Mutation in thesdeSGene Promotes Expression of thesdeABEfflux Pump Genes and Multidrug Resistance in Serratia marcescens

Abstract

ABSTRACTSerratia marcescensgained resistance to both biocides and antibiotics on expressing the SdeAB efflux pump, following exposure to increasingly higher concentrations of a biocide (H. Maseda et al., Antimicrob. Agents Chemother. 53:5230–5235, 2009). To reveal the regulatory mechanism ofsdeABexpression, wild-type cells were subjected to transposon-mediated random mutagenesis, and a mutant with antibiotic resistance, which mimicked the phenotype of the previous biocide-resistant cells, was obtained. The transposon element was found in the chromosomal DNA downstream of thesdeABoperon. Sequencing revealed the presence of an open reading frame (ORF) encoding a protein with 159 amino acid residues that is highly similar to the BadM-type transcriptional repressor, designatedsdeS. The level ofsdeB::xylEreporter gene expression, undetectable in the wild-type cells, appeared to be fully comparable to that in the biocide-resistant cells. Nucleotide sequencing of the mutant revealedsdeSto have a single G-to-A base substitution at position 269 that converted Trp90 to a stop codon. Introduction of a plasmid-borne intactsdeSinto the mutant cells and the biocide-resistant cells resulted in a reduction insdeB::xylEreporter activity to an undetectable level. These results suggested that SdeS functions as a repressor of thesdeABoperon. It was concluded that the original biocide-resistant cells had an impairedsdeSand, therefore, a derepressed level of the SdeAB efflux pump.