Why can microbial resistance to antibiotics considered a genetic phenomenon

Microbial resistance is considered a genetic phenomenon because there are two major ways that bacteria acquire resistance: through mutations of chromosomal genes or through acquisition of new genes carried on R-plasmids. In both cases, the cell gains the ability to resist the activity of a particular drug through the modified structure or activity of proteins that are coded for by the genes in question. For example, some of these altered proteins can be cytoplasmic membrane proteins or porin proteins that no longer allow a drug such as penicillin to enter the cell. Also, some of the proteins coded for by these genes may be enzymes of various sorts that serve to inactivate a particular drug, such as the inactivation of beta-lactam antibiotics by beta-lactamases.

Furthermore, these altered genes and proteins are then heritable by the offspring of the cell that acquired the resistance, leading quickly to entire populations of bacteria or other microbes that are resistant to a drug. When selective pressure is then brought to bear (through the administration of a particular drug) on a population that contains both susceptible and resistant cells, the result is that the susceptible cells die off, leaving the resistant cells to grow and flourish.