lsothiazolone biocides are known to react with nucleophilic materials. This in part affects their stability in the presence of reducing agents as well as defines their mechanism of action for critical cell reaction sites [1]. Thiols are key active sites on many proteins and enzymes. This study demonstrated that a isothiazolones react with protein thiols, destroying both soluble and insoluble types (Table 2). This reactivity may also be linked to the killing effect of the biocides such that few survivors remain after contact with the biocides and loss of thiols. Once the biocide reacts with a thiol, the ring opens, and the isothiazolone is no longer active. Many different reactions are possible and are illustrated in Figure 9. The relationship between loss of viability and loss of thiol content is shown in Figure 10. A strong correlation is associated between CMIT killing and thiol destruction.
Thiol-active sites are common to dehydrogenase enzymes and other proteins as well. Thus, the strong reactivity of isothiazolones to thiols is linked to cell death and inhibition of critical cell functions. Various reduced-sulfur molecules exist in microbial cells including cysteine, cystine, thiogly-collate, and glutathione. These have all been implicated in the mechanism of action of the different isothiazolone biocides [1,4-11].