ABSTRACT Biocides are widely used to control unwanted biological growth. Isothiazolinone biocides are one class of biocides used to stabilize a range of products against microbial contamination and degradation. Although the development of resistance to such biocides is thought to be unlikely, we describe a system capable of detoxifying the common isothiazolinone biocide MIT/CMIT (2-methyl-4-isothiazolin-3-one and 5-chloro-2-methyl-4-isothiazolin-3-one). It was found in Klebsiella oxytoca complex bacteria contaminating a commercial liquid soap that contained the isothiazolinone as a preservative. The isolate inactivates MIT/CMIT via a glutathione-dependent reaction catalyzed by a plasmid-encoded glutathione transferase system. This system comprises a glutathione transferase ( gstA ) and a hydroxyacylglutathione hydrolase ( gloB ), both located in one operon on an 80-kbp plasmid and co-localized with an arsenic resistance cassette. The plasmid can be transferred by conjugation to Escherichia coli . The findings highlight the risk that broadening biocide use can increasingly drive the evolution, selection, and spread of different resistance mechanisms. IMPORTANCE Biocide usage is increasing worldwide to combat organisms causing spoilage or infectious disease. However, the use of biocides is also recognized as a selection factor that shapes microbial communities and drives adaptation. Isothiazolinone biocides are mainly used to stabilize liquid products, personal care products, and fuel against microbial spoilage. The detoxification system described here allowed bacteria to survive in a liquid soap cleaning product containing the widely used biocide preservative MIT/CMIT. Importantly, the biocide detoxification system is mobile between different species and co-localized with a metal resistance operon. The system thereby demonstrates how the spread and co-selection of resistances can occur in bacterial populations exposed to biocides.
Hauf et al. (Fri,) studied this question.