Wikipedia (definition) - Adaptive Response

The adaptive response is a form of direct DNA repair in E. coli that is initiated against alkylation, particularly methylation, of guanine or thymine nucleotides or phosphate groups on the sugar-phosphate backbone of DNA. Under sustained exposure to low-level treatment with alkylating mutagens, E. coli can adapt to the presence of the mutagen, rendering subsequent treatment with high doses of the same agent less effective.