How often have you heard of two or more bacterial species coexisting within the same cell of a host? It’s known to happen with some frequency in some amebas, insects and other invertebrates (including the strange case of the mealybug bacteria, which have an endosymbiotic bacterium that carries another bacterium inside it.) But how widespread are such ménages a trois/quatre/cinq or more outside these groups? One new example is the recently published account of two bacterial species coexisting within a single spore of a fungus. I’m talking about an arbuscular mycorrhizal fungus, Gigaspora margarita, whose cells carry a rod shaped bacterium of Burkholderia group called Candidatus Glomeribacter gigasporarum (CaGg) and coccoid one as yet nameless, related to the Mycoplasma. Neither bacterium has been cultivated.
The association is not obligatory for the fungus because it can do without at least one of the bacteria, although not without affecting its fitness. On the other hand, CaGg appears to be nutritionally dependent on the host, at least by genome analysis. Less is known about the other symbiont. The two bacteria do not occupy the same compartment, CaGg usually being seen within an intracytoplasmic vacuole and the mollicute, free in the cytoplasm. The fungus possesses more mycoplasmas than CaGgs. The two bacteria appear to differ in their ability to carry out recombination as well as in their host range. So, they are quite distinct. Not much is known about what they do for their host or whether they “talk to each other.”
The authors say: "These findings show for the first time that fungi support an intracellular bacterial microbiota, in which distinct types of endobacteria coexist in a single cell." How about green plants or vertebrates?
Desirò A, Salvioli A, Ngonkeu EL, Mondo SJ, Epis S, Faccio A, Kaech A, Pawlowska TE, & Bonfante P (2014). Detection of a novel intracellular microbiome hosted in arbuscular mycorrhizal fungi. The ISME journal, 8 (2), 257-70 PMID: 24008325