Exploring the antimicrobial potential of marine macroalgal phenolic extracts (phycobiotics) against bacterial pathogens in aquaculture and clinical settings

IntroductionThe growing threat of antimicrobial resistance has intensified the search for alternatives to conventional antibiotics in both human and veterinary medicine. Probiotics and their bacteriocins represent promising candidates; however, the complexity of microbial ecosystems and the variability in pathogen susceptibility highlight the need for complementary antimicrobial strategies based on natural compounds with distinct mechanisms of action. In this context, macroalgae have emerged as a potential source of bioactive molecules with antimicrobial activity (phycobiotics).MethodsPhenolic extracts from different marine macroalgae collected along the northern coast of Galicia (Spain) were obtained and quantified. Moreover, macroalgal phenolic extracts were evaluated for antimicrobial activity against clinically and veterinary relevant pathogens, probiotic strains, and bacteriocin-resistant mutants (curvacin G14/curvacin G15-resistant Streptococcus parauberis LMG22252 and LJDL21, nisin Z-resistant Lactococcus garvieae CF00021, and garvicin A/garvicin Q/nisin Z-resistant Lc. garvieae CF00021).ResultsThe highest phenolic contents were quantified in Pelvetia canaliculata, Fucus vesiculosus, and Cystoseira tamariscifolia, which also exhibited the greatest antimicrobial activity against pathogens with minimal effects on probiotic strains, supporting their potential combined use. Their inhibitory effect remained effective against bacteriocin-resistant St. parauberis LMG22252 and LJDL21, and Lc. garvieae CF00021, suggesting different mechanisms of action. Whole-genome sequencing of wild-type and bacteriocin-resistant mutant strains was performed to identify genetic variations associated with bacteriocin resistance, revealing that it was mediated by multifactorial adaptive responses.DiscussionMacroalgal phycobiotics represent a promising antimicrobial strategy, exhibiting broad-spectrum activity against pathogens while exerting minimal effects on beneficial probiotic bacteria. Their efficacy against bacteriocin-resistant mutants further supports their potential for use in combination with probiotics as a complementary or synergistic strategy for pathogen control in both aquaculture and clinical settings.