The secreted staphylococcal biofilm protein Sbp forms biomolecular condensates in the presence of DNA

Abstract

Staphylococcus epidermidis is the leading cause of device-related infections, primarily due to its ability to form biofilms, surface-adherent bacterial communities that confer remarkable resistance to antibiotics and host defenses. Small basic protein, Sbp, is a 16- kDa protein expressed by S. epidermidis that has been shown to be crucial for biofilm formation, but little is known about its function. Sbp features a high proportion of basic residues as well as several predicted regions of intrinsic disorder. Consistent with its high positive charge density, Sbp is shown here to interact with double-stranded DNA, a ubiquitous component of the biofilm matrix, forming soluble complexes or large aggregates with short or longer DNA oligonucleotides, respectively. The observed multivalent interaction of Sbp with DNA along with its predicted disorder suggested that it might form biomolecular condensates with DNA. Confocal and differential interference contrast microscopy revealed that Sbp and dsDNA form phase-separated droplets and/or solid aggregates, depending on the concentration and stoichiometry of Sbp and DNA as well as the DNA oligonucleotide length. Fluorescence recovery after photobleaching experiments demonstrated that Sbp-DNA condensate droplets initially exhibit liquid-like behavior but gradually transition to a gel-like state. This work provides the first evidence that Sbp binds DNA and undergoes biomolecular condensation, revealing a previously unrecognized mechanism that may contribute to biofilm matrix organization in S. epidermidis.