Bacteria strains
Our model synthetic community consists of five bacterial isolates previously cultured and typed (based on 16S rDNA) from potting compost, with the 5 isolates chosen because of their consistent persistence over 6 weeks in multiple replicates in diluted tryptic soy broth (TSB), and ease at which they can be distinguished morphologically on King’s medium B (KB) agar plates (Hesse et al. 2018). The closest sequence matches of the 5 isolates are: Pseudomonas corrugata ,Stenotrophomonas rhizophilia , Achromobacter agilis ,Variovorax guangxiensis and Ochrobactrum daejonense . Competition experiments have shown that the interactions among these species range from growth-enhancing asymmetric cross-feeding to competition (Padfield et al. 2020). We have demonstrated stable coexistence of all species combinations (i.e. 2, 3, 4 and 5 species communities) through the ability of each species to invade from rare for at least 28 days without pulse disturbance and for at least 10 weeks when pulsed weekly (Castledine et al. 2020; Padfield et al. 2020). Indeed, the species have coexisted whilst retaining morphological distinctiveness for over the 13 months we have propagated them to date under weekly pulse disturbance (unpublished). For our invader, we used the opportunistic human pathogen Pseudomonas aeruginosa PA01 (O’Brien et al. 2018) which also grows well in TSB. P. aeruginosa is a fast growing, competitive species capable of invading and surviving in an array of environments including rivers, medical equipment and humans (Chatterjee et al. 2017; Rutherfordet al. 2018). This strain was lacZ-marked, which allows it to be easily identified when plated on agar containing X-gal due to a blue colour change (O’Brien et al. 2018).