The use of lactic acid bacteria (LAB) in the prevention or reduction of fish diseases is receiving increasing attention. In the present study, 47 LAB strains were isolated from farmed seabass ( Dicentrarchus labrax ) and were phenotypically and phylogenetically analysed by 16S rDNA and randomly amplified polymorphic DNA - polymerase chain reaction (RAPD-PCR). Their antimicrobial effect was tested in vitro against a wide variety of pathogenic and spoilage bacteria. Most of the strains isolated were enterococci belonging to the following species: Enterococcus faecium (59%), Enterococcus faecalis (21%), Enterococcus sanguinicola (4 strains), Enterococcus mundtii (1 strain), Enterococcus pseudoavium (1 strain), and Lactococcus lactis (1 strain). An Aerococcus viridans strain was also isolated. The survey of their antimicrobial susceptibility showed that all isolates were sensitive to vancomycin and exhibited resistance to between 4 and 10 other antibiotics relevant for therapy in human and animal medicine. Different patterns of resistance were noted for skin and intestines isolates. More than 69% (32 strains) of the isolates inhibited the growth of the majority of pathogenic and spoilage bacteria tested, including Listeria monocytogenes, Staphylococcus aureus, Aeromonas hydrophila, Aeromonas salmonicida, Vibrio anguillarum, and Carnobacterium sp. To our knowledge, this is the first report of bioactive enterococcal species isolated from seabass that could potentially inhibit the undesirable bacteria found in food systems.

In order to test the ability to produce antibacterial substances within marine bacteria, prior to select potential probiotics for use in shellfish farming, we targeted a large collection of bacterial isolates (132 strains), brought from the clam Ruditapes decussatus and 37 reference strains. First, we proceeded to their biochemical identification and the screening of antibiotic resistance profiles. Else, we investigated their inhibitory activity in vitro against several fish and shellfish pathogens, using two methods: the double-layer agar and the direct simultaneous antagonism methods. The results showed high frequencies of inhibitory producing bacteria (IPB) within the isolates. These bacteria (25%) were aerobic mesophylic bacteria belonging to various bacterial groups: 33.7% oxidase-positive Gram-negative bacteria, 7.4% Enterobacteriaceae and 28% lactic acid bacteria. Besides this group, nine strains produced strong inhibition effect. These bacteria belonged to : Aeromonas hydrophila, Aeromonas sobria, Pseudomonas cepacia, Vibrio sp, Serratia liquefaciens and Lactobacillus rhamnosus. They were active against pathogenic bacteria belonging to the genera: Aeromonas, Pseudomonas and Vibrio. These potential probiotics were submitted to further investigations prior to their introduction in larval shellfish farming.

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