Methylchloroisothiazolone-induced growth inhibition and lethality in Escherichia coli

Exposure of log phase Escherichia coli cells to inhibitory levels of 5-chloro-2-methyl-isothiazolin-3-one (MCI) results in rapid bacteriostasis and a delayed onset of bactericidal activity. Inhibition of respiration occurs within the same time frame as bacteriostasis, and is followed by a decline in intracellular ATP levels. In vitro and in vivo experiments suggest that growth inhibition is the result of selective inhibition of particular targets, with succinate dehydrogenase being identified as a possible target. Such selectivity was not anticipated from this highly reactive molecule. MCI-induced lethality is positively correlated with a loss of reduced protein sulphydryls (r2 = 0.79). A greater than equimolar loss of reduced protein sulphydryls, compared with the number of MCI molecules added, and a reduction in killing by MCI after induction of the OxyR regulon suggest that free radical generation may have a role in the antibacterial activity of MCI. We present an examination of the in vivo effects of MCI exposure on bacterial cells, and evidence that the isothiazolones exhibit selectivity in their cellular targets and antimicrobial effects.