Endosymbiosis was crucial to the development of complex life and it continues to happen to this day. The origin of mitochondria see bacterial guests in archaea hosts, some of which later took in cyanobacteria which became chloroplasts. Modern "eukaryotes" have varying ranges of ancestry from these two domains with animals being about 50-50. More recent endosymbiosis examples include bacterial or protist guests in eukaryotic hosts.
But are there any examples of a bacterial guest endosymbiosis in a bacterial host? Bdellovibrio bacteriovorus enters other bacteria but it destroys them much like the lytic phage cycle. Bacteria inside bacteria inside mealybug cells have been found. However, this isn't an example of a bacterial symbiosis of another free-living bacterium or even of an extracellular animal-associated bacterium. Are there any examples of bacterial endiosymbiosis of bacteria that aren't intimately associated with the intracellular machinery of a eukaryote?