Survival in soil of genetically engineeredEscherichia colias related to inoculum density, predation and competition

The factors responsible for the regulation of engineered Escherichia coli population density were investigated in soil microcosms. In sterile soil, the E. coli populations increased up to an equilibrium level of 108 CFU.g−1 regardless of the inoculum size. In natural soil, E. coli populations exhibited a decrease in number which could be well described by polynomial equations on a log-scale. Increase in the initial density led to enhanced persistence: E. coli viable counts rapidly fell under the detection limit (102 CFU g−1) when introduced at 106 and 107 CFU g−1, or remained constant at this level for a high inoculum (108 CFU g−1). Soil indigenous predators and competitors were both involved in the regulation of the E. coli population density. The role of protozoan predation in E. coli population decline was demonstrated by the simultaneous increase of the indigenous amoeba counts and the decline of E. coli cell number. The absence of amoebae delayed the fall in abundance of E. coli populations. The presence of a protozoa-free microflora resulted in a 104-fold reduction of the final population size reached by E. coli in sterile soil, highlighting the role of soil competitors in E. coli die-out from soil. Although the engineered and wild-type E. coli displayed similar dynamics in sterile and non sterile soil, simultaneous inoculation of both populations led to a competitive disadvantage for the engineered strain in all microcosms tested. The ecological effects of the release of genetically engineered E. coli should be evaluated on a case by case basis for each soil with regards to the structure of the soil microflora. Furthermore, in situ intraspecific competition studies can provide additional knowledge about the relative fitness of a modified bacterium as compared to a wild-type counterpart.