Mechanism of BceAB-type transporter: Resistance by lipid II flipping

Treatment of bacterial infections are the great challenge of our era due to the evolved resistance mechanisms against antibiotics. The Achilles heel of bacteria is the cell wall especially during the needs of its synthesis and cell division. Here lipid II is an essential cell wall precursor component synthesized in the cytosol and flipped into the outer leaflet of the membrane prior to its incorporation into the cell wall. Compounds targeting the cell wall or its biosynthesis precursors have been around for decades and have been used as antibiotics against bacterial infections like meningitis, pneumonia and endocarditis. Antimicrobial peptides (AMPs) have proven to be a promising weapon against multiresistant bacteria. However, the Bacitracin efflux (BceAB)-type ATP binding cassette transporters expressed in the membrane of human pathogenic bacteria have been shown to confer resistance to these alternative antibiotics, thereby hampering their medical development. In Streptococcus agalactiae COH1 the BceAB-type transporter NsrFP (SaNsrFP) confers high-level resistance against the antimicrobial peptide nisin, a member of the lantibiotic subfamily. We showed that SaNsrFP provides a novel resistance mechanism by flipping lipid II back into the cytosol, thereby preventing the binding of nisin as well as other lipid II targeting compounds. This is intriguing since a relatively simple reaction mediates resistance to human pathogenic bacteria to lipid II targeting antibiotics, regardless of their structure. Significance StatementThe ABC-transporter NsrFP from Streptococcus agalactiae (SaNsrFP) belongs to the BceAB-type transporters. Several BceAB-type transporters are known to confer resistance against multiple antimicrobial peptides. In this study a new resistance mechanism was identified, which is based on the reduction of the number of cell wall precursor lipid II molecules on the cell surface mediated by SaNsrFP. SaNsrFP flips lipid II, which are considered to be the target for many antibiotics, back into the cytoplasm. With this newly gained knowledge about the resistance mechanism of BceAB-type transporters, novel strategies can be established to overcome or bypass this resistance in human pathogenic bacteria.