Long-range interactions keep bacterial cells from liquid-solid interfaces: Evidence of a bacteria exclusion zone near Nafion surfaces and possible implications for bacterial attachment.

Hydrophilic surfaces of both abiotic and biological origin have been shown to bear particle-exclusion zones as large as hundreds of micrometers at liquid-solid interfaces. Here we present the first systematic investigation and evidence for bacteria-free exclusion zones for several bacterial strains, including pathogens associated with hospital infections and/or foodborne outbreaks: Staphylococcus aureus, Escherichia coli O157:H7, and Listeria monocytogenes. Tests were carried out both in a phosphate buffer, as well as triptic soy broth (TSB) of high ionic strength. Bacterial cell density distribution at the Nafion-liquid interface was visualized using confocal laser scanning microscopy. A robust image analysis method was developed to generate a profile of cell concentration near the interface and quantify EZ size. Results revealed an exclusion zone (EZ) of 40-60μm and a transition zone (TZ) of 40-80μm for bacterial cells suspended in tryptic soy broth. There were no statistical differences in the size of EZ and TZ for the bacterial strains tested with the same substrate, but differences existed for different substrates tested, implying a physicochemical underpinning for EZ. In a test conducted with E. coli, cells progressively penetrated EZ over 2days. Furthermore, EZ-bearing Nafion had 80% less biomass accumulation of E. coli over 2days compared to an EZ-less, hydrophilic, smooth aluminum oxide surface. This suggests that EZ may represent the first line of defense, spatially and temporally, against bacteria approaching certain hydrophilic surfaces. These findings could have important implications in developing biofouling-resistant material surfaces for applications sensitive to bacterial attachment and biofilm formation.