Button mushrooms (Agaricus bisporus) are rich in protein and are often marketed as a protein power food. However, the protein content in mushrooms depends on the quality of their growth substrate, a selective compost made from wheat straw, poultry manure and gypsum in a microbially mediated composting process. The microbial communities that produce this compost change rapidly during the initial thermophilic stage of composting (80 °C), but pasteurisation (60 °C) and curing (45 °C ) leads to a stable consortium of bacteria and one dominant ascomycete fungus, Mycothermus thermophilus. This climax microbial community is important, as these microbes support selective growth of the Agaricus mycelium. We have characterized how the interactions between M. thermophilus, Pseudoxanthomonas taiwanensis (the dominant bacterium in mushroom compost), and Mycovorax composti, a novel thermophilic chitinolytic bacterium, are important in transforming polymeric compounds from the raw material into a nutritious substrate for the Agaricus mycelium. These insights will promote sustainable food supply, by providing novel pathways for the mushroom industry to produce higher yields of better quality mushrooms.