Long Term Co-Culture in Escherichia coli and Pseudomonas aeruginosa
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Most bacteria live in heterogeneous mixture in nature and as a result, constantly interact with other species. As bacteria are haploid, genetically tractable, and in some cases have generation times of less than one day, they are used as model organisms for evolution. In this study we used Escherichia coli strain MG1655 and two strains of Pseudomonas aeruginosa PA01 and PA14. E. coli and P. aeruginosa strains were grown as monocultures or as mixed cultures over a two month period, with the cultures being transferred daily. Throughout the culture protocol, representative cultures were collected and frozen for later analysis. When grown as batch cultures, E. coli did not survive with P. aeruginosa beyond 6-11 days. In contrast, when transferred regularly, the two organisms co-existed in approximately equal numbers in both planktonic and biofilm populations for two months (estimated 500 generations for monocultures). In co-culture experiments, evolved P. aeruginosa PA14 strain but not PAO1 lost fitness when grown with the parent E. coli strain. The evolved E. coli MG1655 strain lost fitness when grown with the parent PAO1 but not the PA14 strain. Several phenotypes changed during long-term culture. Indole production in E. coli increased during co-culture with both P. aeruginosa strains although the increase was significant only during co-culture with PA14. Pyocyanin production, commonly associated with P. aeruginosa competition, was decreased during long-term co-culture but in strain PAO1, increased significantly during long-term monoculture. Other P. aeruginosa phenotypes associated with quorum signaling were also reduced during long-term co-culture. Overall, this study shows how bacterial competition can be investigated as a component of natural selection during experimental evolution.