This investigation was to determine whether Pseudomonas aeruginosa cou
ld acquire resistance to the bactericide isothiazolone, and what the n
ature of such a resistance mechanism would be. The Pseudomonas was cul
tured in nutrient-limited broth in the presence of sub-inhibitory conc
entrations of isothiazolone (a mixture of 1.15% 5-chloro-N-methylisoth
iazolone (CMIT) and 0.35% N-methylisothiazolone (MIT)). Three cultures
tested in parallel adapted gradually during exposure for 15 d from an
initial minimum inhibitory concentration (MIC) of 300 mu l l(-1) to 6
07 mu l l(-1). The three parallel cultures adapted at similar rates, s
o the adaptation was not ascribed to mutation but to a specific mechan
ism. Resistant cells did not produce any extracellular isothiazolone-q
uenching compounds nor undergo detectable alterations in their lipopol
ysaccharide layer. In wild cells, a 35 kDa outer membrane protein (pro
tein T) was detectable, whereas resistant cells lacked this protein. P
roduction of protein T was suppressed within 24 h of exposure to isoth
iazolone. It was still suppressed after 72 h of growth in isothiazolon
e-free medium. It is proposed that Ps. aeruginosa acquires resistance
to isothiazolone by a process of adaptation where the outer membrane p
rotein T is suppressed.