Studies on the in vivo cellular reactions of insects: Clearance of pathogenic and non-pathogenic bacteria in Galleria mellonella larvae

Abstract

The fate of various doses of bacteria of different pathogenicities injected into Galleria mellonella larvae was monitored over time, from haemocyte and bacterial counts, phagocytic responses and the speed and extent of formation of melanized cell aggregates (nodules). An initial haemocytopenia was recorded in all larvae, probably as a result of wound healing, an increased stickiness of the haemocytes for host tissues and/or cell clump or nodule formation. The results also showed that phagocytosis is the primary cellular defence reaction of this insect for doses of bacteria below ca. 10 3 μl −1 haemolymph while above this level phagocytosis and bacterial clearance are usually rapidly augmented by nodule formation. The extent to which these processes are elicited depends greatly upon the nature of the bacteria injected. In general, the more pathogenic strains produced greater responses than the relatively non-pathogenic forms. This enhanced cellular reactivity was, however, soon overcome by the pathogens which rapidly induced a secondary bacteraemia, a huge drop in haemocyte numbers and death of the larvae. The relative importance of phagocytosis and nodule formation in dealing with various doses of bacteria of differing pathogenicities is discussed.