The degradation of farmland in China underscores the need for developing and utilizing saline-alkali soil. Soil health relies on microbial activity, which aids in the restoration of the land’s ecosystem, and hence it is important to understand microbial diversity. In the present study, two Gram-stain-positive strains HR 1-10^T and J-A-003^T were isolated from saline-alkali soil. Preliminary analysis suggested that these strains could be a novel species. Therefore, the taxonomic positions of these strains were evaluated using polyphasic analysis. Phylogenetic and 16S rRNA gene sequence analysis indicated that these strains should be assigned to the genus Halalkalibacter. Cell wall contained meso-2,6-diaminopimelic acid. The polar lipids present in both strains were diphosphatidyl-glycerol, phosphatidylglycerol, and an unidentified phospholipid. The major fatty acids (>10%) were anteiso-C_15:0, C_16:0 and iso-C_15:0. Average nucleotide identity and digital DNA#x2013;DNA hybridization values were below the threshold values (95% and 70%, respectively) for species delineation. Based on the above results, the strains represent two novel species of the genus Halalkalibacter, for which the names Halalkalibacter flavus sp. nov., and Halalkalibacter lacteus sp. nov., are proposed. The type strains are HR 1-10^T (=GDMCC 1.2946^T = MCCC 1K08312^T = JCM 36285^T), and J-A-003^T (=GDMCC 1.2949^T = MCCC 1K08417^T = JCM 36286^T).

Milbemycins are a group of macrolide pesticides with great potential in the agricultural field owing to their high insecticidal activity and environmental compatibility. Milbemycin A3 and A4 with high bioactivities are the main components of milbemycin-derived products, which require a component ratio A4:A3 of 2.3- to 4.0-fold. Streptomyces bingchenggensis BC04 is a promising milbemycin producer, whereas the component ratio of its products (A4:A3 of 9.0-fold) could not meet the requirement for industrial production. To address this issue, we reconstructed the precursor biosynthetic pathways to fine tune the supply of different acyl-coenzyme A precursors required for milbemycin biosynthesis. Based on an analysis of the intracellular acyl-coenzyme A precursors, we reconstructed stepwise heterogeneous biosynthetic pathways of extender units for milbemycin biosynthesis. Then, we coordinated the supply of milbemycin biosynthetic starter units with temporal promoters. Thanks to these manipulations, we obtained an engineered strain with 39.5% milbemycin titer improvement to 3417.88 mg/L and a qualified component ratio A4:A3 of 3.3-fold. This work demonstrated that coordinating the precursor supply is a simple and effective approach to optimize the component ratio of A4:A3 in milbemycin fermentation products. Moreover, this strategy might also be useful to construct high-yield strains with optimized component ratios of fermentation products in other Streptomyces.