Occurrence and antimicrobial susceptibility of aeromonads from maricultured gilthead seabream (Sparus aurata)

In Europe the demand for gilthead seabream (Sparus aurata) has increased significantly over the past decade as well as its aquaculture production. In aquaculture, antibiotics are commonly used for prevention or cure of diseases. A mayor potential consequence of this use is the development of drug-resistant bacteria. Furthermore, inappropriate use is likely to cause an unnecessary impact on the environment. In this work we studied the antimicrobial susceptibility pattern of Aeromonas strains, considering that aeromonads are found in a diversity of habitats and are responsible for human and fish infections. A total of 80 fish was examined in 2009 in March and September. The fish came from 2 types of mariculture farms in Italy: a traditional, in Ligury (in-shore cages) and an atypical, in Tuscany (initial phase of fish life in cages and then free life in lacoon). Aeromonads were counted from skin, gills, flesh and gut of the fish on Aeromonas Agar (Ryan), GSP Agar and Starch DNA Agar. Means and standard deviations were determined for aeromonads counts and means were compared by T-test. Seventy Aeromonas isolates, preidentified at genus level and confirmed with a genus-specific PCR, were phenotypically identified at group level using API20 E and 20 NE kits, completed by testing gas production from glucose. The strains, belonging to A. caviae, A. hydrophila, A. sobria groups, were studied for their antimicrobial susceptibility pattern considering 20 antimicrobials by the disk diffusion technique, according to the CLSI recommendations and a chi(2) test was performed. The highest mean counts were reached in gills in September in both plants with values over 5 log cfu/g, whereas flesh samples never gained 3 log cfu/g. All strains exhibited resistance against at least 4 antimicrobials. Three strains from the Ligurian farm showed resistance to 10-13 antimicrobials.