Genetic transformation in freshwater: Escherichia coli is able to develop natural competence

Abstract

Until now, Escherichia coli was thought to be unable to develop natural competence, i.e., genetic transformation could be achieved only artificially with the aid of nonphysiological concentrations of calcium ions or by other treatments. We have tested the competence development of E. coli through transformation under natural conditions in river water, springwater, and mineral water which contained between 0 and 11 mM Ca 2؉ , using pUC18 DNA. The presence of calcium ions at concentrations as low as 1 to 2 mM was sufficient to obtain transformants. Variations in the temperature of incubation were not required for competence development but had an influence on the transformation frequency. Using water from mineral springs originating from calcareous regions, we have obtained transformation frequencies with laboratory strains of E. coli similar to those reported for other gram-negative bacteria known to develop natural competence. The competence development of E. coli is most probably internally regulated (as for the other gram-negative bacteria), and inadequate conditions chosen for the transformation tests in the laboratory might impair the detection of higher natural transformation frequencies. The results will enhance our knowledge about the fate of laboratory or production strains of E. coli cells reaching natural aquatic ecosystems.

Horizontal transfer of nonconjugative plasmids in a colony biofilm ofEscherichia coli

“…Many previous reports of bacterial gene transfer in biofilms have described conjugation (Davey & O'Toole, 2000;Roberts et al, 2001;Bushman, 2002;Molin & Tolker-Nielsen, 2003;Sorensen et al, 2005), and several others reported in situ transformation among naturally transformable bacteria (Stewart et al, 1983;Wang et al, 2002;Hendrickx et al, 2003). Although natural transformation of E. coli by artificially added DNA was reported in several experimental systems (Baur et al, 1996;Bauer et al, 1999;Tsen et al, 2002;Woegerbauer et al, 2002;Maeda et al, 2003Maeda et al, , 2004, our result provides the first evidence that the nonconjugative nonviral DNA can be horizontally transmitted between E. coli cells. This result challenges conventional bacterial genetics.…”

“…Escherichia coli is not thought to be naturally transformable; it develops high genetic competence only under artificial conditions, such as exposure to high Ca 21 concentrations (Mandel & Higa, 1970). However, recently, several reports showed that E. coli can express modest genetic competence under certain conditions possible in its environments (Baur et al, 1996;Bauer et al, 1999;Tsen et al, 2002;Woegerbauer et al, 2002;Maeda et al, 2003). We recently revealed that E. coli cells within a colony biofilm developed modest genetic competence under Ca 21 -poor conditions, such as growth and incubation on Luria-Bertani (LB) agar (Maeda et al, 2004).…”

Survival of free DNA encoding antibiotic resistance from transgenic maize and the transformation activity of DNA in ovine saliva, ovine rumen fluid and silage effluent

“…E. coli populations are known to increase rapidly under conditions of fasting [19]. Furthermore, E. coli can be induced to take up DNA and is known to develop natural competence under certain conditions [20].…”

Identification of a novel DNA element that promotes cell-to-cell transformation in Escherichia coli

“…In Escherichia coli, artificial transformation by the calcium chloride (CaCl 2 ) [9,10] or other methods [11,12] is well known as a common laboratory technique, but natural transformation is poorly understood. Several reports [13][14][15][16][17][18] including ours [19][20][21] demonstrated that E. coli can develop modest genetic competence for transformation under conditions occurring in usual natural and human environments.…”