Anthropogenic Disruption Versus Natural Restoration: Enterobacter cloacae ‐Driven Barnacle Larval Settlement and Its Mitigation via Natural Bacteriophages

Coastal regions support approximately 60% of the global population and face escalating anthropogenic pollution, which disrupts the dynamics of marine and coastal ecosystems. The discharge of over 80% of sewage without adequate treatment introduces human pathogenic microorganisms into coastal waters, posing significant risks to ecological integrity and public health. This challenge is exacerbated by cross-resistance between antibiotics and biocides, whereby biocide use for biofilm control in coastal industries may inadvertently select for resistant pathogens of terrestrial origin. While microbial biofilms are known to promote macrofouling by facilitating invertebrate larval settlement, a major operational challenge for marine industries, the role of anthropogenic microbial contamination in influencing macrofouling dynamics remains poorly understood. Here, we provide evidence linking anthropogenic microbial contamination to marine biofouling. We isolated an antibiotic and biocide-resistant Enterobacter cloacae strain from a marine cooling water circuit at an operational power plant and identified it as a potent inducer of barnacle (Amphibalanus reticulatus) larval settlement. Salt-tolerance assays combined with Multi-Locus Sequence Typing (MLST) and AAI/ANI-based comparative genomics against reference strains indicated a likely terrestrial origin for this isolate. Larval settlement and choice assays demonstrated that E. cloacae biofilms increased barnacle settlement by > 70% relative to controls. To develop sustainable mitigation strategies against this biocide-resistant organism, we isolated natural bacteriophages targeting E. cloacae from the same water sample. Phage-mediated selective elimination of E. cloacae from biofilms reduced larval settlement by 80% in plate-based assays, providing proof of concept for bacteriophage-based targeted elimination of biofouling-promoting bacteria. Our findings reveal a previously unrecognised connection between anthropogenic bacterial contamination and biofouling dynamics, establishing bacteriophages as an environmentally sustainable strategy for controlling biofilm-mediated larval settlement in marine industries.