The mechanism of action of isothiazolone biocides is complex and considered a two-step process involving rapid growth inhibition leading to a loss of viability. Growth inhibition is the result of rapid disruption of the central metabolic pathways of the cell by inhibition of several specific enzymes, including dehydrogenases. The specific dehydrogenase enzymes which react with isothiazolones include those involved in the tricarboxylic acid (Krebs) cycle (alpha-ketoglutarate, pyruvate dehydrogenase, succinate dehydrogenase, and lactate dehydrogenase) and energy generation (NADH dehydrogenase). Key physiological activities that are rapidly inhibited in microbial cells are respiration (oxygen consumption) and energy generation (ATP synthesis). Many of these key enzymes are present in both aerobic and anaerobic microorganisms, which explains why CMIT/MIT is such a broad spectrum biocide. Biocide transport studies showed DCOIT and CMIT were rapidly transported inside the cell via a process which requires energy, whereas the MIT component enters the cell more slowly, through a more typical diffusion process.
Inhibition of cellular activity by isothiazolones is rapid and occurs within minutes, whereas cell death (cidal activity) is observed after several hours contact. In general, the higher the concentration of biocide, the shorter the contact time required for more complete kill. Cell death results from the progressive loss of protein thiols in the cell from one of multiple pathways (Figure 13). As critical pathways in cell metabolism are disrupted, free radicals are produced which also results in cell death. Isothiazolones also inhibit ATP synthesis and utilization, which affects the potential of the cells to maintain the energy balance for growth and cell repair. This unique mechanism results in the broad spectrum of activity, low use levels for microbial control, and difficulty in attaining resistance.