Interaction of Xenorhabdus nematophilus (Enterobacteriaceae) with the antimicrobial defenses of the house cricket, Acheta domesticus.

Fifth instar Acheta domesticus nymphs exhibited a decline in total hemocyte counts during the first hour of exposure to dead Xenorhabdus nematophilus; the bacterial level in the hemolymph also declined during this time. Thereafter bacterial numbers in the hemolymph increased as the level of damaged hemocytes increased. The bacteria lowered phenoloxidase activity in vivo by initially reducing the number of hemocytes containing prophenoloxidase and later by inhibiting enzyme activation. Preincubating X. nematophilus in hemolymph with active phenoloxidase in vitro accelerated the removal of the bacteria from the hemolymph in vivo which may be due to modification of the bacterial surface by serine proteases. Lysozyme activity increased in bacteria-injected insects in parallel with an increase in counts of damaged hemocytes; most of the enzyme was located in hemocytes. Lipopolysaccharides of X. nematophilus caused changes in hemocyte counts and phenoloxidase and lysozyme levels comparable to whole bacteria. Lipopolysaccharides also slowed the removal rate of the bacteria from, and accelerated bacterial emergence into, the hemolymph.