FlavoTyper: a genome-based in-silico serotyping tool for the fish pathogen Flavobacterium psychrophilum

Flavobacterium psychrophilum is a devastating pathogen of fish reared in freshwater worldwide. Serotyping is a relevant method for epidemiological surveillance and outbreak detection, as well as for a better understanding of host-pathogen interactions. Serological diversity may also have important consequences for the selection of appropriate strains for vaccine development and for selective breeding for increased disease resistance. F. psychrophilum serotyping relies on structural variations in the O-polysaccharide (O-PS) moiety of the cell surface lipopolysaccharide (LPS). However, conventional serotyping is costly, labor-intensive and requires significant technical expertise. Moreover, divergent scheme proposals highlighted the absence of harmonization among laboratories. In this context, the development of an mPCR-based serotyping scheme targeting wzy genes greatly improved the reliability and standardization of serotyping. Nevertheless, the proposed mPCR scheme did not capture the entire diversity of genomic variability. The aim of this study was to establish a robust and publicly available tool for F. psychrophilum genome-based serotyping. Extensive genome analysis of the O-antigen biosynthesis locus allowed the identification of biomarkers enabling the development of FlavoTyper, an in-silico-based serotyping tool. The FlavoTyper tool was evaluated on all F. psychrophilum genome assemblies publicly available, providing sound and sensitive predictions and easily interpretable results. When applied to a curated collection of publicly available genomes, the in-silico O-types assigned by the tool were statistically significantly associated with host fish species, confirming previous studies (coho salmon with O:0, rainbow trout with O:1 and O:2, and ayu with O:3) and their distribution across MLST clonal complexes revealed that the O-antigen locus is frequently rearranged independently of the core-genome lineage, consistent with the extensive recombination that shapes the evolution and genomic diversity of this species.