Evolutionary aspects of toxin-producing bacteria

Protein toxins that constitute key virulence determinants for pathogenic bacteria share the common feature of having discrete functional domains with a spectrum of biological activities, from mediating cellular uptake to manipulating cellular processes and immune responses, which serve to enhance bacterial survival and dissemination during infection. Toxins and toxic secreted effectors are encoded by genes located on pathogenicity islands. The plethora of genome sequence data now available has revealed the vast extent to which toxin gene evolution has occurred through mutation and genetic exchange via horizontal gene transfer, recombination, and gene-shuffling. These events are mediated through plasmids, phages, transposons, and other transmissible elements. Evidence points to toxin evolution and transmission occuring not only within the human and animal host environments, but also in soil and aquatic environments, the phyllosphere, and the guts of insects, parasites, and other vectors.