In other words, the bacteria display resistance to the inhibitory or killing actions of the antimicrobial. Many bacteria are naturally insensitive to one or more antimicrobials. This is why doctors and veterinarians prescribe different antimicrobial drugs based on which bacterial disease is diagnosed. Antimicrobial resistance and antimicrobial residues are separate issues. The occurrence of residues above acceptable safety limits is very rare but can be a strong indicator of irresponsible use of antimicrobials.
Bacterial strains can be resistant in their normal state to one or more specific antimicrobials.
Resistance can be acquired. Bacteria are constantly changing in order to survive. During the course of an antimicrobial treatment there are some bacteria that are naturally stronger or mutate to become less susceptible to the action of the drug. These genetic mutations, which can occur quite readily in bacteria, can spontaneously produce a new resistant trait or enhance an existing one. Bacteria replicate by cell division, which means they inherit the resistance genes from their forerunners.