Symbiotic bacteria, immune-like sentinel cells, and the response to pathogens in a social amoeba.

Some endosymbionts living within a host must modulate their hosts' immune systems in order to infect and persist. We studied the effect of a bacterial endosymbiont on a facultatively multicellular social amoeba host. Aggregates of the amoeba contain a subpopulation of sentinel cells that function akin to the immune systems of more conventional multicellular organisms. Sentinel cells sequester and discard toxins from aggregates and may play a central role in defence against pathogens. We measured the number and functionality of sentinel cells in aggregates of infected by bacterial endosymbionts in the genus . Infected produced fewer and less functional sentinel cells, suggesting that may interfere with its host's immune system. Despite impaired sentinel cells, however, infected were less sensitive to ethidium bromide toxicity, suggesting that may also have a protective effect on its host. By contrast, infected by did not show differences in their sensitivity to two non-symbiotic pathogens. Our results expand previous work on yet another aspect of the complicated relationship between and , which has considerable potential as a model for the study of symbiosis.