Neisseria gonorrhoea (Ngo) is the causative agent of the second most common STI gonorrhoea. Antimicrobial resistant (AMR) Ngo are classified as a High Priority pathogen for next generation antimicrobial development. Mip is a dimeric surface-exposed outer membrane associated FK506-binding protein (FKBP) in Ngo involved in protein folding and chaperone functions. Mip is shown to be highly conserved in all gonococci and has been shown to be a druggable target in many pathogens.
This study explored the role of gonococcal Mip in resistance to cationic antimicrobial peptides (CAMPs), an essential trait for colonisation of host mucosa and macrophage survival. To do this, a mip mutant strain, NgoΔmip, was constructed in strain FA1090 and demonstrated a survival defect during stationary phase growth at 37°C. To investigate the role of mip in gonococcal resistance to CAMPs, MICs of NgoΔmip against CAMP polymyxin B, and human CAMP LL-37 were determined. The results showed that NgoΔmip is highly susceptible to polymyxin B (MIC 0.07 μg/ml vs WT 96 μg/ml) and LL-37 (3.2 μM vs WT 12.8 μM), a phenotype that was partially restored by complementing the mutant strain. To explore this phenotype further, NgoΔmip was allowed to infect murine and human macrophages and intracellular survival was assayed. NgoΔmip showed reduced survival in murine macrophages (8-fold less than WT) but no significant differences in survival in human macrophages. To explore the association between NgoΔmip survival and CAMP expression, CAMP expression was induced using a histone deacetylase inhibitor (HDACi) in a human macrophage intracellular survival assay. A 14-fold reduction in intracellular counts was seen with NgoΔmip in the presence of HDACi compared to a 2-fold reduction of WT.
In conclusion, Mip has a role in CAMP resistance and hence intracellular survival in phagocytic cells. The mechanism of Mip in CAMP resistance is unclear as it is an outer membrane protein while the primary determinant of CAMP resistance, the lipid A ethanolamine transferase EptA, is located on the inner membrane. Further work is necessary to understand the inter-relations of Mip in pathogenicity of Neisseria and direct/indirect roles in host cell responses.