Microbial Contaminants in the Cultivation of Pleurotus ostreatus

Abstract

The evidence of a successful mushroom cultivation process is the production of high-quality fruit bodies, a process basically dependent on the quality of the spawns and substrates. Microbial contamination at any stage of the cultivation process poses a significant challenge to productivity and yield. The aim of this research was to isolate and identify bacteria and fungi associated with contaminated spawns and substrates in the cultivation of Pleurotus ostreatus in the Mushroom unit of Biotechnology Advanced Research Centre (BARC), Sheda Science and Complex (SHESTCO). The spawns were made using 100% wheat grain (Triticum aestivum) and the substrates were made using different formulations of sawdust (SD) and rice (Oryza sativa) bran (RB). Contamination was initially detected phenotypically through discoloration, unpleasant odor and poor mycelia colonization. The bacteria and fungi were isolated using spread plate method. The identification of the bacteria was done using morphological characteristics and biochemical tests. The fungi were identified based on their microscopic and macroscopic characteristics. The only microbial contaminant in the wheat spawns were identified as Bacillus subtilis. The mean total bacterial count (TBC) for the spawns varied from below detection limit to 2.1 x 106 CFU/g. The wheat spawns had no coliforms and no fungal growth. The spawns were inoculated on rice bran (RC) and sawdust (SD) substrates having different percentage ratios: RC (70%): SD (30%); SD (70%): RC (30%) and RC (50%): SD (50%), 100% sawdust and 100% rice bran. The mean TBC for contaminated substrates varied between below detection limit to 4.5 x 106 CFU/g. The bacterial isolates and percentage of occurrence were: Bacillus subtilis 5(25%), Pseudomonas aeruginosa 10(50%) and Staphylococcus aureus 5(25%). The mean total fungal count for the contaminated substrate ranged from below detection limit to 3.5 x 103. CFU/g. The fungal isolates were Aspergillus niger and Candida albicans. The 50% RB: 50% SD combination formulation which had no contamination also had the best yield. However, this does not necessarily mean that the 50% RB and 50% SD combination formulation would be best for avoiding contamination. Contaminated spawns and substrates along the cultivation line should be quickly removed and discarded aseptically to avoid cross contamination. Stringent sterilization and pasteurization techniques should be adopted to ensure uniformity in order to prevent future contamination. Future studies should focus on the molecular characterization of these contaminants and elucidate their specific effect on mycelia colonization at each cultivation point.