Key Points
The wealth of genome data for host-adapted bacteria have made it possible to re-examine and quantify the level of integration between bacteria and their hosts.
For genera with multiple hosts there seems to be a correlation between host range, host population size and the repertoire of bacterial outer surface proteins.
Shifts in bacterial lifestyle towards greater interaction with the host are mediated by mechanisms such as gain of new functions through horizontal gene transfer and duplication and functional divergence of existing genes.
Bacteria from many different phyla have established relationships with animals, plants and invertebrates. Mutualistic relationships are mostly seen in invertebrates, suggesting that plants and animals present barriers to obligate mutualism that are difficult to overcome.
Secretion systems in bacteria are essential for the interaction with their eukaryotic hosts. Horizontal gene transfer events, functional diversification and innovation through gene duplication of these systems have made it possible for bacteria to adapt to their host in various ways, giving rise to pathogenic or mutualistic relationships.
Obligate mutualistic bacteria with highly eroded genomes have survived owing to selective pressure on the essential function they contribute to eukaryotic fitness. This contribution has resulted in intimate relationships, in which some bacteria have transferred their genes to the host nuclear genome or established consortia with other co-habiting symbionts for the benefit of the host.