IsdC from Staphylococcus lugdunensis Induces Biofilm Formation under Low-Iron Growth Conditions

Staphylococcus lugdunensis is a coagulase-negative staphylococcus that is a commensal of humans and an opportunistic patho-gen. It can cause a spectrum of infections, including those that are associated with the ability to form biofilm, such as occurs with endocarditis or indwelling medical devices. The genome sequences of two strains revealed the presence of orthologues of the ica genes that are responsible for synthesis of poly-N-acetylglucosamine (PNAG) that is commonly associated with biofilm in other staphylococci. However, we discovered that biofilm formed by a panel of S. lugdunensis isolates growing in iron-restricted me-diumwas susceptible to degradation by proteases and not by metaperiodate, suggesting that the biofilmmatrix comprised pro-teins and not PNAG.When the iron concentration was raised to 1 mM biofilm formation by all strains tested was greatly re-duced. Amutant of strain N920143 lacking the entire locus that encodes iron-regulated surface determinant (Isd) proteins was defective in biofilm formation under iron-limited conditions. An IsdC-null mutant was defective, whereas IsdK, IsdJ, and IsdB mutants formed biofilm to the same level as the parental strain. Expression of IsdC was required both for the primary attach-ment to unconditioned polystyrene and for the accumulation phase of biofilm involving cell-cell interactions. Purified recombi-nant IsdC protein formed dimers in solution and Lactococcus lactis cells expressing only IsdC adhered to immobilized recombi-nant IsdC but not to IsdJ, IsdK, or IsdB. This is consistent with a specific homophilic interaction between IsdCmolecules on neighboring cells contributing to accumulation of S. lugdunensis biofilm in vivo. Coagulase-negative staphylococci (CoNS) and Staphylococcusaureus are the predominant etiological agents of medical de