This work was supported by CICYT grants SAF2004-0033 and BFU2007-65781 from the Spanish Ministry of Science and Technology and the FEDER Plan. We warmly thank Laboratorios Normón for the kind gift of the antimicrobials used in this work. N.S. and R.M. are holders, respectively, of a fellowship associated with this grant and a scholarship from FICYT (Principado de Asturias).