Gene transcription in Escherichia coli occurs is catalyzed by the core RNA polymerase associated with one of seven different sigma factors. The main sigma factor, σ⁷⁰, is in control of the transcription of the majority of the bacterial genes, in particular those associated with bacterial growth. RpoS (σ^S) is the second sigma factor in importance, being involved in the transcription of genes associated with the general stress response. Despite its relevance, the rpoS gene is polymorphic and its expression varies a lot from cell to cell. Moreover, in the laboratory, the frequency and fixation of null mutations in rpoS is much higher than expected. This is because there is a constant and strong selective pressure on rpoS and on other genes that govern its expression. In this project, we evaluate the level of rpoS polymorphism in the species E. coli in a longitudinal study. E. coli strains are isolated monthly from a local sewage-contaminated stream. The level of RpoS and its genetic polymorphism are analyzed in each bacterium. In addition, the isolated strains are tested for antibiotic resistance and mutant frequency.