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Fish pathogenic bacteria of the Tenacibaculum genus are a serious emerging concern in modern aquaculture, causing tenacibaculosis in a broad selection of cultured finfish. Data describing their virulence mechanisms are scarce and few means, antibiotic treatment aside, are available to control their proliferation in aquaculture systems. We genome sequenced a collection of 19 putative Tenacibaculum isolates from outbreaks at two aquaculture facilities and tested their susceptibility to treatment with tropodithietic acid (TDA)-producing Roseobacter group probiotics. We found that local outbreaks of Tenacibaculum can involve heterogeneous assemblages of species and strains with the capacity to produce multiple different virulence factors related to host invasion and infection. The probiotic Phaeobacter piscinae S26 proved efficient in killing pathogenic Tenacibaculum species such as T. maritimum , T. soleae , and some T. discolor strains. However, the T. mesophilum and T. gallaicum species exhibit natural tolerance towards TDA and are hence not likely to be easily killed by TDA-producing probiotics. Tolerance towards TDA in Tenacibaculum is likely involving multiple inherent physiological features pertaining to electron and proton transport, iron sequestration, and potentially also drug efflux mechanisms, as genetic determinants encoding such features were significantly associated with TDA tolerance. Collectively, our results support the use of TDA-producers to prevent tenacibaculosis, however, their efficacy is likely limited to some Tenacibaculum species. Importance A productive and sustainable aquaculture sector is needed to meet the UN sustainable development goals (SDGs) and supply the growing world population with high-protein food sources. A sustainable way to prevent disease outbreaks in the industry is the application of probiotic bacteria that can antagonize fish pathogens in the aquaculture systems. TDA-producing Roseobacter group probiotics have proven efficient in killing important vibrio pathogens and protecting fish larvae against infection, yet their efficacy against different fish pathogenic species of the Tenacibaculum genus has not been explored. Therefore, we tested the efficacy of such potential probiotics against a collection of different Tenacibaculum isolates and found the probiotic to efficiently kill a subset of relevant strains and species, supporting their use as sustainable disease control measure in aquaculture.

To counteract the antibiotic resistance crisis, novel anti-infective agents need to be discovered and brought to market. Microbial secondary metabolites have been important sources of inspiration for small-molecule therapeutics.