Biofilms are structured communities characterized by distinctive gene expression patterns and profound physiological changes compared to those of planktonic cultures. Here, we show that many gram-negative bacterial biofilms secrete high levels of a small-molecular-weight compound, which inhibits the growth of only Escherichia coli K-12 and a rare few other natural isolates. We demonstrate both genetically and biochemically that this molecule is the amino acid valine, and we provide evidence that valine production within biofilms results from metabolic changes occurring within high-density biofilm communities when carbon sources are not limiting. This finding identifies a natural environment in which bacteria can encounter high amounts of valine, and we propose that in-biofilm valine secretion may be the long-sought reason for widespread but unexplained valine resistance found in most enterobacteria. Our results experimentally validate the postulated production of metabolites that is characteristic of the conditions associated with some biofilm environments. The identification of such molecules may lead to new approaches for biofilm monitoring and control.Biofilms are matrix-encased bacterial communities that develop on most surfaces and often constitute a reservoir of bacterial pathogens. Detrimental biofilms are difficult to eradicate due to a characteristic tolerance to biocides. They cause serious health and economic problems when they form on medical and industrial devices (11,19,24). While biofilmassociated antibiotic tolerance is considered a major physiological trait that distinguishes free cells from surface-attached cells, biofilms are also characterized by gene expression patterns that are distinctive compared to those of planktonic cultures (4,35,40,41,58). These changes are likely to result from modifications of growth conditions within biofilms, and they have been proposed to correspond to ill-understood responses triggered by the biofilm lifestyle (3,26).Recently, studies conducted with simplified, mixed communities composed of two bacterial species revealed that biofilm bacteria express competitive or cooperative behavior that does not take place within classical planktonic cultures (7,22). These results suggested that biofilm-associated weaponry could contribute to the dynamics of bacterial biofilm populations. However, thus far, the production of molecules involved in antagonistic bacterial relationships within biofilms has been poorly investigated.Here we show that the continuous-flow biofilms formed by many Escherichia coli strains and other gram-negative bacteria accumulate high levels of a small-molecular-weight compound with inhibitory activity against E. coli K-12 strains. We demonstrate both genetically and biochemically that this compound is the amino acid valine. We provide evidence that valine secretion within biofilms could be the cause of the longknown but unexplained widespread valine resistance observed for most enterobacteria. Beyond the biological significance of in-biofilm valin...

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BackgroundAlternative sigma factors trigger various adaptive responses. Lactobacillus sakei, a non-sporulating meat-borne bacterium, carries an alternative sigma factor seemingly orthologous to σH of Bacillus subtilis, best known for its contribution to the initiation of a large starvation response ultimately leading to sporulation. As the role of σH-like factors has been little studied in non-sporulating bacteria, we investigated the function of σH in L. sakei.ResultsTranscription of sigH coding for σH was hardly affected by entry into stationary phase in our laboratory conditions. Twenty-five genes potentially regulated by σH in L. sakei 23 K were revealed by genome-wide transcriptomic profiling of sigH overexpression and/or quantitative PCR analysis. More than half of them are involved in the synthesis of a DNA uptake machinery linked to genetic competence, and in DNA metabolism; however, σH overproduction did not allow detectable genetic transformation. σH was found to be conserved in the L. sakei species.ConclusionOur results are indicative of the existence of a genetic competence state activated by σH in L. sakei, and sustain the hypothesis that σH-like factors in non sporulating Firmicutes share this common function with the well-known ComX of naturally transformable streptococci.