Identification of immunogenic and serum binding proteins of Staphylococcus epidermidis

Staphylococcus epidermidis is a commensal of human skin and a leading cause of nosocomial bloodstream infections. Limited information is available about S. epidermidis proteins that are expressed upon transition to the bloodstream or those involved in host-pathogen interactions. A cell surface fraction from S. epidermidis 0-47 grown in rabbit serum to mimic environmental signals encountered during a bloodstream infection was separated by two-dimensional (2D) gel electrophoresis. Following 2D separation, the proteins were transferred to nitrocellulose and detected with either pooled sera generated in rabbits immunized with live S. epidermidis 0-47 or with biotin-labeled serum proteins eluted from the surface of bacteria grown in rabbit serum. Twenty-nine immunoreactive or serum binding proteins of S. epidermidis were identified by mass spectrometry. Twenty-seven of the corresponding genes were expressed in Escherichia coli, and the purified recombinant proteins were used to immunize mice. In a preliminary screen, 12 of the 27 recombinant proteins induced a response that reduced the number of bacteria recovered from the spleen or bloodstream of infected mice. In subsequent vaccination studies, 5 of the 12 proteins resulted in a statistically significant reduction in the number of bacteria. The identification of five candidate vaccine antigens from the initial screen of only 29 proteins demonstrates the utility of this approach.