Microbial Genetics Practice Questions

Q: In staphylococci, plasmid encoding beta-lactamase production is transmitted by which mechanism?

Transduction. ### Explanation **Correct Option: B. Transduction** In *Staphylococcus aureus*, the transmission of antibiotic resistance, specifically the **plasmid-mediated production of beta-lactamase (penicillinase)**, occurs primarily through **transduction**. This process involves the transfer of genetic material from a donor bacterium to a recipient via a **bacteriophage** (a virus that infects bacteria). In staphylococci, this is the most common natural mechanism for horizontal gene transfer of plasmids. **Analysis of Incorrect Options:** * **A. Conjugation:** While conjugation (cell-to-cell contact via sex pili) is the most common method for spreading multi-drug resistance in **Gram-negative bacilli** (like *E. coli*), it is less common in staphylococci for beta-lactamase plasmids. * **C. Transposition:** Transposons ("jumping genes") are DNA sequences that move within a single genome (from plasmid to chromosome or vice versa). They facilitate the movement of resistance genes but are not the primary mechanism of *inter-cellular* transmission for these specific plasmids. * **D. Transformation:** This involves the uptake of "naked" DNA from the environment. While it occurs in *Streptococcus pneumoniae* and *Neisseria*, it is not a significant mechanism for plasmid transfer in *Staphylococcus*. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Resistance:** Beta-lactamase in *S. aureus* breaks the beta-lactam ring, rendering penicillin G, ampicillin, and amoxicillin ineffective. * **Treatment:** To overcome this, **penicillinase-resistant penicillins** (Oxacillin, Nafcillin) or **Beta-lactamase inhibitors** (Clavulanic acid) are used. * **MRSA Note:** Resistance in MRSA is **not** due to beta-lactamase; it is due to an altered target site (**PBP2a**) encoded by the **mecA gene** on the Staphylococcal Cassette Chromosome (SCCmec).

Q: The discovery of gene transformation originated from studies involving which of the following bacteria?

Streptococcus pneumoniae. **Explanation:** The discovery of **transformation**—the process by which bacteria take up "naked" DNA from their environment—is a landmark event in molecular biology. **Why Streptococcus pneumoniae is correct:** In 1928, **Frederick Griffith** conducted the "Griffith's Experiment" using *Streptococcus pneumoniae* (Pneumococcus). He observed that heat-killed virulent strains (Smooth/S-type with capsule) could transfer genetic material to live non-virulent strains (Rough/R-type without capsule), making the latter virulent. This "transforming principle" was later identified as **DNA** by Avery, MacLeod, and McCarty in 1944. **Analysis of Incorrect Options:** * **Bacillus subtilis:** While it is a model organism for studying Gram-positive transformation and sporulation, it was not the organism used in the original discovery. * **Streptococcus pyogenes (Group A Strep):** Though closely related to *S. pneumoniae*, it is primarily known for causing pharyngitis and rheumatic fever, not for the discovery of transformation. * **Escherichia coli:** *E. coli* is the workhorse of modern genetics and biotechnology (often used in artificial transformation via heat shock), but it was not the organism used in Griffith’s foundational study. **High-Yield Clinical Pearls for NEET-PG:** * **Natural Competence:** Only certain bacteria are "naturally competent" to undergo transformation (e.g., *S. pneumoniae*, *Haemophilus influenzae*, *Neisseria* species). * **Mechanism:** Transformation involves the uptake of DNA via the **Com system** (competence proteins). * **DNAse Sensitivity:** Transformation is the only horizontal gene transfer method that is **inhibited by DNAse** in the environment (unlike conjugation or transduction). * **Virulence Factor:** In *S. pneumoniae*, the **polysaccharide capsule** is the primary virulence factor; transformation allowed the R-strain to acquire the genes to produce this capsule.

Q: Drug resistance in Tuberculosis is due to?

Mutation. **Explanation:** The development of drug resistance in *Mycobacterium tuberculosis* (MTB) is fundamentally different from many other bacteria. **Why Mutation is the Correct Answer:** In *M. tuberculosis*, drug resistance is almost exclusively due to **chromosomal gene mutations**. Unlike many enteric bacteria, MTB does not possess horizontal gene transfer mechanisms like plasmids or transposons. Resistance arises from spontaneous, random mutations in specific genomic loci (e.g., *katG* or *inhA* for Isoniazid, and *rpoB* for Rifampicin). Under the selective pressure of inadequate or monotherapy, these resistant mutants survive and multiply, leading to acquired resistance. **Why Other Options are Incorrect:** * **Transformation, Transduction, and Conjugation:** These are methods of **Horizontal Gene Transfer (HGT)**. While these processes are common in organisms like *Staphylococcus* or *E. coli* (where resistance is often plasmid-mediated), they **do not occur** in *M. tuberculosis* in a clinical setting. MTB is a "clonal" organism that relies on vertical inheritance of genetic errors (mutations). **NEET-PG High-Yield Pearls:** 1. **Rifampicin Resistance:** Over 95% of cases are due to mutations in the **rpoB gene** (beta subunit of RNA polymerase). This is the target for the **CBNAAT/GeneXpert** test. 2. **Isoniazid (INH) Resistance:** Most commonly associated with mutations in the **katG gene** (catalase-peroxidase enzyme) or the **inhA** promoter region. 3. **Multi-Drug Resistant TB (MDR-TB):** Defined as resistance to at least **Isoniazid and Rifampicin**. 4. **The "Fall and Rise" Phenomenon:** This occurs when a single drug is added to a failing regimen; the sensitive bacilli die (fall), but the resistant mutants eventually multiply (rise). This is why TB treatment always requires a multi-drug combination.

Q: Bacterial R factor is transmitted by which method of gene transfer?

Conjugation. **Explanation:** **Why Conjugation is Correct:** The **R factor (Resistance factor)** is a type of plasmid that carries genes for antibiotic resistance [3]. It consists of two components: the **RTF (Resistance Transfer Factor)**, which contains the genes necessary for plasmid replication and transfer, and the **r-determinant**, which carries the actual resistance genes [3]. The primary method of R factor transmission is **Conjugation** [1]. This process involves direct cell-to-cell contact through a **sex pilus**, allowing the rapid horizontal spread of multi-drug resistance (MDR) among Gram-negative bacteria (e.g., *E. coli*, *Salmonella*, *Shigella*) [1], [3]. **Why Other Options are Incorrect:** * **A. Transduction:** This involves gene transfer via a **bacteriophage** (virus) [2], [4]. While some resistance genes can be transduced, the R factor as a whole is classically associated with conjugation [5]. * **B. Transformation:** This is the uptake of **naked DNA** from the surrounding environment [2]. It is a significant mechanism for bacteria like *S. pneumoniae* but is not the primary mode for R factor dissemination. * **D. Fusion:** Protoplast fusion is an artificial laboratory technique used in genetic engineering and is not a natural mechanism of bacterial gene transfer. **Clinical Pearls for NEET-PG:** * **Conjugation** is the most common method for the spread of **multi-drug resistance** in clinical settings [3]. * **Plasmids** are extrachromosomal, self-replicating, double-stranded circular DNA molecules. * **High-Frequency Recombination (Hfr) cell:** Formed when an F-plasmid integrates into the bacterial chromosome. * **Transposons ("Jumping Genes"):** These are DNA sequences that can move from a plasmid to a chromosome (or vice versa), often carrying resistance genes.

Q: A nonspore-forming, gram-positive bacillus was isolated from a throat specimen from a primary school-aged child who had not been vaccinated with the DTaP vaccine. The strain isolated carried a bacteriophage on which the gene for toxin was encoded. How did this strain become toxigenic?

Transduction. ### Explanation **Correct Option: D. Transduction** The clinical scenario describes **Corynebacterium diphtheriae**, the causative agent of Diphtheria. The pathogenicity of this organism depends on the production of the **Diphtheria toxin (DT)**. Crucially, the *tox* gene is not part of the bacterial chromosome; it is carried by a **temperate bacteriophage** (specifically the **Beta-phage**). The process by which a bacteriophage transfers genetic material (like the *tox* gene) into a bacterium is called **Transduction**. Specifically, this is an example of **Lysogenic conversion**, where the integration of the phage DNA (prophage) into the bacterial genome converts a non-toxigenic strain into a toxigenic, virulent one. **Why Incorrect Options are Wrong:** * **A. Conjugation:** This involves the transfer of genetic material (usually plasmids) through direct cell-to-cell contact via a sex pilus. The *tox* gene is phage-mediated, not plasmid-mediated. * **B. Frameshift mutation:** This is a genetic mutation caused by indels (insertions or deletions) of nucleotides. While mutations can alter toxin expression, they do not "introduce" a new gene from a virus. * **C. Homologous recombination:** This is a process where nucleotide sequences are exchanged between two similar or identical molecules of DNA. It is a mechanism for DNA repair or genetic variation, not the primary method of acquiring a viral toxin gene. **NEET-PG High-Yield Pearls:** * **Organism:** *C. diphtheriae* is a Gram-positive, non-motile, non-spore-forming, club-shaped bacillus (Chinese letter pattern). * **Mechanism of Toxin:** The Diphtheria toxin inhibits protein synthesis by **ADP-ribosylation of Elongation Factor-2 (EF-2)**. * **Culture:** Use **Loeffler’s Serum Slope** (rapid growth) or **Potassium Tellurite Agar** (black colonies). * **Test for Toxigenicity:** **Elek’s Gel Precipitation Test** is the gold standard for detecting the toxin.

Q: All of the following statements about bacteriophages are true except:

It transfers only chromosomal genes.. **Explanation:** Bacteriophages are viruses that infect bacteria and play a crucial role in horizontal gene transfer. The correct answer is **D** because bacteriophages do not transfer *only* chromosomal genes; they can also transfer **plasmids and transposons**. **1. Why Option D is the correct answer (The Exception):** In the process of **transduction**, bacteriophages package genetic material from a donor bacterium and inject it into a recipient. This material can include chromosomal DNA, but it frequently includes extra-chromosomal elements like **plasmids** (e.g., antibiotic resistance genes) or **transposons**. Therefore, the statement that they transfer *only* chromosomal genes is factually incorrect. **2. Analysis of Incorrect Options:** * **Option A:** This is the basic definition of a bacteriophage (literally "bacteria eater"). * **Option B:** Transduction is the process by which DNA is transferred from one bacterium to another by a virus. It is divided into *Generalized* (any gene) and *Specialized* (specific genes near the prophage site) transduction. * **Option C:** Through **Lysogenic Conversion**, a temperate phage integrates its DNA into the bacterial chromosome, imparting new phenotypic traits like toxigenicity. **Clinical Pearls & High-Yield Facts:** * **Lysogenic Conversion Examples:** Several major bacterial toxins are encoded by bacteriophages (Mnemonic: **ABCD'S**). * **A:** Group A *Streptococcus* (Pyrogenic exotoxin/Scarlet fever) * **B:** *Botulinum* toxin * **C:** *Cholera* toxin * **D:** *Diphtheria* toxin * **S:** *Shiga* toxin * **Phage Typing:** Used in epidemiology to differentiate bacterial strains (e.g., *S. aureus*, *S. typhi*). * **T4 Phage:** The most commonly studied phage infecting *E. coli*.

Question 1

Transfer of some chromosomal material from one bacterium to another uses which of the following?

Accepted Answer

All of the above

Answer

F factor

Answer

R factor

Answer

Transfer factor

Question 2

In staphylococci, plasmid encoding beta-lactamase production is transmitted by which mechanism?

Accepted Answer

Transduction

Answer

Conjugation

Answer

Transposition

Answer

Transformation

Question 3

What is a simple bacterial test for mutagenic carcinogens?

Accepted Answer

Redox test

Answer

Ames test

Answer

Bacteriophage

Answer

Gene splicing

Question 4

The discovery of gene transformation originated from studies involving which of the following bacteria?

Accepted Answer

Streptococcus pneumoniae

Answer

Bacillus subtilis

Answer

Streptococcus pyogenes

Answer

Escherichia coli

Question 5

Drug resistance in Tuberculosis is due to?

Accepted Answer

Mutation

Answer

Transformation

Answer

Transduction

Answer

Conjugation

Question 6

Which of the following statements is true regarding generalized transducing phages (e.g. phage P1)?

Accepted Answer

The entire bacterial chromosome is carried within a single phage particle.

Answer

Bacterial DNA is carried in every phage particle.

Answer

Only particular host genes are carried in the phage particle.

Answer

The phage will only affect genes that are near each other in the host chromosome.

Question 7

Bacterial R factor is transmitted by which method of gene transfer?

Accepted Answer

Conjugation

Answer

Transduction

Answer

Transformation

Answer

Fusion

Question 8

One bacterium receives a gene for restriction endonuclease from another by horizontal transduction. What is the likely outcome?

Accepted Answer

The bacterium will die as it does not have the protective methylase enzyme

Answer

The proof-reading activity of the bacteria is increased

Answer

The bacterium will cause digestion of host DNA on infection

Answer

The mechanism of repair is excellent

Question 9

A nonspore-forming, gram-positive bacillus was isolated from a throat specimen from a primary school-aged child who had not been vaccinated with the DTaP vaccine. The strain isolated carried a bacteriophage on which the gene for toxin was encoded. How did this strain become toxigenic?

Accepted Answer

Transduction

Answer

Conjugation

Answer

Frameshift mutation

Answer

Homologous recombination

Question 10

All of the following statements about bacteriophages are true except:

Accepted Answer

It transfers only chromosomal genes.

Answer

It is a virus that infects bacteria.

Answer

It helps in transduction of bacteria.

Answer

It imparts toxigenicity to bacteria.

Microbial Genetics — MCQs

Microbial Genetics — MCQs

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