Table 1.

Molecular mechanisms of antibiotic resistance.

↓ Permeability	Outer membrane forms a permeability barrier (Gram positive >Gram negative). Down-regulation of porins or by the replacement of porins with more selective channels.
↑ Efflux	Bacterial efflux pumps actively transport many antibiotics out of the cell (multidrug resistance [MDR] efflux pumps).
Mutation and Transformation in Antibiotic Target Structures	Changes to the target structure that prevent efficient antibiotic binding: Transformation can confer antibiotic resistance by target protein modification through the formation of ‘mosaic’ genes. Acquisition of a gene homologous to the original target. Protection by modification of the target: Erythromycin ribosome methylase (erm). Chloramphenicol–florfenicol resistance (cfr) methyltransferase. Quinolone resistance (qnr) gene.