Characteristics of epidemic and sporadic Flavobacterium psychrophilum sequence types

• MLST data reveals an epidemic F. psychrophilum lineage with a worldwide distribution.
• Characteristics of epidemic and sporadic F. psychrophilum STs are compared.
• Epidemic STs show a higher ability to adhere and to resist antimicrobial agents.
• IS256 is a potential molecular marker for STs of the largest clonal complex CC-ST2.

Population genetic analysis of Flavobacterium psychrophilum using multilocus sequence typing (MLST) data has identified a genetic lineage consisting of sequence types (STs) associated with epidemic spread and high mortality in farmed rainbow trout (Oncorhynchus mykiss). To identify phenotypic characteristics that discriminate epidemic F. psychrophilum STs from sporadically occurring STs, a set of traits of isolates/STs (n = 32) with distinct genetic background and clinical impact was studied in vitro. The results showed that epidemic and sporadic STs could not be discriminated by their resistance to povidone-iodine or iron starvation or by their ability to hydrolyze elastin and gelatin. Antimicrobial resistance against flumequine, oxolinic acid and oxytetracycline was significantly more prevalent (p < 0.05), although not a prerequisite, in epidemic STs. The two groups showed equal biofilm forming abilities, while the epidemic STs were associated with higher adherence (p < 0.05) indicating that adhesion and antimicrobial resistance could be factors contributing to the epidemic potential of F. psychrophilum. All isolates/STs of the largest clonal complex CC-ST2 were distinguished from others by the bacterial insertion sequence IS256 in the collagenase encoding gene. IS256 might thus constitute a novel molecular marker for rapid differentiation of epidemiologically important CC-ST2 genotypes.