Influence of Tn917 insertion on transcription of the icaADBC operon in six biofilm-negative transposon mutants of Staphylococcus epidermidis.

Insertion of Tn917 into the icaADBC operon determines a biofilm-negative phenotype in biofilm-producing Staphylococcus epidermidis due to the inactivation of the genes responsible for the synthesis of the polysaccharide intercellular adhesin. We previously characterized six isogenic biofilm-negative transposon mutants of S. epidermidis 1457 with Tn917 insertions in either icaA or icaC. Northern blot analysis using ica- and Tn917-specific probes revealed that ica sequences located upstream and downstream of the transposon insertion site were still transcribed in five mutants in which Tn917 was inserted in the same transcriptional orientation. Outward-directed transcription initiating from within the transposon resulted in the complete expression of individual ica genes. Our results indicate that not only the inactivation of the entire operon but the isolated interruption of individual icaA and icaC genes led to a biofilm-negative phenotype in S. epidermidis. Tn917 mutagenesis may also result in the activation of genes located downstream of the insertion site.