Nodulation is the predominant insect cellular reaction to
bacterial infections. Nodulation begins with formation of microaggregates
of circulating hemocytes and entrapped bacterial cells. Microaggregates
grow with addition of hemocytes and their attached bacteria, and the
process is completed with addition of a final layer of plasmatocytes. The
mature nodules are darkened by melanization. We reported that nodulation,
along with other cellular defense reactions, is mediated by eicosanoids,
which are signal moieties formed by enzymatic oxygenation of certain
polyunsaturated fatty acids. Most recently we reported that cellular
defense reactions are stimulated by treating insects with the
lipopolysaccharide (LPS) components purified from the gram-negative
bacterium Serratia marcescens. In this paper we report on the
effects of LPS purified from four gram-negative bacteria (Halomonas
elongata, Klebesella, Provedentia 2, Escherichia
coli), three strains of Prototheca portoricensis algae, and two
Lipid A moieties (a component of LPS) on nodulation reactions in
Manduca sexta. We observed different degrees of nodule formation
to challenges with the LPS from the different sources. At the low end,
larvae treated with H. elongata stimulated no more nodules than
controls while those treated with E. coli had very high numbers of
nodules. Larvae treated with three different strains of P.
portoricensis produced similar numbers of nodules and treatment with
the two synthetic Lipid As did not depart from controls. In a second
series of experiments E. coli and Provedentia 2 were
incubated with Polymyxin B (PMB) before injection. PMB binds specifically
to the Lipid A portion of LPS. In both cases larvae treated PMB-bound LPS
had lower numbers of nodules than controls treated with unbound LPS. This
research supports the hypothesis that the Lipid A is a necessary, but not
sufficient component of LPS to stimulate insect nodulation reactions.
Species 1: Lepidoptera Sphingidae Manduca sexta (tobacco hornworm)
Keywords: bacteria, algae
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA