Microbial Genetics Practice Questions

Q: Which of the following toxins is carried by a bacteriophage?

Verocytotoxin of EHEC. ### Explanation The correct answer is **Verocytotoxin of EHEC**. This question tests the concept of **Lysogenic Conversion**, where a bacteriophage (bacterial virus) integrates its DNA into a bacterium's genome, allowing the bacterium to express new traits, such as toxin production. **1. Why Verocytotoxin is correct:** Verocytotoxin (also known as **Shiga-like toxin**) produced by Enterohemorrhagic *E. coli* (EHEC, e.g., O157:H7) is encoded by genes carried by a **Stx-phage** (a temperate bacteriophage). When this phage infects a non-toxigenic *E. coli*, the bacterium gains the ability to produce the toxin, which inhibits protein synthesis by damaging the 28S rRNA. **2. Analysis of Incorrect Options:** * **Cholera toxin (Vibrio cholerae):** While the toxin is indeed phage-encoded (by the **CTXφ phage**), it is often considered a classic example alongside Shiga toxin. However, in many standard MCQ formats, Verocytotoxin or Diphtheria toxin are the primary "textbook" answers for phage-mediated toxins. * **Heat-labile toxin (LT) of E. coli:** This toxin is encoded by **plasmids**, not bacteriophages. * **Anthrax toxin (B. anthracis):** The genes for the three components (PA, LF, EF) are located on the **pXO1 plasmid**, while the capsule is encoded on the **pXO2 plasmid**. **3. NEET-PG High-Yield Pearls (Mnemonic: ABCD'S):** The most important toxins encoded by **Lysogenic Phages** can be remembered by the mnemonic **ABCD'S**: * **A:** Group **A** Streptococcal erythrogenic toxin (Scarlet fever) * **B:** **B**otulinum toxin (Types C and D) * **C:** **C**holera toxin * **D:** **D**iphtheria toxin * **S:** **S**higa-like toxin (Verocytotoxin of EHEC) **Clinical Note:** Shiga-like toxin is the primary mediator of **Hemolytic Uremic Syndrome (HUS)**, characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure.

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: Which of the following methods involves the direct transfer of naked DNA from the environment into a bacterial cell?

Transformation. **Explanation:** **Transformation** is the process by which a "competent" bacterium takes up **naked, free DNA** directly from the surrounding environment. This DNA is usually released into the medium following the lysis of other bacteria. It was first described by **Frederick Griffith** (1928) in *Streptococcus pneumoniae*, demonstrating that non-virulent strains could become virulent by acquiring DNA from heat-killed virulent strains. **Why other options are incorrect:** * **Transduction:** This involves the transfer of bacterial DNA from one cell to another via a **bacteriophage** (virus). It does not involve naked DNA. * **Conjugation:** This is the transfer of genetic material (usually plasmids) through **direct cell-to-cell contact** via a sex pilus. It is often referred to as "bacterial mating." * **Transfection:** This is the process of deliberately introducing naked nucleic acids into **eukaryotic cells** (animal or plant cells), rather than bacteria. **High-Yield Clinical Pearls for NEET-PG:** * **Natural Competence:** Only certain bacteria are naturally competent, notably *Haemophilus influenzae*, *Streptococcus pneumoniae*, and *Neisseria* species (mnemonic: **H**is **S**ecret **N**ame). * **Griffith Experiment:** Proved the "Transforming Principle," which Avery, MacLeod, and McCarty later identified as DNA. * **Calcium Chloride ($CaCl_2$):** Used in laboratory settings to induce "artificial competence" by altering the bacterial cell membrane to allow DNA entry. * **Significance:** Transformation is a key mechanism for the spread of antibiotic resistance genes in clinical environments.

Q: Tetanospasmin encoding genes are located on?

Plasmid. **Explanation:** The correct answer is **B. Plasmid**. In *Clostridium tetani*, the neurotoxin **tetanospasmin** (responsible for the clinical manifestations of tetanus) is encoded by a **large, non-conjugative plasmid** (specifically a 75-kb plasmid). This is a classic example of extrachromosomal inheritance where virulence factors are carried on mobile genetic elements rather than the main bacterial chromosome. **Analysis of Options:** * **A. Chromosome:** While the structural genes for many bacterial toxins are chromosomal (e.g., *Corynebacterium diphtheriae* toxin is encoded by a bacteriophage integrated into the chromosome), tetanospasmin is specifically plasmid-mediated. * **C. Both:** The genetic information for tetanospasmin is exclusively located on the plasmid; it is not found on the chromosome. * **D. Transposon:** Transposons are "jumping genes" that can move between plasmids and chromosomes, but they are not the primary stable location for the tetanospasmin gene. **NEET-PG High-Yield Pearls:** * **Toxin Mechanism:** Tetanospasmin is an **A-B toxin** that acts as a zinc metalloproteinase. It cleaves **synaptobrevin** (a V-SNARE protein), preventing the release of inhibitory neurotransmitters **GABA and Glycine** from Renshaw cells in the spinal cord. * **Clinical Correlation:** This leads to "spastic paralysis," characterized by *risus sardonicus* (grimace), trismus (lockjaw), and opisthotonus. * **Comparison:** Unlike *C. tetani* (plasmid), the toxin of ***Clostridium botulinum*** is typically encoded by a **bacteriophage** (lysogenic conversion), though some strains carry it on plasmids or the chromosome. * **Oxygen Requirement:** *C. tetani* is an obligate anaerobe; germination of spores requires a low oxidation-reduction potential (e.g., necrotic tissue).

Q: Genetic material is transferred from one bacterium to another by all of the following mechanisms EXCEPT?

Transfection. **Explanation:** The question asks for the mechanism that is **not** a natural method of genetic transfer between bacteria. **1. Why Transfection is the Correct Answer:** **Transfection** is the process of deliberately introducing naked nucleic acids (DNA or RNA) into **eukaryotic cells** (animal or plant cells), often using chemical or physical methods in a laboratory setting. While it is a vital tool in molecular biology and gene therapy, it is not a natural mechanism for horizontal gene transfer between bacteria. **2. Analysis of Incorrect Options (Natural Bacterial Gene Transfer):** * **Conjugation:** Often called "bacterial sex," this involves the transfer of genetic material (usually plasmids) through direct cell-to-cell contact via a **sex pilus**. It is mediated by the F-plasmid. * **Transduction:** Genetic material is carried from one bacterium to another by a **bacteriophage** (a virus that infects bacteria). It can be generalized or specialized. * **Transformation:** This is the uptake of "naked" DNA fragments from the surrounding environment by a **competent** bacterium. This was famously demonstrated by Griffith’s experiment. **High-Yield Clinical Pearls for NEET-PG:** * **Drug Resistance:** Conjugation is the most common method for the spread of multi-drug resistance (R-plasmids) among clinical isolates. * **Griffith Effect:** Transformation was the first evidence that DNA is the genetic material (using *Streptococcus pneumoniae*). * **Lysogenic Conversion:** A form of specialized transduction where a phage integrates into the bacterial chromosome, making the bacteria pathogenic (e.g., *Corynebacterium diphtheriae* toxin, Cholera toxin, and Shiga toxin). * **Protoplast/Spheroplast:** If DNA is introduced into bacteria artificially in a lab (similar to transfection), the process is technically called **Transformation** or **Electroporation**.

Question 1

Which of the following toxins is carried by a bacteriophage?

Accepted Answer

Verocytotoxin of EHEC

Answer

Cholera toxin

Answer

Heat-labile toxin of E. coli

Answer

Anthrax toxin

Question 2

What is a simple bacterial test for mutagenic carcinogens?

Accepted Answer

Redox test

Answer

Ames test

Answer

Bacteriophage

Answer

Gene splicing

Question 3

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 4

Drug resistance in Tuberculosis is due to?

Accepted Answer

Mutation

Answer

Transformation

Answer

Transduction

Answer

Conjugation

Question 5

Which of the following methods involves the direct transfer of naked DNA from the environment into a bacterial cell?

Accepted Answer

Transformation

Answer

Transduction

Answer

Conjugation

Answer

Transfection

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

Tetanospasmin encoding genes are located on?

Accepted Answer

Plasmid

Answer

Chromosome

Answer

Both

Answer

Transposon

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

Genetic material is transferred from one bacterium to another by all of the following mechanisms EXCEPT?

Accepted Answer

Transfection

Answer

Conjugation

Answer

Transduction

Answer

Transformation

Question 10

The role of plasmids in bacterial conjugation was first described by Lederberg and Tatum in which organism?

Accepted Answer

Escherichia coli

Answer

Haemophilus influenzae

Answer

Corynebacterium species

Answer

Pseudomonas species

Microbial Genetics — MCQs

Microbial Genetics — MCQs

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