Laboratory and Field Performance of 2-(Thiocyanomethylthio)benzothiazole against Fast-growing Nontuberculous Mycobacteria Found in Metalworking Fluid

Uncontrolled, fast-growing nontuberculous mycobacteria (NTM) in metalworking fluids (MWF) can cause dermal lesions, hypersensitivity pneumonitis, and other health complications in exposed metalworkers. Although biocides are routinely used to control microbial growth in MWF, they are often ineffective because they are unsuitable for the physical or chemical conditions of the system, added infrequently, or ineffective against the microorganisms present. Given the increasing concern for NTM in industrial settings, there is a growing need for MWF-compatible (i.e., nonoxidizing) biocides that are effective against NTM. Here, we evaluated the efficacy of four nonoxidizing biocides against fast-growing NTM species in water-based MWF. Results from a 72-hour laboratory challenge indicated that biocides containing 2-(thiocyanomethylthio)benzothiazole (TCMTB), commonly used as a fungicide, were most effective at controlling Mycobacterium chelonae, M. fortuitum, and field and laboratory strains of M. immunogenum. In subsequent field trials at nine MWF plants where workers had reported symptoms of dermal and lung illnesses, TCMTB consistently reduced viable NTM counts, as measured through both culture-based assays and microscopy. However, dosages needed to be increased two- to four-fold relative to the laboratory experiments to control NTM populations in systems with established biofilms. Due to the observed success of TCMTB against NTM in field context, the manufacturing plants involved in this study continued regular application of TCMTB as a preventative treatment. No additional outbreaks were observed throughout our follow-up period, which, in several cases, spanned approximately 10 years. The results suggest that TCMTB may provide an efficacious method for treating and preventing NTM contamination in MWF systems. Future studies should explore the mechanism of action of the sulfur-rich chemical structure of TCMTB to potentially guide the development and synthesis of related compounds with anti-NTM properties.