LAUSR

Abstract The globally distributed bacterium Streptococcus iniae is responsible for outbreaks of disease resulting in high mortality in a wide range of economically important freshwater and marine fish species. Despite the significance of S. iniae, our understanding of its transmission and infection dynamics remains incomplete. Biofilms are important for the survival and pathogenesis of many bacteria, but there is a paucity of information on their role in the ex-host persistence of S. iniae. This study aimed to compare biofilm formation by isolates representing different S. iniae genotypes and to investigate the effect of biofilm formation on environmental persistence and resistance to common disinfectants and antimicrobials. Eleven clinical isolates of S. iniae representing 4 distinct genetic groups and diverse host types were assessed for their ability to form biofilms. Planktonic bacteria or mature biofilms were exposed to in vitro aquatic microcosms of different temperatures to quantify the number of culturable bacteria in each system over time. The minimum biofilm eradication concentration (MBEC) assay® system was used to determine biofilm resistance to 18 antimicrobials and 4 disinfectants commonly used in food producing animals and aquaculture, respectively. All isolates formed biofilms within 72 h. Bacteria remained culturable notably longer in the biofilm form compared to the planktonic, with a significant impact from temperature and salinity (p