Contribution of chloride channel permease to fluoride resistance in Streptococcus mutans

Abstract

Genes encoding fluoride transporters have been identified in bacterial and archaeal species. The genome sequence of the cariogenic Streptococcus mutans bacteria suggests the presence of a putative fluoride transporter, which is referred to as a chloride channel permease. Two homologues of this gene (GenBank locus tags SMU 1290c and SMU 1289c) reside in tandem in the genome of S. mutans. The aim of this study was to determine whether the chloride channel permeases contribute to fluoride resistance. We constructed SMU 1290c- and SMU 1289c-knockout S. mutans UA159 strains. We also constructed a double-knockout strain lacking both genes. SMU 1290c or SMU 1289c was transformed into a fluoride transporterdisrupted Escherichia coli strain. All bacterial strains were cultured under appropriate conditions with or without sodium fluoride, and fluoride resistance was evaluated. All three gene-knockout S. mutans strains showed lower resistance to sodium fluoride than did the wild-type strain. No significant changes in resistance to other sodium halides were recognized between the wild-type and double-knockout strains. Both SMU 1290c and SMU 1289c transformation rescued fluoride transporter-disrupted E. coli cell from fluoride toxicity. We conclude that the chloride channel permeases contribute to fluoride resistance in S. mutans.