Gene Transfer in Bacteria — MCQs

10 questions

The initial origin of new drug resistance genes in bacteria most commonly occurs due to:

R-factor in bacteria is transferred by:

The genetic ability of a bacterium to grow in the presence of several antibiotics is passed in vivo from one bacterium to another by:

Which of the following is the most widespread method of genetic transfer among bacteria?

Transfer of a portion of DNA from one bacterium to another by a bacteriophage is known as?

Lysogenic conversion is seen in -

False regarding bacterial plasmids is:

What is the primary use of Elek's Gel test?

Which of the following statements about Corynebacterium diphtheriae is NOT true?

Q10

All the following terms are used to describe bacterial chromosome Except

Gene Transfer in Bacteria Indian Medical PG Practice Questions and MCQs

Question 1: The initial origin of new drug resistance genes in bacteria most commonly occurs due to:

A. Translation
B. Mutation (Correct Answer)
C. Conjugation
D. Transduction

Explanation: ***Mutation*** - **Random genetic changes** in bacteria can alter drug targets or introduce drug-inactivating enzymes, leading to resistance. - **Spontaneous mutations** in the bacterial genome are the primary source of new resistance genes that did not previously exist in the bacterial population. - While mutations occur at low frequency, they are the fundamental mechanism by which novel resistance traits first arise. *Translation* - This is the process of synthesizing proteins from mRNA; it is a fundamental cellular process and not a cause of drug resistance. - Errors in translation are generally lethal to the cell and do not typically confer specific drug-resistant phenotypes. *Conjugation* - This is a mechanism for **horizontal gene transfer** where bacteria directly transfer genetic material, including resistance genes, via a pilus. - While conjugation is the **most important mechanism for spreading resistance** in clinical settings, it transfers pre-existing resistance genes rather than creating new ones. *Transduction* - This is another form of **horizontal gene transfer** involving bacteriophages (viruses) carrying bacterial genes, including resistance genes, between bacteria. - Similar to conjugation, transduction is a mechanism for the **transfer** of pre-existing resistance genes, not their original creation.

Question 2: R-factor in bacteria is transferred by:

A. Transduction
B. Conjugation (Correct Answer)
C. Uptake of naked DNA
D. Genetic transfer from parent to offspring

Explanation: ***Conjugation*** - **R-factors** (resistance factors) are plasmids carrying genes for antibiotic resistance, and their primary mode of transfer between bacteria is through **conjugation**. - **Conjugation** involves direct cell-to-cell contact through a pilus, allowing the transfer of the plasmid DNA from a donor bacterium to a recipient bacterium. *Transduction* - **Transduction** involves the transfer of bacterial DNA by a **bacteriophage (virus)**, which can accidentally package bacterial genes instead of viral ones. - While it can transfer resistance genes, it is not the primary mechanism for R-factor spread across diverse bacterial species in the way conjugation is. *Uptake of naked DNA* - The **uptake of naked DNA** from the environment is known as **transformation**, which primarily occurs when bacteria are in a specific physiological state (competence). - This mechanism is less efficient for widespread R-factor dissemination compared to conjugation, which involves active transfer. *Genetic transfer from parent to offspring* - **Genetic transfer from parent to offspring** (vertical gene transfer) refers to the transmission of genetic material during cell division. - While R-factors are replicated and passed on to daughter cells, this does not explain their horizontal spread to new, unrelated bacteria.

Question 3: The genetic ability of a bacterium to grow in the presence of several antibiotics is passed in vivo from one bacterium to another by:

A. Transposition
B. Conjugation (Correct Answer)
C. Transduction
D. Transformation

Explanation: ***Conjugation*** - This process involves the direct transfer of **genetic material** (often plasmids carrying **resistance genes**) from one bacterium to another through direct cell-to-cell contact, typically via a **pilus**. - It is a primary mechanism for the spread of **antibiotic resistance** among bacteria, enabling the rapid acquisition of resistance traits. *Transposition* - This involves the movement of **transposable elements** (jumping genes) within a bacterium's own genome or between a plasmid and the chromosome. - While transposons can carry antibiotic resistance genes, their movement is typically within a single cell's DNA, not directly between different bacteria (though they can be part of transferred plasmids). *Transduction* - This is the process where **bacteriophages** (viruses that infect bacteria) carry bacterial DNA from one bacterium to another. - While it can transfer antibiotic resistance genes, it requires a viral vector rather than direct cell-to-cell contact, making it a distinct mechanism from conjugation. *Transformation* - This is the uptake of **naked DNA** from the environment by a bacterial cell. - The DNA can originate from dead bacterial cells, and while it *can* include resistance genes, it's not a direct, active transfer between living bacteria in the way conjugation is.

Question 4: Which of the following is the most widespread method of genetic transfer among bacteria?

A. Transformation
B. Transduction
C. Lysogenic conversion
D. Conjugation (Correct Answer)

Explanation: ***Conjugation*** - This method involves **direct cell-to-cell contact** between bacteria, typically through a **pilus**, allowing for the transfer of **plasmids** or chromosomal DNA. - Its widespread nature is due to the efficient transfer of large amounts of genetic material, including **antibiotic resistance genes**, among diverse bacterial species. *Transformation* - This process involves the uptake of **naked DNA** from the environment by a competent bacterial cell. - While important, it is less common because bacteria must be in a specific **physiological state** (competent) to take up DNA, and the DNA itself can be fragile in the environment. *Transduction* - This mechanism involves the transfer of bacterial DNA from one bacterium to another via a **bacteriophage** (virus). - Its prevalence is limited by the specific range of bacteria that a phage can infect, making it less widespread compared to conjugation. *Lysogenic conversion* - This refers to the acquisition of new traits by a bacterium due to the integration of a **prophage** (temperate phage DNA) into its genome. - While it can alter bacterial phenotypes, it is a specific outcome of phage infection and not a primary mechanism for broad genetic transfer among bacteria.

Question 5: Transfer of a portion of DNA from one bacterium to another by a bacteriophage is known as?

A. Transduction (Correct Answer)
B. Conjugation
C. Transformation
D. Mutation

Explanation: ***Transduction*** - **Transduction** is a process by which **bacterial DNA** is transferred from one bacterium to another by a **bacteriophage** (a virus that infects bacteria). - During infection, the bacteriophage can inadvertently package fragments of the host bacterial DNA into its **virion particles**, which are then transmitted to a new bacterial host upon subsequent infection. *Uptake of free DNA* - This process is called **transformation**, where bacteria take up **naked DNA** directly from their external environment. - Transformation does not involve a **bacteriophage** as an intermediary for DNA transfer. *Direct DNA transfer through contact* - This describes **conjugation**, a process where DNA is transferred from one bacterium to another via direct cell-to-cell contact through a **pilus**. - Conjugation involves a **sex pilus** and does not rely on a **bacteriophage** for DNA transfer. *Change in DNA sequence* - A change in the DNA sequence is generally referred to as a **mutation**, which is an alteration in the nucleotide sequence of the genome. - While transduction can lead to changes in the recipient bacterium's DNA, the term itself refers specifically to the **mechanism of transfer** rather than the resulting sequence alteration.

Question 6: Lysogenic conversion is seen in -

A. Staphylococcus
B. Corynebacterium diphtheriae (Correct Answer)
C. Salmonella
D. E. coli

Explanation: ***Corynebacterium diphtheriae*** - **Lysogenic conversion** in *Corynebacterium diphtheriae* is the **classic textbook example**, where a bacteriophage (phage β) carries the **tox gene** for the **diphtheria toxin**, converting a non-pathogenic strain into a toxigenic pathogen. - The integration of this **prophage** into the bacterial chromosome allows for stable toxin production, leading to the clinical manifestations of diphtheria. - This is the **most commonly cited example** of lysogenic conversion in medical microbiology education. *Staphylococcus* - While *Staphylococcus* can be infected by bacteriophages, **lysogenic conversion** is not the primary mechanism for acquiring major virulence factors. - Toxins like **TSST-1** and **Panton-Valentine leukocidin (PVL)** are more commonly encoded on **plasmids, pathogenicity islands, or other mobile genetic elements** rather than temperate bacteriophages. *Salmonella* - Some *Salmonella* serotypes can undergo **lysogenic conversion** where prophages confer **O-antigen modifications** or other factors. - However, this is less emphasized in standard medical curricula compared to the diphtheria toxin example. *E. coli* - **Shiga toxin-producing *E. coli* (STEC)** strains acquire **Shiga toxins (Stx1 and Stx2)** through lysogenic conversion via **Stx-encoding bacteriophages**. - While this is indeed a valid and important example of lysogenic conversion, the **diphtheria toxin** case is traditionally presented as the **primary teaching example** in Indian medical PG curricula (Ananthanarayan & Paniker, Jawetz). - For examination purposes, ***Corynebacterium diphtheriae*** is the expected answer when asked about lysogenic conversion.

Question 7: False regarding bacterial plasmids is:

A. Extrachromosomal
B. Transmission to different species
C. Eliminated by treating with radiation
D. Can cause lysogenic conversion (Correct Answer)

Explanation: ***Can cause lysogenic conversion*** - **Lysogenic conversion** is a phenomenon caused by **bacteriophages**, which are viruses that infect bacteria, not by plasmids directly. - It involves the integration of a **phage genome** into the bacterial chromosome, altering the bacterium's phenotype (e.g., toxin production). *Extrachromosomal* - Bacterial plasmids are indeed **extrachromosomal DNA molecules**, meaning they exist independently of the bacterial chromosome. - This characteristic allows them to be easily transferred between bacteria. *Transmission of different species* - Plasmids can be transmitted horizontally between bacteria, even across **different species**, through mechanisms like **conjugation**, **transformation**, or **transduction**. - This interspecies transmission is a major factor in the spread of **antibiotic resistance**. *Eliminated by treating with radiation* - Plasmids, like all DNA, can be eliminated or degraded by treatments such as **radiation** (e.g., UV) or certain chemicals. - Such treatments disrupt the plasmid DNA structure, preventing its replication or function.

Question 8: What is the primary use of Elek's Gel test?

A. Influenza
B. Diphtheria (Correct Answer)
C. Brucellosis
D. Cholera

Explanation: ***Diphtheria*** - **Elek's Gel immunodiffusion test** is a crucial in vitro assay used to detect **toxin production** by *Corynebacterium diphtheriae* strains - This test helps differentiate **toxigenic strains** (which cause diphtheria) from **non-toxigenic strains** - Based on the principle of **immunodiffusion** where antitoxin-impregnated filter paper is placed on agar and bacterial strains are streaked perpendicular to it - **Lines of precipitation** form if the strain produces diphtheria toxin *Influenza* - Influenza diagnosis primarily relies on rapid antigen detection tests, PCR, or viral culture - The Elek test is not used for the identification or toxin detection of *Influenza virus* *Brucellosis* - Brucellosis diagnosis is typically confirmed by serological tests (e.g., agglutination tests) or blood cultures - The Elek test has no role in the diagnosis of *Brucella* infections *Cholera* - Cholera is diagnosed by identifying *Vibrio cholerae* in stool samples through culture and biochemical tests - The Elek test is irrelevant to the diagnosis of cholera

Question 9: Which of the following statements about Corynebacterium diphtheriae is NOT true?

A. Has metachromatic granules
B. Toxin mediated by chromosomal gene (Correct Answer)
C. Toxigenicity demonstrated by elek's test
D. Does not invade deeper tissues

Explanation: ***Toxin mediated by chromosomal gene*** - The **diphtheria toxin** is encoded by the **tox gene**, which is a lysogenic bacteriophage (cornyphage) gene, not a chromosomal gene. - This **bacteriophage** integrates into the bacterial chromosome, making toxigenic *C. diphtheriae* strains lysogenized. *Has metachromatic granules* - *Corynebacterium diphtheriae* is known for possessing **metachromatic granules** (also called Babes-Ernst bodies), which are inclusions that stain differently from the rest of the cell. - These granules are composed of **polyphosphate reserves** and are important for identification. *Does not invade deeper tissues* - *Corynebacterium diphtheriae* remains **localized** to the mucosal surface of the upper respiratory tract or skin, forming a **pseudomembrane**. - Its pathogenicity is primarily due to the **exotoxin** it produces, which then disseminates systemically. *Toxigenicity demonstrated by elek's test* - The **Elek test** is a standard laboratory assay used to determine the **toxigenicity** of *Corynebacterium diphtheriae* strains by detecting the production of diphtheria toxin. - It works by identifying the **immunoprecipitation lines** formed between antitoxin and toxin in an agar medium.

Question 10: All the following terms are used to describe bacterial chromosome Except

A. Diploid (Correct Answer)
B. Circular
C. Nucleoid
D. Haploid

Explanation: ***Diploid*** - Bacterial chromosomes are typically **haploid**, meaning they contain only one copy of each gene, not two sets as in diploid organisms. - While bacteria can sometimes have multiple copies of their chromosome, these copies are usually identical replicas and do not represent distinct sets of genetic information. *Circular* - The bacterial chromosome is characteristically a **single, closed circular DNA molecule**. - This circular structure distinguishes bacterial chromosomes from the linear chromosomes found in eukaryotes. *Nucleoid* - The bacterial chromosome is located in a region of the cytoplasm called the **nucleoid**, as bacteria lack a membrane-bound nucleus. - The nucleoid is where the genetic material is condensed and organized within the bacterial cell. *Haploid* - Bacteria are essentially **haploid organisms**, possessing a single set of chromosomes. - This means they have only one allele for each gene, which can lead to rapid expression of mutations.

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