ADAPTATION OF BACTERIAL CULTURES TO NONOXIDIZING WATER-TREATMENT BACTERICIDES

Bacterial communities in water-cooling systems treated with bactericid es often become resistant to these bactericides. This has been ascribe d to selection for resistant cells. Certain bacteria, having a high in herent susceptibility to water treatment bactericides, become dominant in systems after bactericide treatment. We investigated the idea that bacterial isolates adapt to growth in the presence of bactericides. P ure cultures of Pseudomonas stutzeri and Bacillus cereus were cultured repeatedly in the presence of sub-inhibitory concentrations of 2,2'-m ethylenebis(4-chlorophenol), sodium dimethyldithiocarbamate and isothi azolone. Both isolates adapted to growth in the presence of increasing concentrations of the bactericides. P. stutzeri adapted from 22 mug.m l-1 2,2'-methylenebis(4-chlorophenol) to 80 mug.ml-1, from 12 mug.ml-1 Na dimethyldithiocarbamate to 310 mug.ml-1, and from 50 mul.l-1 isoth iazolone to 250 mul.l-1. B. cereus adapted from 20 mug.ml-1 2,2'-methy lenebis(4-chlorophenol) to 75 mug.ml-1, from 6 mug.ml-1 Na dimethyldit hiocarbamate to 132 mug.ml-1, and from 50 mul.l-1 isothiazolone to 300 mul.l-1. The phenomenon of resistance to water treatment bactericides can be ascribed not only to selection but also to adaptation.