To measure the relative fitness of all mutants in thelibrary for a single environmental condition, we uti-lize previously established methods that leverage DNAbarcoding and high-resolution lineage tracking. Wecompete a pool of barcoded strains against a highly-abundant reference strain, the wild type (WT) ances-tor, which takes up 95% of the culture. We quantifythe frequencies of barcodes over time via amplicon se-quencing. Based on these frequency trajectories, wecan infer the relative fitness of thousands of strains(see Methods for more details) (5, 22, 56).
I appreciate this well-structured and efficient experimental design! However, I wonder about the potential for metabolic interactions between strains potentially confounding the results of this pooled competition set-up. Cells harboring mutations (particularly those in metabolic regulation pathways like Ras and TOR) might show altered secretion of nutritious or toxic compounds that could affect neighboring cells. This seems particularly relevant for interpreting fitness effects in the context of your "limiting functions" model. Perhaps these effects are not as concerning as I am flagging here since the local environment of neighboring cells for each mutant will differ across batch replicates, which could help counter some of the confounding nature of the pooled design.