Tenacibaculum dicentrarchi produce outer membrane vesicles (OMV) that are associated with the cytotoxic effect in rainbow trout head kidney macrophages

Tenacibaculum dicentrarchi is the second most important pathogen in Chilean salmon farming. This microorganism causes severe skin lesions on the body surface of farmed fish. The bacterium can also adhere to surfaces and form biofilm, survive in fish skin mucus, and possess different systems for iron acquisition. However, the virulence mechanisms are still not fully elucidated. Outer membrane vesicles (OMV) are nanostructures released by pathogenic Gram-negative bacteria during growth, but none has been described yet for T. dicentrarchi. In this study, we provide the first reported evidence of the fish pathogen T. dicentrarchi producing and releasing OMV from 24 h after incubation, increasing thereafter until 120 h. Analyses were conducted with T. dicentrarchi TdCh05, QCR29, and the type strain CECT 7612T . The OMV sizes, determined via scanning electron microscopy, ranged from 82.25 nm to 396.88 nm as per the strain and incubation time point (i.e., 24 to 120 h). SDS-PAGE revealed that the number of protein bands evidenced a drastically downward trend among the T. dicentrarchi strains. In turn, the OMV shared five proteins (i.e., 22.2, 31.9, 47.7, 56.3, and 107.1 kDa), but no protein pattern was identical. A heterogeneous amount of protein, RNA, and DNA were obtained, depending on the time at which OMV were extracted. Purified OMV were biologically active and induced a cytotoxic effect in macrophage-enriched cell cultures from rainbow trout (Oncorhynchus mykiss) head kidneys. This is the first step towards understanding the role that OMV could play in the pathogenesis of T. dicentrarchi.

New Zealand rickettsia-like organism and Tenacibaculum maritimum vaccine efficacy study

A cohort of Chinook salmon juveniles was vaccinated, with an autogenous bivalent vaccine against New Zealand RLOs (NZ-RLO1) and Tenacibaculum maritimum. A proportion of the cohort was not vaccinated to act as controls. At smoltification, the fish were challenged with NZ-RLO1, NZ-RLO2, or T. maritimum. We found that challenge with T. maritimum by immersion in (7.5 × 105  cfu/mL of water) did not yield any pathology. Challenge with RLOs produced clinical signs that were more or less severe depending on the challenge route, dose or vaccination status. Survival was significantly higher for vaccinated fish within the groups challenged with NZ-RLO1 by intraperitoneal injection with a relative percent survival (RPS) of 48.84%. Survival was not significantly different between vaccinated and non-vaccinated fish for groups challenged with NZ-RLO2 by intraperitoneal injection or by NZ-RLO1 by immersion. Yet, anecdotally the clinical disease presentation (manifesting as haemorrhagic, ulcerative skin lesions) was more severe for the non-vaccinated fish. This study demonstrates that autogenous vaccine against NZ-RLO is protective against severe disease and death by NZ-RLO1 challenge which warrants implementation and further evaluation under field conditions. Yet, this study also highlights the importance of the route of administration and dose when evaluating pathogenicity and vaccine efficacy.

First report and characterization of Tenacibaculum maritimum isolates recovered from rainbow trout (Oncorhynchus mykiss) farmed in Chile

The present study reports on the first isolation of Tenacibaculum maritimum in rainbow trout (Oncorhynchus mykiss) farmed in Chile. In March 2020, two cages raising rainbow trout (~250 g) in the Los Lagos Region suffered a disease outbreak. In total, 17,554 fish died (3.5%-4.8% accumulated mortality). Microbiological analysis of the diseased fish obtained two representative isolates (i.e. Tm-035 and Tm-036). These were obtained from the external gross skin lesions-typical of tenacibaculosis-of two fish. Phenotyping, PCR tests and sequencing of the 16S rRNA and housekeeping genes confirmed the isolates as T. maritimum. The pathogenic potential of Tm-035 was further assessed by bath challenging Atlantic salmon (Salmo salar), which killed 70 ± 15% of fish within 11 days. Dead fish presented the same external clinical signs as did the farmed rainbow trout specimens. This research further broadens the known host distribution of this pathogen. Furthermore, the virulence experiments demonstrated that T. maritimum does not have a specific host. Additional studies are needed to evaluate the risk of T. maritimum for the O. mykiss farming industry.