced by Shepard (1960), this provides the necessary cool temperature for the bacilli to multiply, though the infection remains localized. * **Nine-banded Armadillo:** Introduced by Kirchheimer and Storrs (1971), armadillos have a low core body temperature and are highly susceptible, developing **generalized lepromatous leprosy**. They serve as a major source for harvesting large quantities of *M. leprae* for research and lepromin antigen production. **2. Why Incorrect Options are Wrong:** * **Option A (Culture media):** Unlike *M. tuberculosis* (which grows on LJ media), *M. leprae* is non-culturable on any cell-free media. * **Options C & D (Liver of guinea pig/Kidney of rabbit):** These animals are not susceptible to *M. leprae*. While guinea pigs are classically used for *M. tuberculosis* inoculation (causing systemic disease), they do not support the growth of the leprosy bacillus. **3. NEET-PG High-Yield Pearls:** * **Doubling Time:** *M. leprae* is the slowest-growing human pathogen, with a generation time of approximately **12–14 days**. * **Viability:** It is stained by **5% Sulfuric acid** (Modified Ziehl-Neelsen) because it is less acid-fast than *M. tuberculosis* (which uses 20% $H_2SO_4$). * **Cultivation Breakthrough:** Recently, *M. lepraemurium* (causing rat leprosy) was grown on specialized media, but *M. leprae* remains unculturable. * **Target Cells:** It has a specific tropism for **Schwann cells**, leading to characteristic nerve thickening and demyelination.

Q: Which of the following is true about Enteropathogenic E. coli?

Causes diarrhea in infants. **Explanation:** **Enteropathogenic *E. coli* (EPEC)** is a major cause of infantile diarrhea, particularly in developing countries. 1. **Why Option A is correct:** EPEC is classically associated with **outbreaks of diarrhea in nurseries and daycare centers**. It primarily affects infants (children <2 years old). The hallmark of its pathogenesis is the **"Attaching and Effacing" (A/E) lesion**. It uses **Bundle-Forming Pili (BFP)** for initial attachment and the **Intimin** protein for intimate adhesion to intestinal epithelial cells, leading to the destruction of microvilli and malabsorptive diarrhea. 2. **Why other options are incorrect:** * **Option B:** EPEC is **non-invasive**. Invasion of intestinal epithelial cells is the characteristic mechanism of **Enteroinvasive *E. coli* (EIEC)**, which mimics Shigellosis. * **Option C:** Adults are rarely affected because they have usually developed acquired immunity. EPEC is a pediatric pathogen. * **Option D:** While immunocompromised hosts are at risk for many infections, EPEC is specifically defined by its predilection for the **pediatric/infant population** regardless of immune status, rather than being an opportunistic infection of the immunocompromised. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** **EPEC** = **P**ediatric/**P**ili (Bundle-forming). * **Pathogenesis:** Mediated by the **LEE (Locus of Enterocyte Effacement)** pathogenicity island. * **Diagnosis:** Characterized by "stacked-brick" appearance is NOT EPEC (that is EAEC); EPEC shows **localized adherence** on HeLa or HEp-2 cells. * **Toxins:** EPEC does **not** produce Shiga-like toxins or Enterotoxins (LT/ST).

Q: Anthrax bacilli differs from anthracoid bacilli by being:

Non-motile. ### Explanation The differentiation between *Bacillus anthracis* (Anthrax bacilli) and *Bacillus anthracoides* (Pseudoanthrax/Anthracoid bacilli) is a classic high-yield topic in NEET-PG Microbiology. **Why "Non-motile" is correct:** *Bacillus anthracis* is characteristically **non-motile**. In contrast, almost all anthracoid bacilli (like *B. cereus* or *B. subtilis*) exhibit active motility (peritrichous flagella). This lack of motility is a key diagnostic feature used to identify the pathogen in a laboratory setting. **Analysis of Incorrect Options:** * **A. Non-capsulated:** Incorrect. *B. anthracis* is **capsulated** (polypeptide capsule made of D-glutamic acid), which is essential for its virulence. Anthracoid bacilli are generally non-capsulated. * **B. Strict aerobe:** Incorrect. Both *B. anthracis* and anthracoid bacilli are **aerobes and facultative anaerobes**. * **D. Haemolytic colonies:** Incorrect. *B. anthracis* is **non-haemolytic** (gamma-haemolysis) on blood agar. Anthracoid bacilli typically produce strong beta-haemolysis. **High-Yield Clinical Pearls for NEET-PG:** | Feature | *B. anthracis* | Anthracoid Bacilli | | :--- | :--- | :--- | | **Motility** | **Non-motile** (Statue-like) | **Motile** | | **Haemolysis** | Non-haemolytic | Usually Beta-haemolytic | | **Capsule** | Present (Polypeptide) | Absent | | **Medusa Head** | Present (on Agar) | Absent/Irregular | | **Salicin Fermentation** | Negative | Positive | | **Penicillin** | Sensitive (McFadyean's reaction) | Resistant | * **Mnemonic:** *B. anthracis* is "Lazy and Boring" (Non-motile, Non-haemolytic). * **McFadyean’s Reaction:** Uses polychrome methylene blue to visualize the **M**auve-colored capsule of *B. anthracis*. * **String of Pearls Reaction:** Seen when *B. anthracis* is grown on agar containing low concentrations of penicillin.

Q: Which organism is known to cause epidemics of puerperal sepsis?

Group A beta-hemolytic streptococci. **Explanation:** **Puerperal sepsis** is defined as an infection of the genital tract occurring at any time between the onset of rupture of membranes or labor and the 42nd day postpartum. **Why Group A beta-hemolytic streptococci (GABS) is correct:** Historically, *Streptococcus pyogenes* (Group A Strep) is the most notorious cause of **explosive epidemics** of puerperal sepsis. It is highly virulent and can be introduced into the birth canal via the hands or respiratory droplets of healthcare workers or the patient herself. While modern hygiene has reduced its frequency, it remains the primary organism associated with rapidly progressing, life-threatening outbreaks in obstetric wards. **Analysis of Incorrect Options:** * **Group B beta-hemolytic streptococci (GBS):** While *S. agalactiae* is the most common cause of neonatal sepsis and a frequent cause of sporadic postpartum infections, it typically exists as normal vaginal flora and does not usually cause the "epidemic" patterns seen with Group A. * **Cytomegalovirus (CMV):** This is a viral pathogen primarily associated with congenital infections (TORCH syndrome) rather than acute postpartum maternal sepsis. * **Toxoplasma gondii:** This is a protozoan parasite. While it can cause congenital toxoplasmosis via transplacental transmission, it is not a causative agent of puerperal sepsis. **High-Yield Clinical Pearls for NEET-PG:** * **Ignaz Semmelweis:** Known as the "Father of Infection Control," he famously discovered that handwashing could drastically reduce the mortality of puerperal sepsis caused by GABS. * **Most common overall cause:** While GABS causes epidemics, **anaerobic bacteria** (e.g., *Bacteroides*, *Peptostreptococcus*) and *E. coli* are common causes of sporadic polymicrobial puerperal infections today. * **Clinical Presentation:** Sudden high fever, pelvic pain, and foul-smelling lochia are classic signs.

Q: Trachoma is one of the leading causes of blindness. Which of the following best typifies the disease?

It is caused by Chlamydia trachomatis. **Explanation:** **1. Why Option A is Correct:** Trachoma is a chronic keratoconjunctivitis caused by specific serotypes of **_Chlamydia trachomatis_ (Serotypes A, B, Ba, and C)**. It is the leading infectious cause of blindness worldwide. The pathogenesis involves repeated cycles of infection leading to subepithelial follicular inflammation, conjunctival scarring (Arlt’s line), trichiasis (inward turning of eyelashes), and eventual corneal opacity. **2. Why the Other Options are Incorrect:** * **Option B:** The WHO-recommended treatment for Trachoma follows the **SAFE strategy**. The medical treatment of choice is a **single dose of oral Azithromycin** (macrolide) or topical Tetracycline. Cephalosporins are not the standard of care for Chlamydial eye infections. * **Option C:** While Trachoma is a global health issue, it is most hyperendemic in the **arid areas of Africa** (Sub-Saharan Africa), the Middle East, and parts of Asia, rather than being specifically characterized by 400 million cases in the Pacific Rim. * **Option D:** Trachoma is a disease of the **conjunctiva and cornea** (keratoconjunctivitis). Uveitis involves the middle layer of the eye (iris, ciliary body, and choroid), which is not the primary site of pathology in Trachoma. **3. NEET-PG High-Yield Pearls:** * **SAFE Strategy:** **S**urgery (for trichiasis), **A**ntibiotics (Azithromycin), **F**acial cleanliness, **E**nvironmental improvement. * **Inclusion Bodies:** Look for **Halberstaedter-Prowazek (HP) bodies** (intracytoplasmic inclusions) in conjunctival scrapings. * **Clinical Signs:** **Herbert’s pits** (scarred limbal follicles) are pathognomonic for Trachoma. * **Serotypes:** Remember **A-C** cause Trachoma; **D-K** cause inclusion conjunctivitis and NGS; **L1-L3** cause Lymphogranuloma Venereum (LGV).

Q: A wound shows crepitation in the subcutaneous tissue and muscle with foul-smelling discharge. Pus culture shows Cl. perfringens. All of the following features about Cl. perfringens are TRUE, EXCEPT?

The most important toxin is hyaluronidase.. **Explanation:** The clinical presentation of crepitation, foul-smelling discharge, and muscle involvement is classic for **Gas Gangrene (Clostridial Myonecrosis)**. **1. Why Option C is the correct answer (The False Statement):** While *Clostridium perfringens* does produce hyaluronidase (an enzyme that aids tissue spread), it is **not** the most important toxin. The primary virulence factor responsible for the pathogenesis of gas gangrene is **Alpha (α) toxin**. Alpha toxin is a **lecithinase (phospholipase C)** that degrades cell membranes, leading to massive tissue necrosis, hemolysis, and toxemia. **2. Analysis of Incorrect Options (True Statements):** * **Option A:** *Cl. perfringens* Type A is indeed the most common cause of gas gangrene worldwide (responsible for 80-90% of cases). * **Option B:** The **Nagler’s reaction** is a biochemical test used to detect Alpha toxin. When the organism is grown on egg yolk agar, the lecithinase activity creates a zone of opalescence around the colonies, which is inhibited by adding specific antitoxin. * **Option D:** Strains causing gas gangrene produce **heat-resistant spores**, which allow the bacteria to survive in soil and dust before entering a traumatic wound. (Note: Food poisoning strains of *Cl. perfringens* also produce highly heat-resistant spores). **Clinical Pearls for NEET-PG:** * **Target Hemolysis:** On blood agar, *Cl. perfringens* shows a "double zone of hemolysis" (inner zone of complete hemolysis due to Theta toxin; outer zone of incomplete hemolysis due to Alpha toxin). * **Stormy Fermentation:** In litmus milk media, it produces acid and gas, leading to a "stormy" appearance of the clot. * **Non-motile:** Unlike most Clostridia, *Cl. perfringens* is non-motile.

Question 1

Toxin production by Corynebacterium diphtheriae is due to which mechanism?

Accepted Answer

Lysogenic conversion

Answer

Chromosomal mutation

Answer

Presence of a plasmid

Answer

Transformation

Question 2

Mycobacterium leprae can be grown on:

Accepted Answer

Foot pad of mouse or Armadillo

Answer

Culture media

Answer

Liver of guinea pig

Answer

Kidney of rabbit

Question 3

Which one of the following gives strong evidence of Typhoid Fever carrier status?

Accepted Answer

Isolation of Vi antigen

Answer

Isolation of core antigen

Answer

Persistence of Vi antibodies

Answer

Demonstration of Typhoid bacilli in stools

Question 4

All of the following statements about Streptococcus pneumoniae are true, EXCEPT:

Accepted Answer

They grow best in 40% CO2

Answer

Spherical gram-positive bacteria

Answer

22 species are recognized

Answer

They reduce iron in hemoglobin to produce a green color in blood agar

Question 5

Which of the following is true about Enteropathogenic E. coli?

Accepted Answer

Causes diarrhea in infants

Answer

Acts by invasion of intestinal epithelial cells

Answer

Adults are mostly affected

Answer

Affects immunocompromised hosts

Question 6

A 22-year-old male presented with fever and increasing dyspnea. He has a history of frequent episodes of hemoptysis, sinusitis, pneumothorax, and fatty stools, for which he has been hospitalized multiple times. Investigations revealed deranged renal and liver function tests, cirrhotic changes in the liver with multiple renal calculi on USG abdomen, signs of necrotizing pneumonia on X-ray chest, and sputum culture showing non-fermenting, mucoid, oxidase-positive colonies. Which of the following drugs can be used to treat the identified organism?

Accepted Answer

IV Meropenem + inhaled tobramycin

Answer

IV plus inhaled colistin

Answer

IM Streptomycin

Answer

IV Ceftriaxone

Question 7

Anthrax bacilli differs from anthracoid bacilli by being:

Accepted Answer

Non-motile

Answer

Non-capsulated

Answer

Strict aerobe

Answer

Haemolytic colonies on blood agar

Question 8

Which organism is known to cause epidemics of puerperal sepsis?

Accepted Answer

Group A beta-hemolytic streptococci

Answer

Cytomegalovirus

Answer

Group B beta-hemolytic streptococci

Answer

Toxoplasma gondii

Question 9

Trachoma is one of the leading causes of blindness. Which of the following best typifies the disease?

Accepted Answer

It is caused by Chlamydia trachomatis

Answer

It is best treated with systemic cephalosporins and ophthalmic tetracycline

Answer

It affects 400 million people in the Pacific Rim

Answer

It is a form of chronic uveitis

Question 10

A wound shows crepitation in the subcutaneous tissue and muscle with foul-smelling discharge. Pus culture shows Cl. perfringens. All of the following features about Cl. perfringens are TRUE, EXCEPT?

Accepted Answer

The most important toxin is hyaluronidase.

Answer

It is the most common cause of gas gangrene.

Answer

Can be detected by Nagler's reaction.

Answer

Gas gangrene strains produce heat-resistant spores.

Bacteriology — MCQs

Bacteriology — MCQs

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