Bacterial interactions with the mycelium of the cultivated edible mushrooms Agaricus bisporus and Pleurotus ostreatus

The cultivated edible mushrooms Agaricus bisporus and Pleurotus ostreatus are valuable food crops and an important source of human nutrition. Agaricus bisporus is the dominant cultivated species in the western hemisphere and in Australia, while in Asian countries P. ostreatus is more prevalent. These two mushroom species are grown on fermented-pasteurized substrates, and bacteria and fungi play an important role in converting feedstocks into a selective medium for the mushroom mycelium. The mushrooms are usually introduced to the substrate as grain spawn, and the actively growing hyphae form a range of direct interactions with the diverse bacterial community in the substrate. Of these interactions, the most well studied is the removal of inhibitory volatile C8 compounds and ethylene by pseudomonads, which promotes mycelium growth and stimulates primordia formation of both A. bisporus and P. ostreatus. Bacterial biomass in the substrate is a significant nutrition source for the A. bisporus mycelium, both directly through bacteriolytic enzymes produced by A. bisporus, and indirectly through the action of extracellular bacterial enzymes, but this is less well studied for P. ostreatus. Apart from their role as a food source for the growing mycelium, bacteria also form extensive interactions with the mycelium of A. bisporus and P. ostreatus, by means other than those of the removal of inhibitory compounds. Although several of these interactions have been observed to promote mycelial growth, the proposed mechanisms of growth promotion by specific bacterial strains remain largely uncertain, and at times conflicting. Bacterial interactions also elicit varying growth-inhibitory responses from A. bisporus and P. ostreatus. This review explores characterized interactions involving bacteria and A. bisporus, and to a lesser degree P.ostreatus, and whilst doing so identifies existing research gaps and emphasizes directions for future work.