This work was supported by grant MEC-06-BI02006-15336-C04-02 from Ministerio de Ciencia e Innovación, Spain and UNOV-08-MB-3 de la Universidad de Oviedo, Spain. The authors would like to acknowledge Professor Jesus Sanchez (Area Microbiologia, Universidad de Oviedo) for their critical reading of the manuscript and help with phylogenetic analysis. A.I.P. was supported by grant CENIT-07-CLEAM, Ministerio de Industria, Turismo y Comercio(CDTI), Spain. A.M. was supported by grant MEC-07-BIO2007-66313 from the DGI, Subdireccion General de Proyectos de Investigacion, MEC, Spain, and M.M.G.S. was supported by grant MEC-06-BIO2006-1533-C04-02. We also thank Priscilla Chase for revising the text.