Comparative Phenotypic and Genotypic Analysis of Edwardsiella Isolates from Different Hosts and Geographic Origins, with Emphasis on Isolates Formerly Classified as E. tarda, and Evaluation of Diagnostic Methods

Abstract

<i>Edwardsiella</i> spp. are responsible for significant losses in important wild and cultured fish species worldwide. Recent phylogenomic investigations have determined that bacteria historically classified as <i>Edwardsiella tarda</i> actually represent three genetically distinct yet phenotypically ambiguous taxa with various degrees of pathogenicity in different hosts. Previous recognition of these taxa was hampered by the lack of a distinguishing phenotypic character. Commercial test panel configurations are relatively constant over time, and as new species are defined, appropriate discriminatory tests may not be present in current test panel arrangements. While phenobiochemical tests fail to discriminate between these taxa, data presented here revealed discriminatory peaks for each <i>Edwardsiella</i> species using matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) methodology, suggesting that MALDI-TOF can offer rapid, reliable identification in line with current systematic classifications. Furthermore, a multiplex PCR assay was validated for rapid molecular differentiation of the <i>Edwardsiella</i> spp. affecting fish. Moreover, the limitations of relying on partial 16S rRNA for discrimination of <i>Edwardsiella</i> spp. and advantages of employing alternative single-copy genes <i>gyrB</i> and <i>sodB</i> for molecular identification and classification of <i>Edwardsiella</i> were demonstrated. Last, <i>sodB</i> sequencing confirmed that isolates previously defined as typical motile fish-pathogenic <i>E. tarda</i> are synonymous with <i>Edwardsiella piscicida</i>, while atypical nonmotile fish-pathogenic <i>E. tarda</i> isolates are equivalent to <i>Edwardsiella anguillarum</i> Fish-nonpathogenic <i>E. tarda</i> isolates are consistent with <i>E. tarda</i> as it is currently defined. These analyses help deconvolute the scientific literature regarding these organisms and provide baseline information to better facilitate proper taxonomic assignment and minimize erroneous identifications of <i>Edwardsiella</i> isolates in clinical and research settings.