Benzisothiazolone (BIT), N-methylisothiazolone (MIT) and 5-chloro-N-methyli
sothiazolone (CMIT) are highly effective biocidal agents and are used as pr
eservatives in a variety of cosmetic preparations. The isothiazolones have
proven efficacy against many fungal and bacterial species including Pseudom
onas aeruginosa. However, some species are beginning to exhibit resistance
towards this group of compounds after extended exposure. This experiment in
duced resistance in cultures of Ps. aeruginosa exposed to incrementally inc
reasing sub-minimum inhibitory concentrations (MICs) of the isothiazolones
in their pure chemical forms. The induced resistance was observed as a grad
ual increase in MIC with each new passage. The MICs for all three test isot
hiazolones and a thiol-interactive control compound (thiomersal) increased
by approximately twofold during the course of the experiment. The onset of
resistance was also observed by reference to the altered presence of an out
er membrane protein, designated the T-OMP, in SDS-PAGE preparations. T-OMP
was observed to disappear from the biocide-exposed preparations and reappea
r when the resistance-induced cultures were passaged in the absence of bioc
ide. This reappearance of T-OMP was not accompanied by a complete reversal
of induced resistance, but by a small decrease in MIC. The induction of res
istance towards one biocide resulted in the development of cross-resistance
towards other members of the group and the control, thiomersal. It has bee
n suggested that the disappearance of T-OMP from these preparations is asso
ciated with the onset of resistance to the isothiazolones in their Kathon(T
M) form (CMIT and MIT).