SUSCEPTIBILITY OF PHOTOBACTERIUM DAMSELAE SUBSP PISCICIDA STRAINS ISOLATED FROM SENEGALESE SOLE, SOLEA SENEGALENSIS KAUP, AND GILTHEAD SEABREAM, SPARUS AURATA L., TO SEVERAL ANTIBACTERIAL AGENTS

The distributions of the MIC values of the five antibacterial agents are also given in Tables 1 and 2. Only the MICs for Te and OT were clearly bimodally distributed, with a group with high MIC values categorized as resistant, and another group with lower MIC values considered to be susceptible. The distribution of MIC values obtained for the strains of P. damselae subsp. piscicida showed that about 88 % and 94 % of the strains isolated from gilthead seabream showed MIC values ≤ 1 μg ml-1 for Te and OT, however only 39.13% and 43.48% of the strains isolated from Senegalese sole showed MIC values ≤ 1 μg ml-1 for both antibacterial agents. The representation of the linear relationship between the Lg MIC values versus inhibition zone diameters of Te and OT (data not showed), theoretical breakpoints for inhibition zone diameters can be derived for these two antibacterial agents (Uhland & Higgins, 2006). The dilutions ≤ 1 μg ml-1 and ≤ 8 μg ml-1 were chosen as the breakpoints of susceptibility to OT and Te, respectively, whilst >32 μg ml-1 was the breakpoint for non-susceptible strains for both antibacterial agents. The value of breakpoint for OT was higher than that obtained for this same antibacterial agent by Kawanishi et al. (2006), who reported a value of breakpoint of 0.5 μg ml-1 for strains of P. damselae subsp. piscicida isolated from farmed seriola. This result could indicate an increase resistance to these two antibacterial agents currently used in aquatic farming worldwide. Rigos, Nengas & Alexis (2006) have reported peaks of OT concentration in muscle and plasma of gilthead sebream, treated by a prolong bath with this antibacterial agent (50 μg ml-1 for 24 h), of 0.096 and 0.047 μg g-1, respectively. These levels in comparison with the MIC values obtained for OT in our study indicate that the administration of this antibacterial agent by bath could be inappropriate to fight against P. damselae subsp. piscicida.