Bacteria isolated from shellfish digestive gland with antipathogenic activity as candidates to increase the efficiency of shellfish depuration process

The aim of the work, as part of the FP7 Beads project, was to isolate potentially probiotic bacteria targeted to shellfish, which could facilitate depuration of commercial molluscs. A total of 365 bacteria were isolated from shellfish digestive gland and screened for activity against different pathogens. One bacterial isolate obtained from Mytilus galloprovincialis, designated as 3M21 and identified as Enterococcus hirae, presented antibacterial activity against Listeria monocytogenes, Listeria innocua, Enterococcus faecalis, and antiviral activity against Hepatitis A ATCC VR-1402 and murine Norovirus (MNV-1), culturable in vitro. This strain was sensitive to chloramphenicol, fusidic acid, ampicillin, vancomycin and gentamicin. The active substance produced by 3M21 was partially characterized and shown to be proteic, with a molecular weight of 6.95 kDa determined by Tris–Tricine electrophoresis, high thermostability (30 min at 100 °C), resistant to a wide pH range (2–9) and to treatment with certain surfactants (Tween 20, 80 and EDTA at 2 mM). After its partial purification, the bacteriocin was encapsulated in 3% of alginate and an alternative using BaCl2 instead of CaCl2 in the hardening bath significantly improved its encapsulation. Although the use of probiotic bacteria is gaining in popularity in the aquaculture industry, its application in shellfish depuration has seldom been studied. The isolation, characterization and microencapsulation of a bacterium with antipathogen activity already acclimatized to seafood environment, as the isolate 3M21, could increase the efficiency of shellfish depuration process.