Background
Development of novel antimicrobial strategies is required to combat the rise of multidrug antibiotic resistant Gram-negative (GN) bacteria. Several antibiotics effective against Gram-positive (GP) bacteria fail to act on GN bacteria because they cannot penetrate the OM. Human immune protection against GN bacteria critically depends on the bactericidal actions of the complement system that perturbs bacterial membranes via the pore-forming Membrane Attack Complex (MAC).
Research aim
Since the MAC forms large pores with an inner diameter of ~10 nm, we hypothesized that it may allow passage of naturally impermeable antibiotics through the bacterial OM.
Results
Here we show that the MAC allows 'GP' specific antibiotics to pass the OM. Proof-of concept was shown for the lantibiotic nisin that specifically binds to lipid II and kills GP bacteria by forming pores in the inner membrane (IM). Whereas nisin did not affect GN bacteria like E. coli and K. pneumoniae, it effectively induced IM pore formation in the presence of human serum. This synergy between complement and nisin was observed in 27% of tested GN clinical isolates. Using flow cytometry and structured illumination microscopy, we show that the MAC rapidly permeabilizes the OM and after a delay the IM. The pores in the OM lead to influx of nisin that subsequently forms pores in the IM. In the presence of nisin, bacterial killing is accelerated and occurs at lower serum concentrations.
Conclusion
In conclusion, we show that complement forms pores in the OM that allow nisin (but also vancomycin) to reach their target sites. This expands the applicability of antibiotics that are considered ineffective for treatment of GN bacteria.