Marine killer yeast Metschnikowia saccharicola active against pathogenic yeast in crab and an optimization of the toxin production

A marine yeast strain, DD21-2 was isolated from sediments in Dandong, China, which has killer activity against yeast Metschnikowia bicuspidata WCY (pathogenic to crab, Portunus trituberculatus). Routine identification, sequence analysis of 26S rDNA and ITS sequencing showed that the strain was M. saccharicola DD21-2, and has not been previously reported as having killer activity against M. bicuspidata WCY. To optimize the production of the killer toxin by M. saccharicola DD21-2, the interaction effects of fermentation process variables were investigated by Response Surface Methodology (RSM). The following reaction factors were selected in screening experiments: Inoculum concentration (2 to 6%), pH (5.0 to 6.0), temperature (20 to 28°C), and fermentation time (2 to 3 days), and the diameter of the inhibition zone produced by the killer toxin was used as a response variable. A quadratic regression model of killer toxin activity was established by regression analysis and significance testing (P test). The results identified the following parameters as optimal for maximal production of the killer toxin by M. saccharicola DD21-2: Fermentation temperature 28°C, pH 5.5, fermentation time 2.7 days, and inoculum concentration 4.1% (v/v). Thus, RSM was effective in determining the best conditions for killer toxin production, suggesting the practical usage of this marine yeast in the investigation of process variables.   Key words: Marine killer yeast, killer toxin, response surface methodology (RSM), fermentation, optimization.