Short Communication Susceptibility of Photobacterium damselae subsp. piscicida strains isolated from Senegalese sole, Solea senegalensis Kaup, and gilthead seabream, Sparus aurata L., to several antibacterial agents E Martínez-Manzanares, S T Tapia-Paniagua, P Díaz-Rosales, M Chabrillón and M A MoriÇigo Department of Microbiology, Faculty of Sciences, University of Ma ´laga, Ma ´laga, Spain Keywords: antibacterial agents, gilthead seabream, minimum inhibitory concentrations, Photobacteri- um damselae subsp. piscicida, Senegalese sole, susceptibility. Diseases of bacterial origin are probably the most significant cause of economic losses in the aquacul- ture industry. Antibiotic treatment of bacterial diseases affecting farmed fish has been applied for many years. However, the occurrence of antibiotic resistance in pathogenic bacteria and the limited number of treatment options in certain countries are limiting the usefulness of these substances (Uhland & Higgins 2006). Photobacterium damselae subsp. piscicida is a fish pathogen responsible for important losses in fish aquaculture worldwide. The importance of extra- cellular products, the presence of iron uptake mechanisms, enzymatic activities such as catalase and superoxide dismutase, and the capsular material as virulence factors in P. damselae subsp. piscicida are well documented (Magarin ˜os, Santos, Romalde, Rivas, Barja & Toranzo 1992; Arijo, Borrego, Zorrilla, Balebona & Morin ˜igo 1998; Bakopoulos, Hanif, Poulos, Galeotti, Adams & Dimitriadis 2004; Dı ´az-Rosales, Chabrillo ´n, Arijo, Martı ´nez- Manzanares, Morin ˜igo & Balebona 2006). Effective prophylactic strategies based on vaccination pro- grammes exist for farmed fish such as gilthead seabream, Sparus aurata L. (Magarin ˜os, Romalde, Santos, Casal, Barja & Toranzo 1994), and sea bass, Dicentrarchus labrax (L.) (Bakopoulos, Volpatti, Gusmani, Galeotti, Adams & Dimitriadis 2003). For some species such as Senegalese sole, Solea senegalensis Kaup, vaccination strategies need to be optimized and for this reason, chemotherapy remains as a useful weapon against this pathogen. Evaluation of the susceptibility and resistance of the pathogen to antibacterial agents must be carried out to achieve successful treatment, and pharmacoki- netic data in target fish species are also necessary for successful treatment together with challenge tests. The objective of this study was to evaluate the susceptibility to several antibacterial agents of 23 P. damselae subsp. piscicida strains isolated from diseased farmed Senegalese sole (weight 50–300 g) during 2000–2005, and 16 strains of the same pathogen recovered during 1996–2000 from several outbreaks affecting farmed gilthead seabream (weight 5–30 g). All strains showed the same biochemical pattern and were virulent with LD 50 values ranging from 10 3 to 2 · 10 5 CFU g )1 of fish for gilthead seabream (Zorrilla, Chabrillo ´n, Arijo, Dı ´az-Rosales, Martı ´nez-Manzanares, Balebona & Morin ˜igo 2003), and from 2.2 · 10 4 to 3.5 · 10 4 CFU g )1 of fish for Senegalese sole (Dı ´az-Rosales et al. 2006). Journal of Fish Diseases 2008, 31, 73–76 Correspondence M A Morin ˜ igo, Department of Microbiology, Faculty of Sciences, University of Ma ´laga, 29071 Ma ´ laga, Spain (e-mail: morinigo@uma.es) 73 Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Publishing Ltd