SignificanceSecond messengers are employed by all organisms to regulate fundamental behaviors, including biofilm formation, motility, metabolism, and pathogenesis in bacteria. We have identified a phospholipase in the El Tor Vibrio cholerae biotype, responsible for the current cholera pandemic, that is directly activated by the second messenger 3′, 3′-cyclic GMP-AMP (cGAMP). Discovery of this proteinaceous bacterial cGAMP effector sheds light on the functions and basic principles of cGAMP signaling. Both this phospholipase and the cGAMP synthase are encoded within the VSP-1 pathogenicity island, unique to the El Tor biotype, and our findings assign a biochemical function to VSP-1 that may contribute to the epidemiological success of El Tor V. cholerae.

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Treatment of bacterial infections is one of the major challenges of our time due to the evolved resistance mechanisms of pathogens against antibiotics. To circumvent this problem, it is necessary to understand the mode of action of the drug and the mechanism of resistance of the pathogen. One of the most potent antibiotic targets is peptidoglycan (PGN) biosynthesis, as this is an exclusively occurring and critical feature of bacteria. Lipid II is an essential PGN precursor synthesized in the cytosol and flipped into the outer leaflet of the membrane prior to its incorporation into nascent PGN. Antimicrobial peptides (AMPs), such as nisin and colistin, targeting PGN synthesis are considered promising weapons against multidrug-resistant bacteria. However, human pathogenic bacteria that were also resistant to these compounds evolved by the expression of an ATP-binding cassette transporter of the bacitracin efflux (BceAB) type localized in the membrane. In the human pathogen Streptococcus agalactiae, the BceAB transporter SaNsrFP is known to confer resistance to the antimicrobial peptide nisin. The exact mechanism of action for SaNsrFP is poorly understood. For a detailed characterization of the resistance mechanism, we heterologously expressed SaNsrFP in Lactococcus lactis. We demonstrated that SaNsrFP conferred resistance not only to nisin but also to a structurally diverse group of antimicrobial PGN-targeting compounds such as ramoplanin, lysobactin, or bacitracin/(Zn)-bacitracin. Growth experiments revealed that SaNsrFP-producing cells exhibited normal behavior when treated with nisin and/or bacitracin, in contrast to the nonproducing cells, for which growth was significantly reduced. We further detected the accumulation of PGN precursors in the cytoplasm after treating the cells with bacitracin. This did not appear when SaNsrFP was produced. Whole-cell proteomic protein experiments verified that the presence of SaNsrFP in L. lactis resulted in higher production of several proteins associated with cell wall modification. These included, for example, the N-acetylmuramic acid-6-phosphate etherase MurQ and UDP-glucose 4-epimerase. Analysis of components of the cell wall of SaNsrFP-producing cells implied that the transporter is involved in cell wall modification. Since we used an ATP-deficient mutant of the transporter as a comparison, we can show that SaNsrFP and its inactive mutant do not show the same phenotype, albeit expressed at similar levels, which demonstrates the ATP dependency of the mediated resistance processes. Taken together, our data agree to a target protection mechanism and imply a direct involvement of SaNsrFP in resistance by shielding the membrane-localized target of these antimicrobial peptides, resulting in modification of the cell wall.

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 highlevel 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. Main Text