Microbial Genetics — MCQs

Microbial Genetics Indian Medical PG Practice Questions and MCQs

Question 11: In transduction, DNA transmitted by a vector to bacteria belongs to which category?

A. Virus
B. Phage
C. Bacteriophage (Correct Answer)
D. Diploid cell

Explanation: ### Explanation **Concept Overview:** Transduction is the process of horizontal gene transfer in bacteria where genetic material is conveyed from a donor to a recipient cell via a viral vector. This process is mediated specifically by **bacteriophages** (viruses that infect bacteria). **Why Option C is Correct:** During the viral replication cycle (lytic or lysogenic), a segment of bacterial DNA may be accidentally packaged into the viral capsid instead of the viral genome. When this "transducing particle" infects a new bacterium, it injects the donor bacterial DNA into the recipient. Because the vector is a virus that specifically targets bacteria, it is termed a **bacteriophage**. **Analysis of Incorrect Options:** * **A. Virus:** While a bacteriophage is a type of virus, "Virus" is a broad general term. In microbiology, when discussing bacterial genetics, the specific term "Bacteriophage" is the most accurate and preferred nomenclature. * **B. Phage:** "Phage" is a common shorthand, but "Bacteriophage" is the complete scientific term used in standard medical examinations. * **D. Diploid cell:** Bacteria are typically haploid. A diploid cell refers to eukaryotic cells with two sets of chromosomes; it is not a vector for DNA transfer in prokaryotes. **High-Yield Clinical Pearls for NEET-PG:** * **Generalized Transduction:** Occurs during the lytic cycle; *any* part of the bacterial genome can be transferred. * **Specialized Transduction:** Occurs during the lysogenic cycle; only specific genes adjacent to the prophage insertion site are transferred (e.g., Shiga-like toxin in *E. coli*, Diphtheria toxin in *C. diphtheriae*, and Erythrogenic toxin in *S. pyogenes*). * **Mnemonic for Specialized Transduction (ABCD'S):** **A** group Strep, **B**otulinum toxin, **C**holera toxin, **D**iphtheria toxin, **S**higa toxin. These are all encoded by lysogenic phages.

Question 12: Multiple drug resistance of bacteria depends on which of the following?

A. Resistance transfer factor (RTF) (Correct Answer)
B. Colicigenic factor (Col)
C. Fertility factor (F+)
D. All of the above

Explanation: ### Explanation **Correct Answer: A. Resistance transfer factor (RTF)** The primary mechanism for the spread of multiple drug resistance among bacteria is through **R-plasmids**. An R-plasmid consists of two distinct components: 1. **Resistance Transfer Factor (RTF):** This is the segment responsible for the initiation and control of plasmid replication and the **conjugative transfer** of the plasmid between bacteria. 2. **r-determinant:** This segment carries the specific genes that code for resistance against various antibiotics (e.g., sulfonamides, streptomycin, chloramphenicol). While the r-determinant holds the resistance genes, the **RTF is essential for the dissemination** of these genes across bacterial populations. Without the RTF, the resistance remains localized to a single cell; with it, multiple drug resistance can spread rapidly via conjugation, even between different species. **Why other options are incorrect:** * **B. Colicigenic factor (Col):** These plasmids code for **colicins** (bacteriocins), which are proteins that inhibit or kill closely related bacterial strains. They do not mediate antibiotic resistance. * **C. Fertility factor (F+):** This is the "sex factor" plasmid that allows for the formation of a sex pilus and DNA transfer. While it facilitates conjugation, it does not inherently carry multiple drug resistance genes. * **D. All of the above:** Incorrect because Col and F factors have distinct physiological roles unrelated to antibiotic resistance. --- ### High-Yield Clinical Pearls for NEET-PG * **R-plasmids** are most commonly found in Gram-negative bacteria (e.g., *E. coli, Shigella, Salmonella*). * **Conjugation** is the most significant clinical method for the horizontal transfer of multi-drug resistance. * **Transposons ("Jumping Genes"):** These are often found within R-plasmids and can move resistance genes between the plasmid and the bacterial chromosome. * **Medical Importance:** R-plasmids are the reason why a patient can become resistant to multiple unrelated antibiotics simultaneously after exposure to just one.

Question 13: Pneumococcal resistance to penicillin G is mainly acquired by:

A. Conjugation
B. Transduction
C. Transformation (Correct Answer)
D. All of the above

Explanation: **Explanation:** **1. Why Transformation is Correct:** *Streptococcus pneumoniae* (Pneumococcus) is naturally **competent**, meaning it has the innate ability to take up exogenous DNA from its environment. Resistance to Penicillin G in Pneumococci is not mediated by beta-lactamase production; instead, it occurs through the **alteration of Penicillin-Binding Proteins (PBPs)**, specifically PBP 2b, 2x, and 1a. The bacteria acquire "foreign" PBP genes from related oral streptococci (like *S. mitis*) via **Transformation**. This results in the formation of **mosaic genes**, which encode PBPs with a significantly decreased affinity for penicillin, leading to resistance. **2. Why Other Options are Incorrect:** * **Conjugation (A):** This involves the transfer of genetic material (usually plasmids) through direct cell-to-cell contact via a sex pilus. While common in Gram-negative enteric bacteria (e.g., *E. coli*), it is not the primary mechanism for penicillin resistance in Pneumococci. * **Transduction (B):** This is the transfer of DNA from one bacterium to another via a bacteriophage (virus). While it occurs in *Staphylococci* (e.g., for penicillinase plasmids), it is not the mechanism for PBP alteration in *S. pneumoniae*. **3. NEET-PG High-Yield Pearls:** * **Mechanism of Resistance:** Remember: Pneumococcus = **Mosaic PBPs** (via Transformation); Staphylococcus = **Beta-lactamase** (via Transduction) or **mecA gene/PBP2a** (for MRSA). * **Griffith Experiment:** The classic "Mouse and Pneumococcus" experiment (Smooth vs. Rough strains) was the first to demonstrate the "Transforming Principle." * **Drug of Choice:** For highly penicillin-resistant Pneumococci, **Vancomycin** or **Linezolid** are typically used. * **Other naturally competent bacteria:** *Haemophilus influenzae*, *Neisseria gonorrhoeae*, and *Bacillus subtilis*.

Question 14: Non-toxigenic C. diphtheriae is converted to toxigenic C. diphtheriae with the help of a bacteriophage. By which method does this conversion occur?

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

Explanation: **Explanation:** The conversion of a non-toxigenic strain of *Corynebacterium diphtheriae* into a toxigenic one is a classic example of **Lysogenic Conversion**, which is a specialized form of **Transduction**. **1. Why Transduction is Correct:** Transduction is the transfer of bacterial DNA from one cell to another via a **bacteriophage** (a virus that infects bacteria). In the case of *C. diphtheriae*, the gene responsible for toxin production (the *tox* gene) is not part of the bacterial chromosome itself; it is carried by a specific temperate bacteriophage called the **Beta-phage**. When this phage infects a non-toxigenic bacterium and integrates its genome into the bacterial chromosome (lysogeny), the bacterium begins to produce the diphtheria toxin. **2. Why Other Options are Incorrect:** * **Transformation:** This involves the uptake of "naked" DNA directly from the surrounding environment. It does not require a viral vector. * **Conjugation:** This is "bacterial sex" involving direct cell-to-cell contact through a sex pilus to transfer genetic material (usually plasmids). * **Recombinant Technology:** This is an artificial laboratory technique used to manipulate DNA; it is not the natural biological process by which *C. diphtheriae* acquires virulence. **Clinical Pearls for NEET-PG:** * **Key Concept:** "No Phage = No Toxin." Only strains of *C. diphtheriae* lysogenized by the Beta-phage cause the clinical disease Diphtheria. * **Other examples of Lysogenic Conversion:** * *Vibrio cholerae* (CTX phage) * *Streptococcus pyogenes* (Erythrogenic toxin/Scarlet fever) * *Clostridium botulinum* (Botulinum toxin) * *Shigella dysenteriae* (Shiga toxin) * **High-Yield Test:** The **Elek’s Gel Precipitation Test** is used to detect the production of this toxin in vitro.

Question 15: Which bacteria was used in Griffith's experiment?

A. Pneumococci (Correct Answer)
B. Neisseria
C. Acinetobacter
D. Pseudomonas

Explanation: **Explanation:** **Griffith’s Experiment and Transformation:** The correct answer is **Pneumococci (*Streptococcus pneumoniae*)**. In 1928, Frederick Griffith conducted a landmark experiment that provided the first evidence of the "transforming principle." He used two strains of *S. pneumoniae*: 1. **Smooth (S) strain:** Possesses a polysaccharide capsule, making it virulent (pathogenic). 2. **Rough (R) strain:** Lacks a capsule and is non-virulent. Griffith observed that when heat-killed S-strain bacteria were mixed with live R-strain bacteria and injected into mice, the mice died. He recovered live S-strain bacteria from the blood, concluding that the R-strain had been "transformed" by picking up genetic material from the dead S-strain. This process is now known as **Natural Transformation**. **Analysis of Incorrect Options:** * **Neisseria, Acinetobacter, and Pseudomonas:** While these genera are also capable of natural transformation (the ability to take up exogenous DNA from the environment), they were not the organisms used in Griffith’s original discovery. Griffith specifically chose *S. pneumoniae* due to the clear phenotypic marker of the capsule. **NEET-PG High-Yield Pearls:** * **Avery, MacLeod, and McCarty (1944):** Later proved that the "transforming principle" Griffith discovered was **DNA**. * **Competence:** The ability of a bacterium to take up extracellular DNA. *S. pneumoniae*, *H. influenzae*, and *Neisseria* are naturally competent. * **Clinical Correlation:** The capsule of *S. pneumoniae* is its chief virulence factor; non-capsulated strains are generally non-pathogenic. This is the basis for the Pneumococcal Polysaccharide Vaccine (PPSV23).

Question 16: Phage conversion is a mechanism by which genetic material is transferred between bacteria. Which of the following bacterial infections requires phage conversion for its pathogenesis?

A. Tularemia
B. Diphtheria (Correct Answer)
C. Gonorrhoea
D. All of the above

Explanation: ### Explanation **1. Why Diphtheria is Correct:** The pathogenesis of **Diphtheria** is primarily due to the production of the **Diphtheria toxin (DT)**. This toxin is not encoded by the bacterial chromosome of *Corynebacterium diphtheriae* itself, but by the **tox gene** carried by a temperate bacteriophage (specifically the **Beta-phage**). * **Phage Conversion (Lysogenic Conversion):** This occurs when a non-pathogenic bacterium becomes pathogenic after being infected by a temperate phage, which integrates its genome into the bacterial chromosome (prophage). Only strains of *C. diphtheriae* lysogenized by the Beta-phage cause the clinical disease Diphtheria. **2. Why Other Options are Incorrect:** * **Tularemia (*Francisella tularensis*):** Pathogenesis is driven by its ability to survive as an intracellular pathogen within macrophages and its unique capsule. It does not rely on phage-encoded toxins. * **Gonorrhoea (*Neisseria gonorrhoeae*):** Pathogenesis involves pili for attachment, Opa proteins, and IgA protease. While it undergoes high rates of **transformation** and antigenic variation, it does not require phage conversion for its primary virulence. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Phage-Encoded Toxins (COBEDS):** * **C** – **C**holera toxin (*Vibrio cholerae*) * **O** – **O** antigen of Salmonella * **B** – **B**otulinum toxin (*Clostridium botulinum*) * **E** – **E**rythrogenic toxin (*Streptococcus pyogenes* - Scarlet fever) * **D** – **D**iphtheria toxin (*C. diphtheriae*) * **S** – **S**higa toxin (*Shigella dysenteriae*) * **Mechanism of DT:** It inhibits protein synthesis by catalyzing the ADP-ribosylation of **Elongation Factor-2 (EF-2)**. * **Elek’s Gel Precipitation Test:** Used to detect the production of this phage-encoded toxin in *C. diphtheriae* isolates.

Question 17: Which of the following statements about plasmids is false?

A. Plasmids are extrachromosomal double-stranded circular DNA.
B. Plasmids are essential for life. (Correct Answer)
C. Plasmids are capable of replicating independently.
D. Curing is the process of eliminating plasmids from bacteria.

Explanation: ### Explanation **Why Option B is the Correct Answer (The False Statement):** Plasmids are defined as **non-essential** genetic elements. While they provide significant survival advantages—such as antibiotic resistance or toxin production—they are not required for the basic growth, metabolism, or reproduction of the bacterial cell under normal conditions. The essential genetic information required for life is housed in the bacterial chromosome. **Analysis of Other Options:** * **Option A:** This is a correct definition. Plasmids are typically double-stranded, circular DNA molecules found in the cytoplasm, physically separate from the genomic DNA. * **Option C:** Plasmids are **replicons**; they possess an "Origin of Replication" (ori) which allows them to replicate autonomously using the host cell's machinery, independent of the chromosomal division cycle. * **Option D:** **Curing** is the term used for the loss of a plasmid from a bacterial lineage. This can occur spontaneously or be induced by chemical agents (e.g., acridine orange) or physical stressors (e.g., UV light, high temperature) that inhibit plasmid replication without affecting chromosomal replication. **High-Yield Clinical Pearls for NEET-PG:** * **R-Plasmids:** Carry genes for antibiotic resistance (e.g., $\beta$-lactamase). These are the most clinically significant plasmids. * **F-Plasmids (Fertility factors):** Mediate the process of **Conjugation** (horizontal gene transfer) via sex pili. * **Col-Plasmids:** Encode **Bacteriocins** (e.g., Colicins), which are proteins that kill other closely related bacteria. * **Virulence Plasmids:** Carry genes for toxins (e.g., Anthrax toxin, Enterotoxins of *E. coli*). * **Size:** Plasmids vary in size from 1 kb to over 200 kb, but they are always significantly smaller than the bacterial chromosome.

Question 18: Transfer of some chromosomal material from one bacterium to another uses which of the following?

A. F factor
B. R factor
C. Transfer factor
D. All of the above (Correct Answer)

Explanation: ### Explanation The transfer of chromosomal material between bacteria primarily occurs through **Conjugation**, a process requiring direct cell-to-cell contact mediated by specific genetic elements called **Plasmids**. **1. Why "All of the above" is correct:** All three factors listed are types of plasmids or genetic elements that facilitate the transfer of DNA: * **F factor (Fertility factor):** This is the classic conjugative plasmid. In **Hfr (High-frequency recombination) cells**, the F factor integrates into the bacterial chromosome. When conjugation occurs, it drags a portion of the host chromosome into the recipient cell. * **R factor (Resistance factor):** These plasmids carry genes for antibiotic resistance. They consist of two parts: the **r-determinant** (resistance genes) and the **RTF (Resistance Transfer Factor)**. The RTF behaves similarly to the F factor, enabling the transfer of the plasmid and occasionally adjacent chromosomal DNA. * **Transfer factor:** This is a functional term often used interchangeably with the RTF component of R plasmids or any genetic element (like the F factor) that initiates the conjugation process and DNA transfer. **2. Analysis of Options:** * **A & B:** Both are specific types of conjugative plasmids capable of mobilizing chromosomal material. * **C:** This is the generic functional unit required for the physical transfer of genetic material. ### Clinical Pearls for NEET-PG * **Hfr State:** Occurs when the F plasmid integrates into the *E. coli* chromosome via insertion sequences. It is the most efficient way chromosomal genes are transferred. * **R Plasmids:** These are the most common cause of **multi-drug resistance (MDR)** in clinical settings (e.g., *Shigella*, *Salmonella*). * **Directionality:** DNA transfer in conjugation is always **unidirectional**, from a donor (F+, Hfr, or R+) to a recipient (F- or R-). * **Transformation vs. Transduction:** Remember that Transformation involves "naked" DNA uptake, while Transduction is mediated by a bacteriophage. Conjugation is the only one requiring **pili** and cell contact.

Question 19: The Hershey-Chase experiment was performed on which of the following?

A. Lactobacillus
B. Tuberculous bacteria
C. Bacteriophages (Correct Answer)
D. Mycoplasma

Explanation: ### Explanation **Correct Answer: C. Bacteriophages** The **Hershey-Chase experiment (1952)**, also known as the "blender experiment," provided definitive proof that **DNA, not protein, is the genetic material**. **Underlying Concept:** Alfred Hershey and Martha Chase utilized **T2 bacteriophages** (viruses that infect bacteria) for their study. They exploited the chemical differences between DNA and proteins: * **DNA** contains Phosphorus but no Sulfur. They labeled it with radioactive **$^{32}P$**. * **Proteins** contain Sulfur (in cysteine/methionine) but no Phosphorus. They labeled it with radioactive **$^{35}S$**. When the labeled phages infected *E. coli*, only the $^{32}P$ (DNA) was found inside the bacterial cells, while the $^{35}S$ (protein coat) remained outside. This proved that DNA is the molecule responsible for carrying genetic information into the host. **Why Incorrect Options are Wrong:** * **A & B (Lactobacillus & Tuberculous bacteria):** While bacteria were used as the *host* (*E. coli*), the experiment was specifically designed to observe the material injected *by* a virus into a cell. * **D (Mycoplasma):** These are the smallest free-living organisms and lack a cell wall, but they were not involved in this landmark molecular biology study. **High-Yield Clinical Pearls for NEET-PG:** * **Griffith’s Experiment (1928):** Discovered the "Transforming Principle" using *Streptococcus pneumoniae* in mice. * **Avery, MacLeod, and McCarty (1944):** First to suggest DNA was the transforming principle (preceded Hershey-Chase). * **Bacteriophage Structure:** Consists of a nucleic acid core (DNA/RNA) and a protein coat called a **capsid**. * **Transduction:** The process by which a bacteriophage transfers genetic material from one bacterium to another (a key mechanism of antibiotic resistance transfer).

Question 20: What genetic peculiarity distinguishes pathogenic/toxigenic strains of Corynebacterium diphtheriae from nonpathogenic strains?

A. Presence of R-factor
B. Presence of gene
C. Integrated temperate phage (Correct Answer)
D. Presence of MDR gene

Explanation: **Explanation:** The pathogenicity of *Corynebacterium diphtheriae* is primarily determined by its ability to produce the **Diphtheria Toxin (DT)**. The gene responsible for this toxin (the *tox* gene) is not part of the bacterial chromosome itself. Instead, it is introduced into the bacterium via **lysogenic conversion** by a **temperate bacteriophage** (specifically the **Beta-phage**). When this phage integrates its DNA into the bacterial genome (becoming a prophage), the previously non-toxigenic strain becomes toxigenic and pathogenic. **Analysis of Options:** * **Option C (Correct):** The integration of the Beta-phage carries the *tox* gene. Without this specific temperate phage, the organism cannot cause the systemic manifestations of diphtheria. * **Option A & D (Incorrect):** R-factors (Resistance factors) and MDR (Multi-Drug Resistance) genes are associated with antibiotic resistance, not the primary virulence or toxin production of *C. diphtheriae*. * **Option B (Incorrect):** While a "gene" is technically involved, the "peculiarity" lies in the *source* of that gene—the integrated phage—making Option C the more specific and accurate answer. **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 concentrations inhibit toxin production via the **DtxR** (Diphtheria toxin repressor) protein. * **Diagnosis:** Toxigenicity is confirmed using the **Elek’s Gel Precipitation Test** or PCR for the *tox* gene. * **Culture:** Use **Loeffler’s Serum Slope** (rapid growth) or **Potassium Tellurite Agar** (black colonies).

Practice by Chapter

Bacterial Genome Organization

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Plasmids and Mobile Genetic Elements

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Bacterial Gene Expression

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Mutation and Mutagenesis

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Gene Transfer in Bacteria

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Transposons and Insertion Sequences

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Bacterial Genetic Recombination

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Regulation of Gene Expression

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CRISPR-Cas Systems

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Bacterial Stress Responses

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Genetics of Antimicrobial Resistance

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Genetic Basis of Bacterial Virulence

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