Which subgroup of proteobacteria contains many species that are closely associated with eukaryotic hosts in mutualistic or parasitic relationships?

Recall the characteristics of each subgroup: Alpha proteobacteria are known for their close associations with eukaryotic hosts. Many species within this subgroup form mutualistic relationships, such as Rhizobium, which fixes nitrogen in symbiosis with plants. Others, like Rickettsia, are parasitic.

Compare with other subgroups: Beta proteobacteria often include species involved in nitrogen fixation but are not primarily known for close associations with eukaryotic hosts. Gamma proteobacteria include many well-known pathogens like Escherichia coli but are not specifically noted for mutualistic relationships. Delta proteobacteria are known for their predatory behavior and complex life cycles, while Epsilon proteobacteria include some pathogens but are not primarily associated with mutualistic relationships.