Mobile Genetic Elements and Bacterial Pathogenesis

Abstract

Horizontally transferred genes were frequently disseminated among bacterial populations as components of mobile genetic elements, such as plasmids, phages, and transposons. This chapter focuses on bacterial pathogens, and in particular, on the role of sequence-specific mechanisms for intercellular gene transfer in the evolution of such organisms. It concentrates on virulence factors encoded by genes on mobilizable or formerly mobilizable genetic elements, especially plasmids, bacteriophages, and pathogenicity islands (PAIs). Finally, analyses based on the different classes of mobile elements can highlight the processes by which virulence genes have been transported and illuminate the evolutionary relationships between types of mobile elements. In the chapter, the last-mentioned approach is used, and bacteriophages, plasmids, and PAIs are sequentially discussed. Finally, some plasmids, such as the large Yersinia virulence plasmid and the enterohemorrhagic Escherichia coli (EHEC) virulence plasmid, contain remnants of DNA transfer systems, suggesting that they were previously capable of self-mobilization. Although the virulence genes acquired as components of mobile elements may not have been subject to host regulatory processes immediately after acquisition, they clearly have not remained autonomous agents, independent of host functions. Mobile genetic elements have clearly distributed a diverse collection of virulence genes and thereby played essential roles in the evolution of bacterial pathogens. However, a subset of virulence factors does not seem to confer any advantages on the bacterial hosts, so the forces underlying their continued production are a mystery.