During human disease, Haemophilus ducreyi leads a primarily extracellular lifestyle, in which the organism is under constant pressure from the immune system. To survive in this environment, H. ducreyi expresses multiple mechanisms that counteract various antimicrobial activities of innate immunity. Key among these is secretion of LspA proteins to prevent phagocytosis, allowing H. ducreyi to reside extracellularly. When phagocytes cannot engulf bacteria, they secrete granule contents, including antimicrobial peptides (APs) such as cathelicidin and defensins, to kill the pathogens extracellularly. APs bind and destabilise cell membranes to lyse bacteria. Our laboratory is studying two transporter systems that protect H. ducreyi from human APs, including cathelicidin LL37 and beta-defensins. To prevent lethal interactions between LL37 and the inner membrane, H. ducreyi utilises the Sap (sensitive to antimicrobial peptides) transporter, which takes up periplasmic LL37 for cytoplasmic degradation. By mutagenizing structural components of the Sap transporter, we have found a direct correlation between the effectiveness of Sap-mediated LL37 resistance in vitro and the contribution of the transporter to virulence in humans. Further, we found that H. ducreyi OppA (oligopeptide binding protein A), the periplasmic component of another uptake transporter, appears to cooperate with the Sap transporter for LL37 uptake. For beta-defensin resistance, H. ducreyi utilises the MTR efflux transporter. MTR is a member of the resistance-nodulation-division family of multidrug resistance transporters that pump hydrophobic agents from the periplasm and cytoplasm out of the cell. Our data demonstrate that the H. ducreyi MTR transporter confers resistance to both LL37 and beta-defensins. Interestingly, we also found that the MTR transporter affects activation of CpxRA, which globally regulates virulence factors in H. ducreyi. The role of MTR in human virulence is under investigation. Together, these studies highlight the significance of AP resistance mechanisms to pathogen survival in the human host.
- antimicrobial peptide resistance
- Haemophilus ducreyi