Test: Bacterial Genetics and Antimicrobial Resistance - NEET PG MCQ

• Capsulated dead S. pneumoniae + non-capsulated live S. pneumoniae →
• Transformation of the gene responsible for capsule production from dead to live pneumococci →
• Yields capsulated live pneumococci.

Plasmids are self-replicating, extra-chromosomal entities that are often exchanged through the process of conjugation.

• Lysogenic conversion would serve as a more accurate term in this context.
• This process involves the integration of phage DNA into bacterial DNA, where it persists as a lysogenic phage.
• In these instances, specific phage genes (for example, the gene responsible for diphtheria toxin) confer toxigenicity to the bacteria.

Transformation refers to the process whereby free DNA is transferred from one bacterium to another.

Conjugation refers to the mechanism through which genetic elements are transferred from one bacterium (male) to another (female) via a sex pilus or a conjugation tube. This process is a form of horizontal genetic transfer.

Transfer of resistance through conjugation is the predominant method by which bacterial resistance genes are shared.

Enzyme inactivation is the most frequently encountered mechanism of drug resistance in bacteria.

• Pneumococcal resistance is chiefly caused by alterations to the target, specifically the Penicillin-binding protein (PBP).
• Resistance to Vancomycin results from the complete removal of the target D-alanyl-D-alanine, which is present in the bacterial cell wall. This is the binding site for Vancomycin, which inhibits cell wall synthesis.

Resistance (R) factors are extrachromosomal plasmids that facilitate the dissemination of multiple drug resistance in bacteria. They are composed of circular double-stranded DNA that encodes genes for a range of enzymes capable of deactivating antibiotics. The R factor comprises two main elements:

• Resistance transfer factor (RTF), which is responsible for the conjugational transfer.
• Each r determinant that confers resistance to a specific antibiotic.

ESBL (Extended spectrum beta-lactamases) exhibit resistance to all penicillins and 1st, 2nd, and 3rd generation cephalosporins, as well as monobactams. This resistance can be countered by adding a β-lactamase inhibitor such as clavulanic acid. Other alternatives that may be utilised include:

• Carbapenems such as imipenem and meropenem,
• Cephamycins like cefoxitin and cefotetan,
• Various classes of antibiotics, including aminoglycosides.

Extended Spectrum-β-Lactamases (ESBL) producing Pseudomonas can be managed using an antipseudomonial β-lactam, such as:

• piperacillin

alongside a β-lactamase inhibitor like:

• tazobactam combination therapy.

Kirby Bauer’s disk diffusion technique is employed to determine the zone of inhibition around the disk where the streaked organism resides, allowing us to ascertain whether the organism is sensitive or resistant to the antibiotic disk. However, this method does not provide information on the MIC.

The MIC (Minimal Inhibitory Concentration) of an antibiotic is defined as the smallest concentration of an antimicrobial substance that will prevent the visible growth of a microorganism. The MIC can be determined through the following methods:

• Agar dilution method,
• Broth dilution method,
• Epsilometer (E test).

• A 0.5 McFarland standard is created by combining 0.05 mL of barium chloride dihydrate with 9.95 mL of 1% sulfuric acid.
• This mixture corresponds to approximately 150 million bacteria/mL in a broth.
• For the antibiotic sensitivity test, the broth of the organism should align with the 0.5 McFarland standard.

Beta-lactamase enzymes are encoded by plasmids and are generated by both gram-positive and gram-negative organisms.