Killing of Mycolic Acid-Containing Bacteria Aborted Induction of Antibiotic Production by Streptomyces in Combined-Culture

Co-culture of Streptomyces with mycolic acid-containing bacteria (MACB), which we termed "combined-culture," alters the secondary metabolism pattern in Streptomyces and has been a useful method for the discovery of bioactive natural products. In the course of our investigation to identify the inducing factor(s) of MACB, we previously observed that production of pigments in Streptomyces lividans was not induced by factors such as culture extracts or mycolic acids. Although dynamic changes occurred in culture conditions because of MACB, the activation of pigment production by S. lividans was observed in a limited area where both colonies were in direct contact. This suggested that direct attachment of cells is a requirement and that components on the MACB cell membrane may play an important role in the response by S. lividans. Here we examined whether this response was influenced by dead MACB that possess intact mycolic acids assembled on the outer cell membrane. Formaldehyde fixation and γ-irradiation were used to prepare dead cells that retain their shape and mycolic acids of three MACB species: Tsukamurella pulmonis, Rhodococcus erythropolis, and Rhodococcus opacus. Culturing tests verified that S. lividans does not respond to the intact dead cells of three MACB. Observation of combined-culture by scanning electron microscopy (SEM) indicated that adhesion of live MACB to S. lividans mycelia were a significant interaction that resulted in formation of co-aggregation. In contrast, in the SEM analysis, dead cells were not observed to adhere. Therefore, direct attachment by live MACB cells is proposed as one of the possible factors that causes Streptomyces to alter its specialized metabolism in combined-culture.