We have employed the Sequential Chip Analysis of Library Enrichment (SCALEs) technique as a powerful tool to identify the genes responsible for increased resistance in E. coli to a selective stress. Multiple genomic libraries of distinct sizes were created in E. coli for use in the SCALES technique. The SCALES technique was used to select for E. coli clones conferring increased resistance to mildly low pH levels that were toxic to wild-type strains. Three viable clones containing the rsx gene were identified from the selection. The rsx clones showed a significant increase in viability compared w/ wild type E. coli in low levels of 3-hydroxypropanoic acid. The rsx clones were mutated and cultured under acidic conditions of pH 1. The mutant clones selected outgrew native rsx clones in a solution of pH 3 by over four times.