Streptomyces turgidiscabies and Streptomyces reticuliscabiei: one genomic species, two pathogenic groups.

Three strains of Streptomyces reticuliscabiei and two strains of Streptomyces turgidiscabies were analysed, together with reference and type strains of other Streptomyces species, for phenotypic traits, DNA-DNA relatedness, comparison of 16S rRNA gene sequences and presence of necrotic protein gene (nec1) homologues in order to clarify their phylogenetic relationships. A numerical analysis of phenotypic characteristics showed that S. reticuliscabiei and S. turgidiscabies belong to the same cluster and share almost all morphological and biochemical traits that are important in the identification of Streptomyces species. DNA-DNA hybridization and phylogenetic comparisons of 16S rRNA gene sequences confirmed that the two species are genomically closely related. In contrast, pathological data showed that S. turgidiscabies and S. reticuliscabiei cause two distinct diseases. Gene homologues of nec1 were detected in S. turgidiscabies and other common scab species (Streptomyces scabiei, Streptomyces europaeiscabiei and Streptomyces stelliscabiei), but not in S. reticuliscabiei. To avoid confusion between agents causing separate diseases, it is proposed that the existing distinct species names are retained: S. turgidiscabies involved in common scab and S. reticuliscabiei involved in netted scab.