Phenotypic Resistance Occurs Without Mutations

Resistance can arise without changes in DNA. The stress of antibiotic treatment causes cells to produce protective proteins using existing genes. An example is induced expression of ß-lactamase genes by some Gram-negative bacteria. Release of protective enzymes into the growth medium then enables neighboring cells to grow in the presence of the antibiotic. As pointed out in Box 5-3, many ß-lactamase genes exist. Some are inducible, whereas many others are always active.