Abstract
The extracellular polymer substances (EPS) generated by biofilms confers resistance to antimicrobial agents through electrostatic and steric interactions that hinder molecular diffusion. This resistance mechanism is particularly evident for antibacterial nanomaterials, which inherently diffuse more slowly compared to small organic antibacterial agents. The aim of this study was to determine if a biofilm's resistance to antibacterial nanomaterial diffusion could be diminished using electrolytes to screen the EPS's electrostatic interactions. Anionic (+) alpha-tocopherol phosphate (α-TP) liposomes were used as the antimicrobial nanomaterials in the study. They self-assembled into 700 nm sized structures with a zeta potential of -20 mV that were capable of killing oral bacteria (S. oralis growth inhibition time of 3.34 ± 0.52 h). In a phosphate (-ve) buffer the -ve α-TP liposomes did not penetrate multispecies oral biofilms, but in a Tris (hydroxymethyl)aminomethane (+ve) buffer they did (depth - 12.4 ± 3.6 μm). The Tris did not modify the surface charge of the α-TP nanomaterials, rather it facilitated the α-TP-biofilm interactions through electrolyte screening (Langmuir modelled surface pressure increase of 2.7 ± 1.8 mN/ m). This data indicated that EPS resistance was mediated through charge repulsion and that this effect could be diminished through the co-administration of cationic electrolytes.
Highlights
Bacterial biofilms are structured communities that co-exist within an extracellular matrix [1]
The extracellular polymer substances (EPS) generated by biofilms confers resistance to antimicrobial agents through electrostatic and steric interactions that hinder molecular diffusion. This resistance mechanism is evident for antibacterial nanomaterials, which inherently diffuse more slowly compared to small organic antibacterial agents
The increase in fluorescence intensity upon the titration of low concentrations of Tris into the disperse phase of the (+) α-TP suggested that aggregate surface potential decreased upon the addition of Tris [37] and this translated into a lower zeta potential of the aggregates in Tris compared to the phosphate buffer
Summary
Bacterial biofilms are structured communities that co-exist within an extracellular matrix [1]. When a biofilm is formed, the bacteria within it become up to 1000 times more resistant to antimicrobial treatment compared to the planktonic organisms [2] This resistance originates from the creation of subpopulations in the biofilm [3], a higher mutation rate [4], the upregulation of efflux pumps [5], modifications in bacterial lipopolysaccharide (LPS) and a reduction in the diffusion rates of antimicrobial agents in the biofilm matrix, which effectively dilutes the administered agents. These characteristics render it problematic to control biofilm growth once they are established on the surface of materials. Because each biofilm can show significant variability with respect to the organisms and extracellular components that it contains [12] and nanomaterial diffusion is inherently slower than small organic antimicrobials, designing a nanomaterial that has the surface properties to allow it to efficiently diffuse into a multispecies biofilm after deposition onto a material surface is not a trivial task [13,14,15]
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