Genetic analysis of <i>Salmonella enteritidis</i> biofilm formation: critical role of cellulose

<jats:title>Summary</jats:title><jats:p>We report here a new screening method based on the fluorescence of colonies on calcofluor agar plates to identify transposon insertion mutants of <jats:italic>Salmonella enteritidis</jats:italic> that are defective in biofilm development. The results not only confirmed the requirement of genes already described for the modulation of multicellular behaviour in <jats:italic>Salmonella typhimurium</jats:italic> and other species, but also revealed new aspects of the biofilm formation process, such as two new genetic elements, named as <jats:italic>bcs</jats:italic>ABZC and <jats:italic>bcs</jats:italic>EFG operons, required for the synthesis of an exopolysaccharide, digestible with cellulase. Non‐polar mutations of <jats:italic>bcs</jats:italic>C and <jats:italic>bcs</jats:italic>E genes and complementation experiments demonstrated that both operons are respon‐sible for cellulose biosynthesis in both <jats:italic>S. enteritidis</jats:italic> and <jats:italic>S. typhimurium</jats:italic>. Using two different growth media, ATM and LB, we showed that the biofilm produced by <jats:italic>S. enteritidis</jats:italic> is made of different constituents, suggesting that biofilm composition and regulation depends on environmental conditions. Bacterial adherence and invasion assays of eukaryotic cells and <jats:italic>in vivo</jats:italic> virulence studies of cellulose‐deficient mutants indicated that, at least under our experimental conditions, the production of cellulose is not involved in the virulence of <jats:italic>S. enteritidis</jats:italic>. However, cellulose‐deficient mutants were more sensitive to chlorine treatments, suggesting that cellulose production and biofilm formation may be an important factor for the survival of <jats:italic>S. enteritidis</jats:italic> on surface environments.</jats:p>