4.1. Azalomycin F5a

Azalomycin F_5a (purity, 98.2%) was isolated from the fermentation of Streptomyces hygroscopicus var. azalomyceticus according to our published methods [1], and widely used in our previous works [11,37,51]. The stock solution of azalomycin F_5a, stored at −20 °C, was prepared by dissolving in dimethyl sulfoxide (DMSO) to obtain a concentration of 2048 μg/mL. The stock solution was diluted to the desired concentrations with tryptic soy broth (TSB) (Haibo Biotechnology Co., Ltd., Qingdao, China) immediately before use. In another, the DMSO concentrations in all the test systems were kept to less than or equal to 1.56%, and all those in the blank controls or 0× MIC azalomycin F_5a groups were 1.56%.