Boosting antibiotics to fight resistance: investigating the effects of combining biosurfactants with antibiotics to treat antibiotic resistant pathogens causing healthcare associated infections

Abstract

Enterococcus faecalis remains a major cause of healthcare-associated infections and is particularly problematic due to its ability to form resilient biofilms and its increasing involvement in antimicrobial resistance. The limited development of novel antibiotics highlights the urgent need for alternative strategies that enhance the performance of existing therapies. Biosurfactants, including rhamnolipids, have attracted growing interest as potential antimicrobial adjuvants due to their ability to disrupt microbial adhesion, destabilise biofilm matrices, and improve drug penetration.

In this study, the antimicrobial and antibiofilm activity of a purified extract of monorhamnolipid was investigated against E. faecalis, both alone and in combination with amoxicillin. Antimicrobial susceptibility was assessed using broth microdilution and checkerboard assays, while biofilm inhibition and disruption were evaluated using standard viability-based methods. Advanced nanoscale characterisation was performed using Atomic Force Microscopy, supported by Scanning Electron Microscopy, to relate biological effects to changes in biofilm structure and mechanical properties.

Purified mono-rhamnolipid demonstrated intrinsic antibiofilm activity, significantly reducing biofilm formation and disrupting established biofilms in a concentration-dependent manner. When combined with amoxicillin, enhanced antibiofilm effects were observed, with fractional inhibitory concentration indices confirming synergistic interactions. AFM nanomechanical mapping revealed a marked reduction in biofilm stiffness following combined treatment compared with untreated controls, indicating mechanical weakening of the biofilm structure. SEM analyses further supported these findings by showing altered biofilm organisation and reduced surface coverage. Collectively, these findings demonstrate that purified mono-rhamnolipid acts as an effective antibiofilm agent and antibiotic adjuvant against E. faecalis, providing both biological and biophysical evidence of biofilm destabilisation. This work supports the potential of biosurfactant-based adjuvant strategies as a promising approach for addressing biofilm associated infections in the context of rising AMR.

Thesis is embargoed until 31st May 2028

Date of Award	May 2026
Original language	English
Supervisor	James Dooley (Supervisor), Nigel Ternan (Supervisor), Patrick Lemoine (Supervisor), Ibrahim Banat (Supervisor) & Patrick Naughton (Supervisor)