The authors would like to express their gratitude to the Clinical and Microbiological staff of Jove Hospital (Fundacion Hospital de Jove, Gijon, Spain) and Dr Kevin Dalton for revising the text. This work was conducted with the financial support of MEC-06-BIO2006-1533-C04-02 and MEC-07-BIO2007-66313. A.M. was supported by a postdoctoral grant from Ministerio de Ciencia e Innovación, Spain; A.I.P. was supported by grant FC-05-IB-130 from Fundacion para el Fomento en Asturias de la Investigacion Cientifica Aplicada y la Tecnologia; and M.M.G.S. was supported by grant MEC-06-BIO2006-1533-C04-02.