Escherichia coli is not assumed to be naturally transformable. However, several recent reports have shown that E. coli can express modest genetic competence in certain conditions that may arise in its environment. We have shown previously that spontaneous lateral transfer of non-conjugative plasmids occurs in a colony biofilm of mixed E. coli strains (a set of a donor strain harbouring a plasmid and a plasmid-free recipient strain). In this study, with high-frequency combinations of strains and a plasmid, we constructed the same lateral plasmid transfer system in liquid culture. Using this system, we demonstrated that this lateral plasmid transfer was DNase-sensitive, indicating that it is a kind of transformation in which DNase-accessible extracellular naked DNA is essential. However, this transformation did not occur with purified plasmid DNA and required a direct supply of plasmid from co-existing donor cells. Based on this feature, we have termed this transformation type as ‘cell-to-cell transformation’. Analyses using medium conditioned with the high-frequency strain revealed that this strain released a certain factor(s) that promoted cell-to-cell transformation and arrested growth of the other strains. This factor is heat-labile and protease-sensitive, and its roughly estimated molecular mass was between ∼9 kDa and ∼30 kDa, indicating that it is a polypeptide factor. Interestingly, this factor was effective even when the conditioned medium was diluted 10–5–10–6, suggesting that it acts like a pheromone with high bioactivity. Based on these results, we propose that cell-to-cell transformation is a novel natural transformation mechanism in E. coli that requires cell-derived DNA and is promoted by a peptide pheromone. This is the first evidence that suggests the existence of a peptide pheromone-regulated transformation mechanism in E. coli and in Gram-negative bacteria.

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We previously showed that non-conjugative, non-viral lateral plasmid transfer occurs in a colony biofilm of mixed Escherichia coli strains cultured on common laboratory media, such as LB agar. In this report, to investigate the possibility of this plasmid transfer under conditions possible outside the laboratory, we examined the activities of foodstuffs and mixed food extracts, which are possible nutrients for bacteria in human environments, for supporting lateral plasmid transfer. Lateral plasmid transfer occurred in colony biofilms grown on several foodstuffs (roasted meats) and on agar media containing mixed food extracts, which consisted of sugar, milk, and extracts of several foodstuffs (vegetables, fruits, and meats). Lateral plasmid transfer did not occur in liquid culture consisting of the same mixed food extracts, suggesting the importance of colony-biofilm formation. These results suggest the possibility that lateral transfer of non-conjugative plasmid between bacterial cells occurs in biofilms grown with foods or food-like nutrients in the environment.

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Edited by Renee Tsolis Keywords:Cell-to-cell transformation DNA uptake sequence Horizontal gene transfer pHSG299 R6-5 Escherichia coli a b s t r a c t Recently, we discovered a novel phenomenon, ''cell-to-cell transformation'' by which non-conjugative plasmids are transmitted horizontally in co-cultures of Escherichia coli F À strains. In this study, we aimed to identify the DNA element responsible for the high cell-to-cell transformability of pHSG299. By transplanting pHSG299 DNA fragments into pHSG399, a plasmid showing low transformability, we discovered that a specific 88 bp fragment of pHSG299 significantly promoted pHSG399 transformability. Although several short motif-like repetitive sequences (6-10 bp) were present in the 88 bp sequence, no known DNA motifs were recognized, suggesting that this 88 bp sequence (cell-to-cell transformation promoting sequence, CTPS; Accession number: AB634455) is a novel DNA element.