An electron microscope study of wheat straw composted as a substrate for the cultivation of the edible mushroom (Agaricus bisporus)

The degradation of wheat straw, during composting, to produce the growth substrate for the edible mushroom ( Agaricus bisporus ), and subsequent colonisation by the fungal hyphae, was studied by electron microscopy which revealed an ecological succession of micro-organisms, initially dominated by a largely bacterial flora with few fungi. Later in the composting process actinomycetes were dominant. The initial rise in numbers of vegetative bacterial cells was followed by a steady decline and the appearance of spore forms. Several modes of microbial attack were observed. The most rapid degradation occurred initially on the cuticle and in the phloem and spread to a general degradation of all the plant tissue types present. Microbial attachment on the plant cell walls was non-uniform. As a result of these processes many of the plant fibres became separated but the final material still retained considerable structural integrity. Agaricus bisporus mycelium rapidly covered the surface of the straw but colonised the internal straw tissues more slowly. Surface-growing hyphal cells were encrusted with needle-like crystals presumed to be calcium oxalate.