Fate of bacteria injected into naive and immunized larvae of the tobacco hornworm Manduca sexta

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The susceptibility of Manduca sexta larvae to infection by several Gram-negative bacteria has been examined. Injected Escherichia coli D31 caused no mortality at any dose tested. Serratia marcescens and Pseudomonas aeruginosa ATCC 9027 were moderately pathogenic and caused low mortality at long times after injection of doses exceeding 10 6 bacteria/insect. P. aeruginosa P11-1 was quite virulent, killing all injected insects at doses exceeding 10 3 bacteria/insect. The period between treatment with P11-1 and death was dose-dependent. The fate of injected viable cells of P. aeruginosa ATCC 9027 and P11-1 and of E. coli D31 was examined in naive (nonimmunized) larvae of M. sexta (bacterial clearance). The concentrations of viable cells of all three bacteria were reduced significantly during the first hour following injection at all doses tested. This initial reduction in concentration was followed by a strain and dose-dependent multiphasic outcome. Immediately after the initial reduction there was a period of no change in bacterial concentration followed by either complete elimination of viable bacteria or bacterial multiplication and host death. The fate of viable cells of P. aeruginosa (P11-1 and ATCC 9027) was also examined in larvae immunized with bacterins of both strains of P. aeruginosa and of E. coli D31. In these immunized insects, the initial reduction in concentration of viable bacteria of all strains was followed by subsequent reductions to low levels of bacteria which were maintained for at least 24 hr. Viable cells of E. coli D31 were comparable to homologous bacterin as an immunogen against P. aeruginosa P11-1. It is hypothesized that the initial reduction in bacterial concentration observed with all strains was a result of nodule formation at or near the injection site. Subsequent control of bacterial multiplication in naive and immunized larvae may reflect efficient phagocytosis by hemocytes and susceptibility of bacteria to induced humoral bactericidal factors. The pathogenicity of P. aeruginosa P11-1 in naive insects may be due to relative resistance of this strain to phagocytosis resulting in multiplication prior to the appearance of bactericidal factors and the accumulation of cytotoxins excreted by the bacterium.