Nanoscale Observations of Extracellular Polymeric Substances Deposition on Phyllosilicates by an Ectomycorrhizal Fungus

Microorganisms colonizing surfaces can exude a wide range of substances, generally called Extracellular Polymeric Substances (EPS). While EPS has often been visualized as thick mature strata embedding microbes, the initial phases of EPS production, its structure at the micro- and nanoscale and the microbial wall areas involved in its exudation are less known. In this work we use Atomic Force Microscopy to image EPS produced by the fungus Paxillus involutus on phyllosilicate surfaces. Hyphal tips initially deposit EPS which assumes the shape of a halo surrounding hyphae. The fusion of adjacent EPS halos is likely responsible for the creation of EPS monolayers covering mineral surfaces. It is also proposed that a specific region of hyphae initiates the formation of mineral channels produced by fungi. The results presented here permit for the first time to propose a model for the initial stages of EPS accumulation in fungi and filamentous microorganisms in general.