The auxotrophic aroA mutant of Aeromonas hydrophila as a live attenuated vaccine against A. salmonicida infections in rainbow trout (Oncorhynchus mykiss) Jose Vivas 1 , Jorge Rian ˜o 1 , Begon ˜ a Carracedo 1 , Blanca E. Razquin 1 , Pilar Lo ´ pez-Fierro 1 , Germa ´ n Naharro 2 , Alberto J. Villena 1 * 1 Departmento de Biologı ´a Celular, Facultad de Ciencias Biolo ´gicas y Ambientales, Universidad de Leo ´n, 24071 Leo ´n, Spain 2 Departamento de Patologı ´a Animal (Sanidad Animal), Universidad de Leo ´n, 24071 Leo ´n, Spain Received 30 January 2003; accepted 27 May 2003 Abstract An auxotrophic aroA mutant of the Aeromonas hydrophila AG2 strain is a live attenuated vaccine against A. hydrophila infection in rainbow trout (Oncorhynchus mykiss). The protection conferred by the live attenuated vaccine against A. salmonicida strains is reported here, and several parameters of the specific and non-specific immune response in vaccinated trout were characterised. Vaccination with a dose of 10 7 cells/fish of the aroA mutant elicited significant protection against the Hooke and DK30 strains of A. salmonicida (relative percent survival RPS >60%). This cross-protection correlated moderately with the activation of the humoral and cellular specific immune responses, which show cross-reactivity against antigens shared by the two bacterial species, and a moderate increase in the lysozyme and antiprotease activities in the serum of vaccinated trout.  2003 Elsevier Ltd. All rights reserved. Keywords: Live vaccine; Cross protection; Immune response; Aeromonas hydrophila; Aeromonas salmonicida; Oncorhynchus mykiss; Rainbow trout 1. Introduction Diseases caused by species of the genus Aeromonas, especially by A. salmonicida (furunculosis, and ulcerative dermatitis) and A. hydrophila (haemorrhagic septicaemia and fin/tail rot) are of major impact in aquaculture [1]. A. hydrophila may also represent a risk of infection for human handlers and consumers [2,3]. The pathogenicity of A. hydrophila appears to involve several extracellular products including proteases, haemolysin, enterotoxins, acetylcholinesterase, as well as a surface array protein layer (S layer). * Corresponding author. Tel./fax: +34-98729-1487 E-mail address: dbcavc@unileon.es (A.J. Villena). Fish & Shellfish Immunology 16 (2004) 193–206 Fish & Shellfish Immunology www.elsevier.com/locate/fsi 1050-4648/04/$ - see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1050-4648(03)00078-0