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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 Dictyostelium discoideum 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 D. discoideum aggregates and may play a central role in defence against pathogens. We measured the number and functionality of sentinel cells in aggregates of D. discoideum infected by bacterial endosymbionts in the genus Paraburkholderia. Infected D. discoideum produced fewer and less functional sentinel cells, suggesting that Paraburkholderia may interfere with its host’s immune system. Despite impaired sentinel cells, however, infected D. discoideum were less sensitive to ethidium bromide toxicity, suggesting that Paraburkholderia may also have a protective effect on its host. By contrast, D. discoideum infected by Paraburkholderia 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 D. discoideum and Paraburkholderia, which has considerable potential as a model for the study of symbiosis. 

The social amoeba Dictyostelium discoideum engages in a complex relationship with bacterial endosymbionts in the genus Paraburkholderia, which can benefit their host by imbuing it with the ability to carry prey bacteria throughout its life cycle. The relationship between D. discoideum and Paraburkholderia has been shown to take place across many strains and a large geographical area, but little is known about Paraburkholderia’s potential interaction with other dictyostelid species. We explore the ability of three Paraburkholderia species to stably infect and induce bacterial carriage in other dictyostelid hosts. We found that all three Paraburkholderia species successfully infected and induced carriage in seven species of Dictyostelium hosts. While the overall behaviour was qualitatively similar to that previously observed in infections of D. discoideum, differences in the outcomes of different host/symbiont combinations suggest a degree of specialization between partners. Paraburkholderia was unable to maintain a stable association with the more distantly related host Polysphondylium violaceum. Our results suggest that the mechanisms and evolutionary history of Paraburkholderia’s symbiotic relationships may be general within Dictyostelium hosts, but not so general that it can associate with hosts of other genera. Our work further develops an emerging model system for the study of symbiosis in microbes. 

Bacterial endosymbionts can provide benefits for their eukaryotic hosts, but it is often unclear if endosymbionts benefit from these relationships. The social amoeba Dictyostelium discoideum associates with three species of Paraburkholderia endosymbionts, including P. agricolaris and P. hayleyella. These endosymbionts can be costly to the host but are beneficial in certain contexts because they allow D. discoideum to carry prey bacteria through the dispersal stage. In experiments where no other species are present, P. hayleyella benefits from D. discoideum while P. agricolaris does not. However, the presence of other species may influence this symbiosis. We tested if P. agricolaris and P. hayleyella benefit from D. discoideum in the context of resource competition with Klebsiella pneumoniae, the typical laboratory prey of D. discoideum. Without D. discoideum, K. pneumoniae depressed the growth of both Paraburkholderia symbionts, consistent with competition. P. hayleyella was more harmed by interspecific competition than P. agricolaris. We found that P. hayleyella was rescued from competition by D. discoideum, while P. agricolaris was not. This may be because P. hayleyella is more specialized as an endosymbiont; it has a highly reduced genome compared to P. agricolaris and may have lost genes relevant for resource competition outside of its host. 

Protists are a diverse group of typically single cell eukaryotes. Bacteria and archaea that form long-term symbiotic relationships with protists may evolve in additional ways than those in relationships with multicellular eukaryotes such as plants, animals, or fungi.