Adaptation and diversification of Escherichia coli K12 MC1000 in a complex environment

An inherent characteristic of natural as well as industrial environments is complexity. Scientific studies have revealed the flexible genetic and phenotypic capacities of microorganisms to cope with such complexity. However, most experiments have been conceptually simple, as they compare populations adapting to rather uniform environments. Therefore, the present work addressed bacterial evolution in a complex environment. The emphasis was on unraveling the level of diversification in respect of the genetic and physiological changes that the organism underwent, which allowed it to either acquire superior fitness or occupy a different niche.The long-term (~1000 generations) adaptive responses of E. coli K12 MC1000 in Luria-Bertani (LB) broth under aerobic, fluctuating and anaerobic conditions were evaluated. Several genetic solutions led to adaptation and a number of metabolic pathways were activated. Reproducibility of changes on genuine targets of selection was observed in parallel populations, suggesting a response triggered by medium. A specific response occurred in genes related to the metabolisms of galactose (galR and galE). Considerable heterogeneity was also found between and within populations. Differential phenotypic outcomes, suggested that parallel responses were affected by differing genomic backgrounds. Analysis of the polymorphisms in one evolved population revealed the existence of two main metabolic and interactive types. The emergence of additional specific phenotypic traits (stress resistance and metabolic properties) was confirmed. The interactive and stable coexistence of these forms revealed the presence of trade-offs and niche partitioning. The complexity of the environment has the potential to trigger the establishment of adapted and coexisting forms.