Growth of Proteus on an agar plate, starting from
an inoculum in the center. Source.
When one cell meets another, it must make decisions. This extends itself to practically every attribute of living things, from avoiding mating with self to the establishment of territorial boundaries. Do we fuse? Will you attack me? Are you a potential mate or are you lunch? And, in the bodies of vertebrates, are you something that my immune system should recognize as foreign? Microbes are not exempted from these quandaries, being that they carry out lively conversations for purposes as diverse as feeding, differentiating into biofilms or fruiting bodies, moving, surviving in a host, or sensing their own numbers. As with the social insects, microbes converse in a chemical language. As Strassman et al. say in an insightful review: Recognition seems likely to be particularly important to microbes because they undertake many processes extracellularly in the public sphere that larger organisms privatize inside.
In studying such microbial interactions, one can choose among a number of interesting models. The Gram-negative bacillus, Proteus mirabilis, practically begs us to select it.