Pseudoalteromonas spp. A novel probiotic protective against vibrios?

Since the EU ban in 2006 of antimicrobials for prophylactic and sub-therapeutic purposes in aquaculture to counter the spread of antibiotic resistance alternative methods have been sought to increase yields and combat disease. Probiotics that positively influence the microbial community within a rearing environment, or within a target species itself leading to increased disease resistance or/and upregulated immune responses, and improved growth rate/feed conversion are becoming increasingly popular functional components of modern aquaculture.

Aranda et al. (2012), investigated the antimicrobial activity of Pseudoalteromonas spp. - isolated from Chilean mussels (Perumytilus purpuratus) - against the opportunistic fish, shellfish, and human pathogen Vibrio parahaemolyticus. Pseudoalteromonas spp. have previously been identified as producing biologically active antibacterial, antifouling, and antibiofilm compounds and the authors wished to identify a possible use as a probiotic in Chilean aquaculture.

18 culturable Pseudoalteromonas spp. bacterial isolates from the mussel were found to inhibit growth in V. parahaemolyticus. 16S rRNA gene sequence identification and phylogenetic analysis of the three most prolific isolates (DIT09, DIT44, DIT46) indicated a well-supported clade of Pseudoalteromonas with close identity to P. prydzensis (99 % identity) and P. mariniglutinosa (98 % identity). Antimicrobial activity for these strains was also positive against V. cholerae and V. anguillarum but not against other bacterial genera indicating a specificity for vibrios.

The antimicrobial compounds released by the DIT strains were extracellular and permeable through membranes with a 3 kDa cut-off, were ethanol soluble, and produced in late exponential growth phase (24 hrs). Interestingly after late exponential phase antimicrobial activity decreased until it was undetectable (96 hrs). High performance liquid chromatography (HPLC) analysis revealed that the active antibiotic compound was likely amphiphilic and identified 3 bioactive molecules for strain DIT09, 6 bioactive molecules for DIT44, and 5 bioactive molecules for strain DIT46.

The mode of action of the bioactive compounds was suggested as bacteriostatic, but not bactericidal as growth was inhibited but the bacterium not killed. A suggested mode of action of this bacteriostatic was that it interfered with the iron acquisition and siderophore machinery of V. parahaemolyticus in a competitive manner, as addition of iron into the culture medium reversed the bacteriostatic action.

Finally, all three DIT strains were subject to ecotoxicology tests to indicate their usefulness as a potential probiotic. Acute (cell free extracts) and chronic (3.3 x 10³ cfu/ml) ecotoxicological tests utilising Artemia franciscana nauplii did not cause any significant mortality. However, the inferences of this ecotoxicological test must be very limited as organism responses are likely to be highly species specific, and a lack of mortality does not necessarily mean a lack of harm.

This study is a useful contribution in the search for novel probiotic bacteria. The characterisation of novel anti-vibrio compounds is an exciting development as vibrios cause numerous diseases and severe mortality in aquaculture. The potential application for human medicine in treating vibrio diseases should also be considered. However, before the probiotic application of this bacterium should be considered further research into its interactions with other beneficial bacteria and host organism should be considered. The authors did note that although the DIT strains were sensitive to most antibiotics it was intermediately resistant to erythromycin. The potential for the dissemination of erythromycin resistance to potentially pathogenic bacteria may preclude the strains in this study from being applicable for probiotic use.

Reference:
Aranda, C. P., Valenzuela, C., Barrientos, J., Paredes, J., Leal, P., Maldonado, M., ... & Osorio, C. G. (2012). Bacteriostatic anti-Vibrio parahaemolyticus activity of Pseudoalteromonas sp. strains DIT09, DIT44 and DIT46 isolated from Southern Chilean intertidal Perumytilus purpuratus. World Journal of Microbiology and Biotechnology, 28(6), 2365-2374.