Effects of associated bacteria on the pathogenicity and reproduction of the insect-parasitic nematode Rhabditis blumi (Nematoda: Rhabditida)

Abstract

Three bacteria, Alcaligenes faecalis, Flavobacterium sp., and Providencia vermicola, were isolated from dauer juveniles of Rhabditis blumi. The pathogenic effects of the bacteria against 4th instar larvae of Galleria mellonella were investigated. Providencia vermicola and Flavobacterium sp. showed 100% mortality at 48 h after haemocoelic injection, whereas A. faecalis showed less than 30% mortality. Dauer juveniles showed 100% mortality against G. mellonella larvae, whereas axenic juveniles, which do not harbor associated bacteria, exhibited little mortality. All of the associated bacteria were used as a food source for nematode growth, and nematode yield differed with bacterial species. Among the bacterial species, P. vermicola was most valued for nematode yield, showing the highest yield of 5.2 × 104 nematodes/mL in the plate. In bacterial cocultures using two of the three associated bacteria, one kind stimulated the other. The highes total bacterial yield of 12.6 g/L was obtained when the inoculum ratio of P. vermicola to A. faecalis was 10:1. In air-lift bioreactors, the nematode growth rate increased with an increasing level of dissolved oxygen. The maximum nematode yield of 1.75 × 105 nematodes/mL was obtained at 192 h with an aeration rate of 6 vvm.