Influence of culture medium composition, dissolved oxygen concentration and harvesting time on the production of Serratia entomophila, a microbial control agent of the New Zealand grass grub

The bacterium Serratia entomophila (Enterobacteriaceae) has been developed as a commercially available biopesticide for control of the pasture pest Costelytra zealandica. The influence of culture medium composition, dissolved oxygen (DO) concentration and harvesting time were investigated in order to optimise the production of S. entomophila. In batch fermentations, highest yields were achieved using sucrose (40 g L^-1) as the carbon source, followed closely by fructose and molasses. The effect of yeast extract (YE), marmite and bakery yeast as cell growth enhancers was also examined in both batch and fed-batch mode. Culture medium containing 20 g L^-1 of YE (fed-batch) produced the highest cell density. No significant effect on cell yield was detected when cultures were supplemented with bakery yeast or marmite. The DO concentration influenced biomass production: a 5-fold increase in cell density was achieved when the concentration of DO was maintained in the range of 20-50% (5.7×10¹⁰ CFUs mL^-1) in comparison with 1% (1.2×10¹⁰ CFUs mL^-1). In cultures maintained at 1 and 20% DO concentration, cells harvested from the exponential growth phase survived for less than 2 weeks when stored at 4°C. In contrast, high cell survival (85-100%) was achieved when cells were harvested after they had entered the stationary growth phase. Recommendations are provided for the production of robust, high cell density cultures of S. entomophila.