IsdC from Staphylococcus lugdunensis induces biofilm formation in low-iron

33 Staphylococcus lugdunensis is a coagulase-negative Staphylococcus that is a commensal of 34 humans and an opportunistic pathogen. It can cause a spectrum of infections including those that are 35 associated with the ability to form biofilm such as endocarditis or indwelling medical devices. The 36 genome sequences of two strains revealed the presence of orthologues of the ica genes that are 37 responsible for synthesis of poly-N-acetyl glucosamine (PNAG) that is commonly associated with 38 biofilm in other staphylococci. However, here we discovered that biofilm formed by a panel of S. 39 lugdunensis isolates growing in iron-restricted medium was susceptible to degradation by proteases 40 and not by metaperiodate suggesting that the biofilm matrix comprised proteins and not PNAG. 41 When the iron concentration was raised to 1mM biofilm formation by all strains tested was greatly 42 reduced. A mutant of strain N920143 lacking the entire locus that encodes iron-regulated surface 43 determinant (Isd) proteins was defective in biofilm formation under iron-limited conditions. A IsdC 44 null mutant was defective whereas IsdK, IsdJ and IsdB mutants formed biofilm to the same level as 45 the parental strain. Expression of IsdC was required both for the primary attachment to 46 unconditioned polystyrene and for the accumulation phase of biofilm involving cell-cell 47 interactions. Purified recombinant IsdC protein formed dimers in solution and Lactococcus lactis 48 cells expressing only IsdC adhered to immobilized recombinant IsdC but not to IsdJ, IsdK or IsdB. 49 This is consistent with a specific homophilic interaction between IsdC molecules on neighbouring 50 cells contributing to accumulation of S. lugdunensis biofilm in vivo. 51