Towards New Topical Treatments For Bacterial Infection

Soft skin and tissue infections represent the most common bacterial infections, however in some cases these infections fail to heal. The presence of surface-attached communities known as biofilms has been observed on >60% of wounds that fail to heal and adversely impact wound healing. However, many of the existing antibacterial agents used to treat non-healing wounds have limited activity against biofilms. This thesis sought to identify and characterise compounds with antibacterial and antibiofilm activity against significant wound pathogens.
Initial experimentation sought to determine whether combining antibacterial agents could improve their antibiofilm activity. Synergism studies identified the combination of chlorhexidine and cetrimide, which displayed improved biofilm eradication against mono-species S. aureus and P. aeruginosa biofilms. Unfortunately, this combination had limited activity against more robust mixed species biofilm. Although this combination did not display improved activity against this mixed species biofilms, the methodology used in this chapter could be used in future to identify and evaluate other combinations with regards to their antibiofilm activity.
The void in broad spectrum antibiofilm agents could be filled by repurposed agents. Bronidox (BX) and bronopol (BP) were identified as compounds that are able to eradicate mono- and mixed biofilms at concentrations that are regarded for safe use. Investigation into their antibiofilm mode of action (MOA) revealed that BP and BX are able to kill stationary phase bacteria, it was additionally shown that both agents target biofilm cells directly and that biofilm matrix disruption is a secondary MOA.
Batumin displays potent antistaphylococcal activity (MIC= 0.0625 µg/mL) and may prove a useful topical antibacterial agent for skin infections; however its MOA is under debate. Initial studies revealed that the batumin exerts its antistaphylococcal effect through the inhibition of fatty acid synthesis. Characterisation of batumin-resistant mutants revealed mutations which were hypothesised to increase FabI expression and reduce susceptibility to batumin. Indeed, the over expression of FabI in S. aureus resulted in reduced susceptibility to batumin. Finally the inhibition of purified S. aureus FabI by batumin was confirmed in vitro. Batumin inhibited saFabI with similar potency to triclosan- another FabI inhibitor. The studies provide the basis to explore batumin further as a topical antistaphylococcal agent.