FIG 9.

Selection of antibiotic resistance. (A) In the preantibiotic era, the presence of antibiotic-resistant microorganisms was rare. The introduction of antibiotics led to killing of antibiotic-susceptible bacterial populations; the selection of the minority antibiotic-resistant populations, which were present in the preantibiotic era; the dispersion of plasmid-encoded mechanisms of resistance; as well as the generation of new resistant strains (by selecting chromosomal mutations). This phenomenon led to the current high levels of antibiotic resistance. (B) Antibiotic resistance may be stable, with a minimal or null effect on bacterial fitness, thus being easily fixed in the bacterial population. The antibiotic-resistant microorganisms tend to be present even in the absence of antibiotic pressure. Antibiotic resistance may also be unstable, affecting bacterial fitness; the absence of antibiotic pressure tends to dilute in the general bacterial population. Nonetheless, antibiotic resistance may act in an indirect manner by other antibacterial agents favoring the fixing of genetic structures carrying multiple mechanisms of antibiotic resistance. Compensatory mutations are mutations which have no direct effect on antibiotic resistance but lead to the recovery of lost fitness. When selected, the microorganisms act as those with stable resistance.