The ability of the strain Pseudomonas aeruginosa PA14, originally isolated from a human patient, to infect plants allowed for high throughput screenings of bacterial mutants attenuated in virulence in more than one host. The mutants first selected in lettuce and Arabidopsis thaliana were further tested in the mouse model, revealing several bacterial genes that are required for pathogenicity in both plants and animal hosts (see figure below).
Besides studying the pathogenicity genes, our group is also interested in studying the hosts' responses against infection and to identify the commonalities among them. For these studies, Arabidopsis is the plant model of choice, since its complete genome is known and DNA microarrays to study global expression are readily available. Furthermore, responses to specific bacterial gene products can be assessed, by comparing wild type and mutant strains.
An example of a nature protocol

Injection of a bacterial suspension
in Arabidopsis leaves