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Sequence variation in a region of the virulence array protein gene (vapA; A-layer) was assessed in 333 ('typical' and 'atypical') isolates of the fish pathogenic bacterium Aeromonas salmonicida. Resulting similarity dendrograms revealed extensive heterogeneity, with nearly all isolates belonging to either of 14 distinct clusters or A-layer types. All acknowledged A. salmonicida subspecies (except ssp. pectinolytica, from which no vapA sequence could be obtained) were clearly separated, and notably, all isolates phenotypically identified as ssp. salmonicida formed a distinct and exclusive A-layer type. Additionally, an array of un-subspeciated atypical strains formed several equally prominent clusters, demonstrating that the concept of typical/atypical A. salmonicida is inappropriate for describing the high degree of diversity evidently occurring outside ssp. salmonicida. Most representatives assessed in this study were clinical isolates of spatiotemporally diverse origins, and were derived from a variety of hosts. We observed that from several fish species or families, isolates predominantly belonged to certain A-layer types, possibly indicating a need for host-/A-layer type-specific A. salmonicida vaccines. All in all, A-layer typing shows promise as an inexpensive and rapid means of unambiguously distinguishing clinically relevant A. salmonicida subspecies, as well as presently un-subspeciated atypical strains.

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