Both bacterial and fungal endophytes reside in healthy tissue and play important roles in plant growth and pathogen defense. Beneficial fungal endophytes have been shown to inhibit the growth and spread of other microbes, such as plant pathogens, through competitive interactions including the production of secondary metabolites that influence microbial growth. Spinach and lettuce plants have been major sources of E. coli O157:H7 (EHEC) outbreaks, and this bacterium was recently shown to adhere to the plant phylosphere as well as adopt an endophytic lifestyle within the tissue of these plants. In an effort to identify endophytes that hold the potential to suppress growth of E. coli O157:H7 in plantae, fungal endophytes in spinach were isolated and tested for their ability to inhibit bacterial growth. Of the endophytic fungi examined, two fungal isolates (PNW-2016-02 and PNW-2016-03) demonstrated particularly strong bacterial growth suppressive properties and were identified based on DNA sequencing. The Stemphylium isolate PNW-2016-03 limited the growth of a broad range of Gram-positive and Gram-negative bacteria, including E.coli O157:H7 in direct competition and indirect growth assays in vitro. While several Stemphylium species are known as plant pathogens, the development of a Stemphylium-spinach plant model system has shown the fungal endophyte positively influences the growth of spinach, suggesting a beneficial mutualistic relationship. Ongoing research is aimed at identifying specific bioactive compounds produced by Stemphylium PNW-2016-03 via chemical characterization including MS/MS-HPLC, and elucidating bioactive related metabolic pathways in the PNW-2016-03 genome. Additionally, the spinach model can be used to examine microbial endosphere/phylosphere community interactions as an endophytic fungus colonizes plant tissue, as well as possible Stemphylium mediated competitive inhibition dynamics of EHEC in plantae.
