Insights into physiological and genetic mupirocin susceptibility in bifidobacteria.

Mupirocin is an antibiotic commonly used in selective media for the isolation of bifidobacteria. However, little is known about the genetic traits responsible for bifidobacterial resistance to mupirocin. Our investigation demonstrates that all of the bifidobacteria tested exhibit a phenotype of generally high resistance to this antibiotic. The genotypic reason for bifidobacterial mupirocin resistance was further characterized by sequencing of the isoleucyl-tRNA synthetase gene (ileS) coupled with three-dimensional modeling of the encoded protein and cloning of the ileS gene of Bifidobacterium bifidum PRL2010 in a mupirocin-sensitive Escherichia coli strain. These analyses revealed key amino acid residues of the IleS protein that apparently are crucial for conferring a mupirocin resistance phenotype to bifidobacteria.