Figure 2

Bacterial pathogens. Bacterial phylogeny is based on work by Hugenholtz et al. [69]. Bacterial phyla are shown on the leaves, with the Firmicutes and the Proteobacteria subdivided to the class level (taxa are shown in italics). Their phylogeny is further broken down to the order level with the use of smaller branches. These two phyla account for roughly three quarters of the described infectious bacterial species (with the noticeable exception of deltaproteobacteria pathogens). Proteobacteria contain both important plant and animal pathogens. Proteobacterial plant pathogens are not clustered, but are observed in alpha, beta, and gamma subdivisions. Some plant and animal pathogen species share the same genus classification (e.g. Ralstonia and Pseudomonas), and for at least one species, Burkholderia cepacia, different subspecies are plant and human pathogens. Of the 20 bacterial phyla currently recognized in the NCBI taxonomy, thirteen are not present in Figure 2 due to the absence of noteworthy pathogens. Some of these missing phyla are relatively important, like the Cyanobacteria, but most of the remaining phyla are restricted to a handful of species and/or environmental niches. Also of interest is the relative weight of the infectious agents within their respective phylum: While virtually all Spirochaetes and Chlamydiae constitute potential infectious agents due to their parasitic lifestyle, the phylum Actinobacteria has few pathogens relative to the overall diversity of the phylum. It should be cautioned that our current view of bacterial diversity is biased towards cultivatable organisms, which represent a small fraction of bacterial diversity [69]. Therefore, this compilation represents the well-recognized infectious bacterial agents, but should in no way be considered exhaustive or complete.