Rhodococcus pedocola sp. nov. and Rhodococcus humicola sp. nov., two antibiotic-producing actinomycetes isolated from soil.

Two novel actinobacterial strains, UC12T and UC33T, were isolated from forest topsoil in Suwon, Gyeonggi-Do, South Korea. Comparative analysis of nearly full-length 16S rRNA gene sequences of UC12T and UC33T revealed close pairwise similarity with species of the genus Rhodococcus, and the UC12T and UC33T sequences were most closely related to Rhodococcus canchipurensis MBRL 353T (98.91 % 16S rRNA gene sequence similarity) and Rhodococcus triatomae IMMIB RIV-085T (97.71 %), respectively. DNA-DNA hybridization showed 33.05-35.60 % genomic similarity between strains UC12T and UC33T, while strain UC12T shared DNA-DNA relatedness values of 32.71-41.29 % with the closest species of the genus Rhodococcus and strain UC33T shared 29.12-37.91 % genomic relatedness with the closest species of the genus Rhodococcus. Both strains showed similar chemotaxonomic characteristics. The major fatty acids were C16 : 0, summed feature 3, C18 : 1ω9c and C18 : 0 10-methyl. The polar lipids were diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol and phosphatidylinositol mannoside. The major quinone derived was MK-8(H2). The cell-wall peptidoglycan contained meso-diaminopimelic acid, and galactose, glucose, arabinose and ribose were detected in whole cells. Mycolic acids were detected. The DNA G+C content of strains UC12T and UC33T was 72.7 mol% and 68.8 mol%, respectively. Both strains produced antibiotic(s) that inhibited bacterial pathogens but not fungi. Based on the physiological, biochemical and genotypic features and the DNA-DNA hybridization between the isolates and type strains of closely related species, we propose that these bacteria be classified as novel species of the genus Rhodococcus with the names Rhodococcus pedocola sp. nov. (type strain UC12T=KACC 18499T=NBRC 111580T) and Rhodococcus humicola sp. nov. (type strain UC33T=KACC 18500T=NBRC 111581T).