Bacteria may evolve complete immunity to the particular antibiotic by first passing through a stage of partial immunity. Partially resistant bacteria can survive a concentration of the antibiotic sufficient to kill susceptible bacteria but will succumb to a greater concentration.
Imagine a scenario in which no single mutation can render the bacteria completely immune to a particular antibiotic, but some mutations convey partial resistance. These mutations are favored only when antibiotic concentrations are low; such conditions lead to a population of partially resistant bacteria. Then another single mutation could result in the partially resistant bacteria becoming either more or completely resistant to the antibiotic.
Partial resistance is the reason why doctors tell you to finish a course of antibiotics — even if you're feeling better and even if you've decided that the last couple of pills are unnecessary. The initial pulse of antibiotics kills all the sensitive bacteria (which is why you feel better), but some of the remaining bacteria may be partially resistant to the antibiotic. Rather than let the partially resistant bacteria off the hook, don't give them a chance to get a leg up on the antibiotic. After all, they're the ones you really want to control. Stop them while you still can!
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