Micromonospora antibiotica sp. nov. and Micromonospora humidisoli sp. nov., two new actinobacterial species exhibiting antimicrobial potential.

Two novel actinobacterial strains, designated MMS20-R2-23 ^T and MMS20-R2-29 ^T , were isolated from riverside soil and subjected to taxonomic characterization. Both strains were Gram-stain-positive, aerobic, non-motile and filamentous, and formed orange to strong orange-brown coloured colonies, which later turned black. Both strains grew optimally at mesophilic temperatures, neutral to slightly alkaline pH and in the absence of NaCl. Analysis of 16S rRNA gene sequences indicated that the two novel strains fell into phylogenetic clusters belonging to the genus Micromonospora . Strains MMS20-R2-23 ^T and MMS20-R2-29 ^T showed the highest 16S rRNA gene sequence similarity to Micromonospora phytophila SG15 ^T (99.3 %) and Micromonospora humida MMS20-R1-14 ^T (99.4 %), respectively. Based on the comparative genome analysis, strain MMS20-R2-23 ^T had the highest orthologous average nucleotide identity (orthoANI) value of 92.70 % with Micromonospora matsumotoense DSM 44100 ^T , and MMS20-R2-29 ^T shared 94.99 % with Micromonospora wenchangensis CCTCC AA 2012002 ^T . Besides, the digital DNA-DNA hybridization (dDDH) values of MMS20-R2-23 ^T and MMS20-R2-29 ^T with the same species were 47.6 and 59.2% respectively, which were also highest among the compared species, thus confirming the separation of each strain at species level from related species. The orthoANI and dDDH values between MMS20-R2-23 ^T and MMS20-R2-29 ^T were 92.18 and 44.9% respectively. The genomes of strains MMS20-R2-23 ^T and MMS20-R2-29 ^T were estimated as 7.56 Mbp and 7.13 Mbp in size, and the DNA G+C contents were 72.5 and 72.9 mol%, respectively. The chemotaxonomic properties of both strains were consistent with those of the genus. The novel strains showed antimicrobial activity against a broad range of microbes, in particular Gram-positive bacteria and yeasts. It is evident that each of the isolated strains merits recognition as representing novel species of Micromonospora , for which the names Micromonospora antibiotica sp. nov. (type strain=MMS20-R2-23 ^T =KCTC 49542 ^T =JCM 34495 ^T ) and Micromonospora humidisoli sp. nov. (type strain=MMS20-R2-29 ^T =KCTC 49543 ^T =JCM 34496 ^T ) are proposed.