Abstract
Characterization of Edwardsiella strains isolated from fish in the Netherlands (1995-2020) shows a relationship between Edwardsiella species and source.
Edwardsiella species are motile, facultative anaerobic, rod-shaped Gram-negative bacteria responsible for the disease edwardsiellosis, which leads to mass mortality in various populations and age groups of fish. Furthermore, E. tarda can infect mammals and cause severe disease in humans after consumption of contaminated food. Five different species have been identified so far, of which four are considered fish pathogens. However, it is not clear which Edwardsiella species are pathogenic to which fish species. The aim of this study was to characterise Edwardsiella species isolated from different fish species in the Netherlands. Furthermore, the Matrix Assisted Laser Desorption Ionization – Time of Flight Mass Spectrometry (MALDI-TOF MS) database was updated with profiles from Edwardsiella species to facilitate correct and rapid diagnosis.
Thirty-six Edwardsiella strains isolated from fish along a 25-year period were identified using Multilocus Sequence Analysis (MLSA) based on sequence analysis of three genes: gyrB , pgi and phoU . A selection of these strains was used to create a new Edwardsiella MALDI-TOF database by analyzing peptide spectra of strains belonging to the four fish pathogenic species.
MALDI-TOF MS analysis of the Edwardsiella isolates using the newly created database revealed a 100% agreement with MLSA. Furthermore, the presence of Edwardsiella species was associated with specific fish species: 1) E. piscicida was only present in sole, turbot, koi carp and eel, 2) E. tarda was solely isolated from ornamental fish generally kept at a higher water temperature and 3) E. ictaluri was recovered from both catfish and electric eel. In addition, E. anguillarum was not isolated from fish in the Netherlands and one strain isolated from electric eel was clearly different in both sequence analysis and MALDI-TOF profile from those of the other four species.
Thus, both MLSA and MALDI-TOF analysis can be used for determination of Edwardsiella species. Moreover, our results may indicate a host or environmental preference of the different species. Finally, it would be interesting to further investigate the pathogenic potential of the different Edwardsiella species in different fish hosts and under various environmental conditions.