In this study, we isolated, identified and characterized isolates of Tenacibaculum dicentrarchi in Atlantic salmon (Salmo salar) farmed in Chile for the first time. In 2010 and 2014, mortalities were observed in Atlantic salmon (average weight 25-30 and 480-520 g, respectively) at an aquaculture centre in Puerto Montt, Chile. Severe tail rots, frayed fins and, in some cases, damaged gills were detected. Wet smear analyses of these lesions revealed a high occurrence of Gram-negative, filamentous bacteria. Microbiological analysis of infected gill and tail tissues yielded six bacterial isolates. All were identified as T. dicentrarchi through polyphasic taxonomy, which included phenotypic characterization, 16S rRNA sequencing and multilocus sequence typing. The latter method revealed a close relationship of the Chilean genotype with the T. dicentrarchi type strain and two Norwegian Atlantic cod (Gadus morhua) isolates. The pathogenic potential of the TdChD05 isolate was assessed by challenging Atlantic salmon and rainbow trout (Oncorhynchus mykiss) for one hour, which resulted in mean cumulative mortality rates of 65% and 93%, respectively, as well as clinical signs 14 days post-challenge. However, challenged Coho salmon (Oncorhynchus kisutch) presented no mortalities or clinical signs of infection. These findings indicate that the geographical and host distribution of T. dicentrarchi is wider than previously established and that this bacterium may have negative impacts on salmonid cultures.

No abstract

Renibacterium salmoninarum is the causative agent of bacterial kidney disease, which significantly affects salmonid farming worldwide. Despite this impact, there is scarce data on its iron uptake ability, a factor of pathogenesis. This study investigated the iron acquisition mechanisms of R. salmoninarum and its capacity to uptake iron from different sources. Thirty-two Chilean isolates and the DSM20767 type strain grew in the presence of 2,2'-Dipyridyl at varying concentrations (250-330 μm), and all isolates positively reacted on chrome azurol S agar. Subsequently, inocula of four Chilean isolates and the type strain were prepared with or without 200 μm of 2,2'-Dipyridyl for uptake assays. Assay results revealed differences between the isolates in terms of iron acquisition. While a prior iron-limited environment was, for most isolates, not required to activate the uptake of iron (II) sulphate, ammonium iron (III) citrate or iron (III) chloride at higher concentrations (100 μm), it did facilitate growth at lower iron concentrations (10 μm and 1 μm). An exception was the H-2 isolate, which only grew with 100 μm of iron sulphide. In turn, 100 μm of haemin was toxic when isolates were grown in normal KDM-2. In silico R. salmoninarumATCC 33209 genome analysis detected various genes coding iron uptake-related proteins. This is the first study indicating two iron acquisition systems in R. salmoninarum: one involving siderophores and another involving haem group utilization. These data represent a first step towards fully elucidating this virulence factor in the pathogenic R. salmoninarum.