Previous studies have proposed that Streptomyces caeruleus is an earlier heterotypic synonym for Streptomyces niveus and Streptomyces spheroides. In this study, phylogenetic analysis of the almost complete 16S rRNA gene sequences of the Streptomyces caeruleus type strains NBRC 13344(T), JCM 4014(T) and NRRL B-2194(T) revealed that S. caeruleus was closely related to Actinoalloteichus cyanogriseus and not to S. niveus, S. spheroides or any other species of the genus Streptomyces. Moreover, the diagnostic cell-wall diamino acid was found to be meso-diaminopimelic acid in S. caeruleus and DNA-DNA hybridization studies revealed that S. caeruleus NBRC 13344(T) was a member of the same species as A. cyanogriseus NBRC 14455(T). Based on these chemotaxonomic and phylogenetic data, it is proposed that Streptomyces caeruleus (Baldacci 1944) Pridham et al. 1958 be reclassified as a heterotypic synonym of Actinalloteichus cyanogriseus Tamura et al. 2000. Furthermore, based on phylogenetic, morphological and MALDI-TOF MS analyses, it is proposed that the species Streptomyces laceyi Manfio et al. 2004 and Streptomyces spheroides Wallick et al. 1956 are reclassified as later heterotypic synonyms of Streptomyces niveus Smith et al. 1956.

The taxonomic positions of soil isolates known as Streptomyces groups A, B and C were clarified. Comparative 16S rDNA sequence studies indicated that representatives of all three taxa formed distinct phyletic lines within the Streptomyces tree though the group A strains were shown to be related to Streptomyces griseus and associated validly described species. The taxonomic integrity of all three groups was highlighted by DNA:DNA relatedness and ribotype data though the group A strains encompassed a higher degree of genetic variation than the group B and C strains. In light of these and earlier phenotypic data it is proposed that Streptomyces groups A, B and C be given species status as Streptomyces sanglieri sp. nov., Streptomyces aureus sp. nov. and Streptomyces laceyi sp. nov., respectively.