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.
Transfer of plasmid-borne antibiotic resistance genes in Escherichia coli wild-type strains is possible by transformation under naturally occurring conditions in oligotrophic, aquatic environments containing physiologic concentrations of calcium. In contrast, transformation is suppressed in nitrogen-rich body fluids like urine, a common habitat of uropathogenic strains. Current knowledge indicates that transformation of these E. coli wild-type strains is of no relevance for the acquisition of resistance in this clinically important environment.Horizontal gene transfer is a recognized process that allows the rapid spread of antibiotic resistance genes inside and outside of hospitals, thereby impeding antimicrobial chemotherapy (17). Resistance determinants are readily acquired and disseminated within and among bacterial populations by (i) conjugation, (ii) transduction, and (iii) transformation, virtually without barriers between species (6, 12). Natural genetic transformation is characterized by the uptake of free DNA by a recipient bacterium, its chromosomal integration or extrachromosomal stabilization, and its expression, which leads to a new phenotype (7,11). Calcium appears to play a pivotal role for the development of bacterial competence, an inducible property of many bacterial species and a prerequisite for transformation, in both gram-positive and gram-negative bacteria (18,19,25). Laboratory strains of Escherichia coli were shown to be transformable by a nonphysiological Ca 2ϩ concentration of 100 mM (9, 22). Recombination-and DNase-deficient laboratory strains are generally used, with a temperature shift from 0 to 37°C after the addition of free DNA (9,22,27). Such conditions are never encountered in the original biotope of E. coli (i.e., the mammalian gastrointestinal and urogenital tracts). Consequently, this species was not considered to be transformable in its natural habitats (11, 26). However, there have been indications that the induction of competence in E. coli is controlled physiologically rather than physicochemically (4), and natural transformation of laboratory strains of E. coli has been demonstrated to occur in river, spring, and mineral water (2) and in foodstuffs (1) but not yet in the body fluids of mammals. Our intention was to evaluate whether wild-type clinical isolates of E. coli were naturally transformable in freshwater, as has already been reported for laboratory strains, and to evaluate the impact of transformation on the dissemination of antibiotic resistance genes under conditions of clinical significance.Bacterial strains and plasmid DNA. Eight ampicillin-sensitive, nonpathogenic E. coli isolates from sputum or swab samples of the oropharynxes of hospitalized patients with sinusitis or otitis were chosen at random without additional criteria of selection. In addition, we tested four pathogenic strains which were isolated from two patients with urogenital infections. From each patient, a single strain was isolated from feces (S 988 and S 1018) and from urine (U 988 and U 10...
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