Biocides/antibiotics reduction of sensitivity in Streptococcus thermophilus mutant NTG6

Biocides have been widely used for decades to preserve materials, including food, to decontaminate surfaces, to disinfect instruments, for personal hygiene, and to prevent infections transmission(Langsrud et al., 2003). Nevertheless, when used in large volumes or at high concentrations, biocides have toxic effects and extensive use is dangerous for the environment, including animals and humans. Despite this widespread and ever increasing use, most bacterial and fungal species remain susceptible to biocides. However, decreased susceptibility of bacteria and fungi to disinfectants has been reported and occasionally linked to antibiotic resistance, mainly in human and veterinary pathogens (Saleh et al., 2011). These reports on mechanisms of cross-resistance, and evidences for genetic linkage of these two types of resistances, have raised concern about a risk of increasing antibiotic resistance connected to the use of disinfectants in the environment, hospitals and industry (Chapman, 2003). Taking into account these considerations, GRAS microorganisms commonly used in food processes may represent a reservoir of potential biocide and biocide/antibiotic resistances transferable by horizontal gene transfer to spoilage and/or pathogenic microorganisms. In this study, <em>S. thermophilus</em> mutants showing a reduced susceptibility to biocide chlorhexidine were induced to verify if the genetic background of this species had the potentiality to develop a biocide tolerance. In this context, a NTG6 mutant was selected after several subcultures of the wild-type DSM20617 in M17 broth in presence of sub-inhibiting concentrations of chlorhexidine. Metabolic activity monitored through Phenotype Microarray (PM) high-throughput technology and the analysis of the growth-kinetic parameters revealed that NTG6 mutant showed a reduced susceptibility to chlorhexidine but also to ethidium bromide, furaltadone and nitrofurantoin at low inoculum concentration, i.e. at high biocide-antibiotic/cells ratio, compared to the wild-type. Comparative genome analysis carried out between the wild-type and the NTG6 mutant revealed several point mutations in genes whose relevance in the biocides/antibiotics reduction of sensitivity will be further addressed. Key words: <em>Streptococcus thermophilus</em>, biocide, antibiotics, co-resistance, cross-resistance. Langsrud S., Sidhu M.S., Heir, E., Holck, A.L. (2003). I Biodet Biodegrad 51: 283-290. Chapman J.S. (2003). I Biodet Biodegrad 51: 271-276. Saleh S., Haddadin R.N.S., Baillie S., Collier P.J. (2011). Lett Appl Microbiol 52: 87-95