抄録
In the production culture of milbemycin α11 and α14 by Streptomyces hygroscopicus subsp. aureolacrimosus SANK 60393 (strain MA3-6126), two types of morphological mutants appeared in the late production phase, in which the milbemycin production rate had diminished. One strain was a bald type, which formed no spores and aerial mycelia, and the other was a black pigment-producing type. Taxonomic study indicated that, in addition to morphological differences, these two mutants possessed several physiological properties, such as acid production and carbon source utilization, distinctive from the original strain. The appearance of the morphological mutants during cultivation was stimulated by not only the mechanical shear stress caused by the addition of glass beads to a flask or by using a buffled flask but also by the addition of supernatant obtained from a 12-day flask-cultured broth. The morphological mutants were defective in their ability to produce and to convert milbemycins. These results suggested that the appearance of morphological mutants led to the decrease in production in the later phases of milbemycin production.
