Mechanistic studies of the antibiofilm activity and synergy with antibiotics of isosorbide mononitrate.

The use of nitric oxide (NO), a naturally occurring antimicrobial agent, as an alternative strategy to combat bacterial biofilms has recently gained considerable momentum in light of the global threat of emerging antibiotic resistance. While previous NO-based anti-biofilm approaches were aimed at killing bacterial cells within biofilms, NO has also been recently identified as a key mediator of biofilm dispersal, causing the release of cells from the biofilm community. This is of great interest towards the design of more effective anti-biofilm strategies but further studies are warranted to validate this concept. Therefore, in the present study we investigated whether a NO precursor, isosorbide mononitrate (ISMN) or its analogue D-isosorbide can induce bacteria cell dispersal from Staphylococcus aureus (S. aureus) biofilms and explored the potential synergy of ISMN and the antimicrobial compounds mupirocin and ciprofloxacin in biofilm eradication. This study demonstrate that ISMN causes dispersal of S. aureus biofilm bacteria, particularly when exposed to high levels of drug. ISMN at 60mg/mL increased the number of colony forming units (CFU) (~3log10 and ~5log10) of planktonic bacteria after 6 and 24-h exposure respectively, compared to control biofilms. This suggests that ISMN induces the transition of sessile biofilm cells to free-swimming planktonic cells. In addition, ISMN exhibits synergistic effects against S. aureus biofilms with ciprofloxacin when tested above its minimum inhibitory concentration (MIC). Specifically, exposure to ISMN significantly enhanced the efficacy of ciprofloxacin by reducing the number of CFU (~3log10 or ~2log10) of biofilm-associated and planktonic bacteria respectively, compared to drug alone. Combined exposure to both ISMN and certain antimicrobial agents may therefore offer an innovative approach to control persistent biofilm and biofilm-associated infections.