Metabolism of <em>Streptomyces avermitilis </em>Isolated from Agricultural Soils Evaluated during the Four Seasons of the Year

Streptomyces avermitilis exhibits a highly complex metabolism, and any drastic environmental change can activate or suppress its biosynthetic pathways. Strains of Streptomyces avermitilis were isolated from four soil types during the four seasons: cultivated, dormant, from a silvopastoral system, and from a dairy farm. Isolation was performed using ISP-3 oatmeal agar with 1% nystatin as an antifungal agent. Plates were inoculated using dipsticks to isolate the bacteria and cross-streaks to generate a pure culture. Plates were incubated for 10 days at 30±2°C in complete darkness. Colony counts were performed daily to analyze growth curves. The species was validated by morphological characteristics from the taxonomic keys in Bergey’s Manual, volume 5, and diffusible pigments were examined on each plate. The highest proportion of diffusible pigments (white, yellow, orange, red, pink, purple, gray, and black) was present in spring and summer than in autumn and winter. The doubling time was 35.30–62.92 h and 1.47–2.62 days. The metabolism of Streptomyces avermitilis varied according to the season and the abiotic stress used (complete darkness).