Microbial dynamics of different casing materials in the production of white button mushrooms (Agaricus bisporus)

Abstract

Peat is a suitable casing layer most commonly used as a substrate in commercial production of white button mushrooms. However, further exploitation of this substrate in South Africa is restricted due the environmental status of this scarce and protected natural resource. Therefore, finding an easily available and economically viable casing material that can either partially or completely replace peat is important for sustainable mushroom production. In the process of finding a replacement material, information regarding the microbiological properties of peat and alternative casing materials is required to profile a natural stable ecosystem since the presence of bacteria in the casing layer is important for pinning and fruit body formation. Pseudomonad bacteria in the casing layer are known to play a role in the fructification of mushroom mycelium. The aim of this study was to investigate the microbial dynamics and profiles of peat, industrial by-products and peat-based mixtures with byproducts using cultural and a culture-independent molecular technique such as denaturing gradient gel electrophoresis (DGGE). Other aims of this study were to investigate the overall microbial profiles of compost, casing and mushrooms in a commercial mushroom production cycle and to evaluate effects of isolated micro-organisms on mycelial growth, mushroom yield and in vitro disease control. Total bacterial population densities in peat-based mixtures at pinning were comparable to that of peat alone and pseudomonads constituted part of bacterial populations in these mixtures. Other bacteria in peat and the mixtures included Ensifer spp., Sinorhizobium spp., Bacillus spp., Sporosarcina spp., Microbacterium spp. Arthrobacter spp. and Sphingobacterium spp. Bacterial profiles of peat at pinning also showed dominant bacteria other than pseudomonads including Bacteroidetes, alpha-Proteobacterium, beta-Proteobacterium, gamma-Proteobacterium, delta-Proteobacterium and uncultured species. In viewing the overall microbial profile of compost, casing and harvested mushrooms, bacterial profiles of mushrooms were more similar to those of casing than to compost and were dominated by pseudomonads. Fungal profiles of compost, casing at pinning and freshly harvested and stored mushrooms were completely dominated by Agaricus bisporus. Bacterial and yeast populations increased significantly during mushroom storage at 4oC while fungal populations remained low. Pseudomonas spp. and an Arthrobactor sp. isolated from casing resulted in 100% in vitro growth stimulation of A. bisporus. Several bacteria and yeasts isolated from compost, casing and mushrooms inhibited in vitro growth of the fungal mushroom pathogens Verticillium fungicola var aleophilum, V. psalliotae, Mycogone perniciosa, Cladobotryum dendroides, C. mycophilum, Trichoderma aggressivum f. europaeum, T. aggressivum f. aggressivum and T. harizianum. In conclusion, peat-based casing mixtures with industrial by-products harbour bacteria important in A. bisporus fructification. Bacteria in these casings are also important in other aspects of mushroom production such as growth stimulation. Based on results in this study, mushroom bacterial profiles are similar to those of the casing, suggesting postharvest quality may be controlled by manipulating casing bacteria.