A transferrable, plasmid-mediated mechanism of colistin resistance was first described in Escherichia coli isolates from food animals in China. What is its mechanism?

• A Hydrolysis of the polymyxin ring by a periplasmic protease
• B Methylation of the 23S ribosomal RNA target by a methyltransferase
• C Increased expression of the outer membrane porin OprD, accelerating drug influx
• D Addition of phosphoethanolamine to lipid A of lipopolysaccharide by a transferase enzyme, reducing colistin binding ✓

Explanation

The mcr-1 gene encodes a phosphoethanolamine transferase that modifies the phosphate groups of lipid A, decreasing the net negative charge of lipopolysaccharide and lowering colistin affinity for its binding site. Its location on a conjugative plasmid makes horizontal spread possible, unlike earlier chromosomal mutations in the pmrAB and phoPQ regulatory systems. Proteolytic degradation of polymyxins is not a recognised resistance route, OprD loss causes carbapenem resistance in Pseudomonas, and 23S rRNA methylation is the mechanism of linezolid resistance.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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