Extracellular polymeric substances mediate the coaggregation of aquatic biofilm-forming bacteria

Coaggregation, a phenomenon contributing to biofilm formation, occurs among biofilm bacteria from different aquatic environments. However, not much is known about molecules involved in aggregation. In this study, freshwater, estuarine and marine biofilm bacteria were evaluated for aggregation capabilities, and their cell-bound extracellular polymeric substances (CB-EPS), known to play an important role in biofilm formation, were characterized for functional groups, and sugar composition via Fourier-transform infrared spectroscopy and high-pressure liquid chromatography. Biofilm-forming potential of estuarine and freshwater biofilm bacteria was higher as indicated by their coaggregation scores, attributed to CB-EPS with distinct sugar types, compared to marine. Most of the biofilm bacteria lost their ability to coaggregate after removal of CB-EPS, indicating its importance in coaggregation. Estuarine (Bacillus indicus, Bacillus cereus), and freshwater (Exiguobacterium spp., B. cereus) bacterial pairs, retained their aggregation capability probably via expression of lipids and proteins, suggesting their ability to rebuild themselves by expressing specific biomolecules under stressed conditions. A similar expression pattern was observed when these strains were exposed to abrupt salinity change (environmental stressor), indicating modulation of cell surface chemistry as a strategy to protect biofilm bacteria in harsh conditions. Unravelling role of these biomolecules as cues for settlement of macrofoulers is a step ahead.