Effects of Quorum Quenchers on Aspergillus fumigatus Conidia Aggregation, Adhesion to Surfaces, and Biofilm Formation

Aspergillus fumigatus can produce in vitro an extracellular hydrophobic matrix with biofilm features under static growth conditions. Microbial quorum sensing (QS) system regulates genetic competence and biofilm formation. Notwithstanding, triclosan, as a synthetic antimicrobial, disrupts QS signal in some bacteria, yeasts, and dermatophytes through blocking the biosynthesis of amino acid and fatty acids in the microbes. In this study, triclosan quorum quenching role as well as the effects of microbial quorum quenchers, furanone, farnesol and tyrosol were investigated against A. fumigatus for the first time. Amphotericin B (AMB), a common antifungal agent used against A. fumigatus infection, carries adverse effects against human health. So, triclosan’s antifungal effect in combination with AMB was also studied.

Microbial cells’ detrimental attachment and biofilm formation on the indwelling medical implants, healthcare and hospital facilities, prompts interests to introduce feasible and cost-effective methods for eliminating/reducing microbial attachment to the surfaces. Using water contact angle (WCA) measurements, hydrophobicity of some hydrophilic surfaces (glass, acrylic, HDPE, nylon 6 and UPVC), and hydrophobic surfaces (PTFE and silicone) was measured following coating them with the mentioned antimicrobial agents.

A. fumigatus conidia were exposed to the agents and the subsequent changes in the conidial viability, biofilm biomass production, total and EPS-related protein/ polysaccharide/ and nucleic acid were studied. The effect of AMB was weaker than the other agents on mitigating the biofilm formation. eDNA release was observed from A. fumigatus mycelia treated with triclosan, tyrosol and farnesol as compared with untreated control group. This eDNA release appeared to be due to necrosis occurrence in the treated samples verified by gel electrophoresis and protein quantity analysis. As a novel finding, the necrosis induced by triclosan followed by apoptosis induced by AMB resulted in a synergistic interaction to reduce the conidia viability even at the sub-MIC doses of triclosan and AMB. Confocal microscopy of the A. fumigatus exposed to triclosan-AMB combination confirmed the synergistic interaction between them against the fungus biofilm. Assaying for hydrophobicity of the conidia cell wall revealed a change from hydrophobic to hydrophilic property. ags3 down-regulation approved with real-time PCR assay showed triclosan’s quorum sensing inhibitory role against A. fumigatus. This was through inhibiting conidia aggregation, and hence prohibiting initiation of quorum sensing signalling pathway. Through SDS-PAGE assay it was established that the conidia cell wall hydrophobic rodlet layer was absent in farnesol-treated sample.

This study revealed some of the physio-chemical properties that are involved in A. fumigatus conidia attachment to the surfaces. This work for the first time established that hydrophobic surfaces, PTFE and silicone, coated with farnesol showed hydrophilicity, while AMB changed UPVC surface charge from hydrophilic to hydrophobic. Taken together, besides influencing the conidia cell wall hydrophobicity, farnesol and AMB can also be used to coat the surfaces in clinical healthcare settings to diminish hydrophobic/hydrophilic microbial cells attachment to the surfaces.