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Microbial communities use quorum sensing to coordinate group behaviour. Among them, the parasite Trypanosoma brucei — which causes African trypanosomiasis in humans — mediates quorum sensing through an unidentified factor that induces the development of cell-cycle arrested 'stumpy forms' of trypanosomes during the blood stage of the life cycle, in preparation for transmission. T. brucei can co-infect animals together with Trypanosoma congolense and Trypanosoma vivax; however, whether interspecies interactions occur and whether they are dependent on quorum sensing was unknown.

In a recent study, Silvester et al. investigated trypanosome interspecies relationships and found that these interactions can regulate parasite growth and development.

a conserved quorum sensing signalling pathway regulates interspecies crosstalk

First, the authors showed that the growth of T. congolense in the bloodstream was controlled by cell density. They found that T. congolense encoded conserved quorum sensing regulatory genes that were orthologous to those involved in the stumpy formation pathway in T. brucei. Functional analyses confirmed complementarity between these genes, as a T. brucei null mutant for one specific quorum sensing component (TbHYP2) was rescued by complementation with the orthologous gene from T. congolense (TcHYP2). Next, the authors found that T. congolense-conditioned culture medium could promote growth arrest and stumpy formation in T. brucei, and that this was mediated through quorum sensing. By contrast, T. congolense was not responsive to T. brucei signalling, but they could not establish whether signalling was unidirectional or whether unidirectionality was a result of the parasite isolate used. Finally, they confirmed these results in a mouse co-infection model in vivo.

Altogether, this study shows that a conserved quorum sensing signalling pathway regulates interspecies crosstalk in trypanosome co-infections and provides novel insights into trypanosome virulence and transmission.