The isolation and characterization of inflammatory polypeptides from Staphylococcus epidermidis

Abstract

Septic shock is the result of an overwhelming immune response to bacterial products, or modulins. Lipopolysaccharide is the primary modulin involved in Gram negative sepsis, but an analogous agent from Gram positive bacteria has not yet been identified. In order to characterize what such a modulin might be, a hospital strain of S. epidermidis which had been observed to secrete or shed potent inflammatory agents was examined more closely. Sephadex gel chromatography revealed that the active component in bacterial supernatants had an apparent molecular weight of about 35 kDa. This active material partitioned into the organic layer following hot, aqueous phenol extraction and was found to consist of a complex or aggregate of three polypeptides. These were designated phenol soluble modulins (PSM) alpha, beta, and gamma. The PSMs were isolated by phenol extraction, dialysis and High Pressure Liquid Chromatography and characterized by Edman degradation, mass spectrometry, and genetic sequencing. PSM$\gamma$ is a 25 amino acid protein identical to the previously characterized S. epidermidis delta toxin, while PSM$\alpha$ and PSM$\beta$ were 22 and 44 amino acid polypeptides, respectively, with more distant homology to known staphylococcal toxins. PSM$\alpha$ was found to be the most active and least abundant of the three modulins. All three PSMs are highly hydrophobic and lack arginine, cysteine, histidine, proline, and tyrosine, and only the gene for PSM$\gamma$ is located within the agr virulence locus.