Non-spore-Forming Bacterial Entomopathogens: Their Toxins, Hosts and the Environment: Why Be a Pathogen

Abstract

This chapter discusses the wide range of variables that can affect the virulence of a bacterial pathogen, as well as its ability to survive. These factors are diverse and operate both externally and internally to the bacterial cell. Similarly, the host is influenced by an array of environmental parameters. These processes in turn affect the efficacy of the pathogen. Factors that impart stress on the host will increase its susceptibility to a pathogen. When not in the presence of a host, the pathogen needs to be able to maintain itself, allowing it to persist and infect subsequent hosts. Pathogens are varied in both their ability to cause disease and in their host specificity. These factors affect pathogen dispersal and transfer to another host. In some instances, spread of a pathogen may be enabled by pathogen-induced changes in host behaviour. At the subcellular level, a pathogen is subjected to natural rates of gene mutation and/or large genomic changes resulting from processes such as horizontal gene transfer, whereby new virulence determinants might be acquired. Whether inside or outside of the host, selective biotic and abiotic forces act on the pathogen and its non-pathogenic counterparts. Competition and the process of natural selection may lead to the emergence of either more benign or more virulent strains. This chapter focuses on non-spore-forming bacterial insect pathogens (entomopathogens) which, due to the absence of a spore-like survival structure, are more likely to be influenced by environmental changes. Entomopathogens may therefore provide greater clues for defining the driving forces behind pathogen evolution. A broad overview of external and subcellular variables that may influence both the pathogen and its host is given. The mechanisms of pathogen entry to the host and bacterial factors allowing a pathogen to survive within the host are then outlined. A range of pathogens, including those that are free-living, animal-vectored or plant-associated, are documented. Shared components of some multicomponent toxin systems that reside in both pathogens and symbionts are discussed, allowing us to propose a possible origin of the toxin transport machinery. In the final sections of the chapter, two case studies are presented: (1) the species-specific and chronic disease in grass grubs ( Costelytra zealandica) caused by the bacterium Serratia entomophila and (2) the rapidly killing broad host-range pathogen Yersinia entomophaga. In both these instances the respective strategies of the pathogen are discussed in relation to their environment, applying the knowledge from earlier in the chapter to help define why these two different types of pathogen reside in the same environment and ask why be a pathogen?