Insect-Pathogen Relationships

Abstract

The insects, the most abundant and diverse animal group on earth, are associated with a tremendous number of microorganisms. As with other eukaryotic systems, the symbiotic associations among different insects and microbes may be classified as mutualistic, commensalistic, saprophytic, or parasitic. Mutualistic relationships provide benefit to both the host insect and the microbe. In fact, some of the best-studied models of animal-microbe mutualism involve insects (Schwenamber and Gassner, 1989). The mutualistic microbes, the endosymbionts(endocytobionts), the exocellular tissue-associated symbionts, and the ectosymbionts, are often associated with insects which possess specialized feeding habits. For example, virtually all of the plant-sucking Homopterans and many of the cellulose-feeding and blood-feeding insects harbor endosymbionts (Table 1-1). Various termites utilize exocellular tissue-associated symbionts to assist in the processing of ingested cellulose. The mushroom farmers, leaf-cutting ants, macrotermites, ambrosia beetles, and siricid wasps consume ectosymbionts as a major component of their diet. Nutrient supplementation is the function commonly affiliated with these insect-microbe interactions. However, these microbes may play a key role in the development, speciation, and sex determination of host insects. For example, the endosymbiotic Wolbachia, commonly found in many insects, are believed to play a key role in speciation (see Chapter 6). The endosymbionts exist within specialized cells, mycetocytes, or the tissues, mycetomes, and are intimately associated with various host tissues. These associations are common and are found in ∼ 10% of insect species. The majority of endosymbionts are characterized as gram negative pleomorphic bacteria. Genetic analysis suggests that these primitive organisms may be linked to eukaryotic organelles (mitochondria). Treatment of insects with heat shock, antibiotics, or lysozyme may produce symbiont-free aposymbionts which have been useful in deciphering the contributions made by these microbes. For example, aposymbiotic aphids lacking the nutrients provided by the microbes fail to reproduce.

“Pathogenicity is not the rule. Indeed, it occurs so infrequently and involves such a small number of species, considering the huge population of bacteria (microbes) on the earth, that it is a freakish aspect. Disease usually results from inconclusive negotiations for symbiosis, an overstepping of the line by one side or the other, a biologic misinterpretation of borders. ”...

L. Thomas,The Lives of a Cell, 1974