The genus Pseudolateromonas is known to have antibacterial activities which suggest that they could be potential candidates as probiotics in aquaculture. Several strains of bacteria have been selected in the surrounding marine environment of New Caledonia. After discrimination of the strains according to their antibacterial activities, one of them was selected and tested in larval rearing and during the juvenile phase in the blue shrimp Penaeus stylirostris. Called NC201, this Pseudoalteromonas showed inhibitory activities in vitro against various pathogenic Vibrio and more particularly those which affect local grow-out farms.
To understand the mechanisms that could explain these results, the NC201 complete genome was analyzed and more than 95% of identity was found with three other Pseudoalteromonas strains but 216 genes were specific to NC201 only. Two genes coding for proteins belonging to the LodA are involved in synthesis of antimicrobial molecules, via H2O2 production.
In vivo trials were conducted both in P. stylirostris larvae and juveniles to confirm the probiotic role of NC201. In larval rearings, animals which have received Pseudoalteromonas obtained higher survival rates than those with no treatment. The enumeration of the Vibrio load in the post-larvae showed a lower concentration in animals which received NC201 regularly.When administrated in juveniles, NC201 has improved survival rates in animals confronted to hypo and hypersaline stress as well as to a Vibrio nigripulchritudo infection challenge. Monitoring of hemolymph invasion by the pathogenic bacterium showed that NC201 slowed infection and could modulate the levels of several of immune and stress responses, thus, with an increase of the overall health status in the blue shrimp.
To allow a commercial outlet for this probiotic, freeze-drying trials were conducted in order to reduce the size of the strain’s production and its ease of use. The NC201 strain was able to be concentrated at more than 1010 cfu/g and has been stored at -20°C for 4 years without loss of viability and alteration of its antibacterial properties in vitro. Further trials are now needed to confirm the probiotic properties of the lyophilized NC201 strains in vivo conditions in shrimp culture.