RESISTANCE OF PSEUDOMONAS-AERUGINOSA TO ISOTHIAZOLONE

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.