Acquired Bacitracin Resistance in Enterococcus faecalis Is Mediated by an ABC Transporter and a Novel Regulatory Protein, BcrR

Abstract

Bacitracin resistance (bacitracin MIC, >256 g ml ؊1 ) has been reported in Enterococcus faecalis, and in the present study we report on the genetic basis for this resistance. Mutagenesis was carried out with transposon Tn917 to select for E. faecalis mutants with decreased resistance to bacitracin. Two bacitracin-sensitive mutants (MICs, 32 g ml ؊1 ) were obtained and Tn917 insertions were mapped to genes designated bcrA and bcrB. The amino acid sequences of BcrA (ATP-binding domain) and BrcB (membrane-spanning domain) are predicted to constitute a homodimeric ATP-binding cassette (ABC) transporter, the function of which is essential for bacitracin resistance in E. faecalis. The bcrA and bcrB genes were organized in an operon with a third gene, bcrD, that had homology to undecaprenol kinases. Northern analysis demonstrated that bcrA, bcrB, and bcrD were transcribed as a polycistronic message that was induced by increasing concentrations of bacitracin but not by other cell wall-active antimicrobials (e.g., vancomycin). Upstream of the bcrABD operon was a putative regulatory gene, bcrR. The bcrR gene was expressed constitutively, and deletion of bcrR resulted in a bacitracin-sensitive phenotype. No bcrABD expression was observed in a bcrR mutant, suggesting that BcrR is an activator of genes essential for bacitracin resistance (i.e., bcrABD). The bacitracin resistance genes were found to be located on a plasmid that transferred at a high frequency to E. faecalis strain JH2-2. This report represents the first description of genes that are essential for acquired bacitracin resistance in E. faecalis.

Role of two-component systems in the resistance ofStaphylococcus aureusto antibacterial agents

“…Interestingly, this TCS regulates not only a downstream operon encoding an ABC transporter (BceAB; MW2543-42), but also another operon encoding a transporter (VraDE; MW2620-2621), which is located separately from the TCS genes. In addition, transporters showing homology with BceAB have been reported in Gram-positive bacteria, including BcrAB from Bacillus licheniformis, 12 BcrAB from Enterococcus faecalis, 13 and MbrAB from Streptococcus mutans. 14 and as a survival strategy in the bacterial community.…”

Molecular Analysis of BcrR, a Membrane-bound Bacitracin Sensor and DNA-binding Protein from Enterococcus faecalis

“…Because UPP acts as a carrier of peptidoglycan monomeric units across the cell membrane (2), the antibacterial nature of bacitracin results from its ability to block cell wall synthesis. High level bacitracin resistance in Enterococcus faecalis is encoded by the bcrABD operon that is under the control of a putative membrane-bound DNA-binding protein, BcrR (3). In the presence of bacitracin, transcription of the bcrABD operon leads to the production of BcrA and BcrB, which are proposed to act as an ABC exporter of bacitracin, thus conferring high level bacitracin resistance (minimum inhibitory concentration Ն 256 g/ml) to the cell (3).…”

“…High level bacitracin resistance in Enterococcus faecalis is encoded by the bcrABD operon that is under the control of a putative membrane-bound DNA-binding protein, BcrR (3). In the presence of bacitracin, transcription of the bcrABD operon leads to the production of BcrA and BcrB, which are proposed to act as an ABC exporter of bacitracin, thus conferring high level bacitracin resistance (minimum inhibitory concentration Ն 256 g/ml) to the cell (3). BcrD is proposed to function as an undecaprenyl pyrophosphate phosphatase that functions to increase the amount of undecaprenyl available to the cell (3).…”

Tripropeptin C Blocks the Lipid Cycle of Cell Wall Biosynthesis by Complex Formation with Undecaprenyl Pyrophosphate

“…TPPC shows potent activity against staphylococci, enterococci, streptococci, and bacilli, while enterococci, some streptococci, and bacilli show resistance to BAC, because of their BAC efflux systems (1,(16)(17)(18)(19). From our study using mutants with defects in the BAC efflux system, such as bceR, bceS, and ywoA (bcrC) mutants, it was revealed that TPPC is equally active against B. subtilis strain 168, despite the presence of these efflux systems (Table 4).…”