The zone of inhibition is an area around a plate treated with antibiotics where microbes do not grow. The phenomenon is useful when evaluating a microbe's resistance to an antibiotic.
The zone of inhibition not only shows whether a species of microbe is resistant to an antibiotic, but it also shows the degree of resistance. If the antibiotic is especially effective against a species of microbe, a large zone of inhibition is observed around the treated area. On the other hand, if the species of microbe is highly resistant to the antibiotic, the observed zone of inhibition is small or nonexistent.