Killing of mycolic acid-containing bacteria aborted induction of antibiotic production by streptomyces in combined-culture

Abstract

Co-culture of Streptomyces with mycolic acid-containing bacteria (MACB), which wetermed "combined-culture," alters the secondary metabolism pattern in Streptomyces andhas been a useful method for the discovery of bioactive natural products. In the course ofour investigation to identify the inducing factor(s) of MACB, we previously observed that productionof pigments in Streptomyces lividans was not induced by factors such as cultureextracts or mycolic acids. Although dynamic changes occurred in culture conditionsbecause of MACB, the activation of pigment production by S. lividans was observed in a limitedarea where both colonies were in direct contact. This suggested that direct attachmentof cells is a requirement and that components on the MACB cell membrane may play animportant role in the response by S. lividans. Here we examined whether this response wasinfluenced by dead MACB that possess intact mycolic acids assembled on the outer cellmembrane. Formaldehyde fixation and ?-irradiation were used to prepare dead cells thatretain their shape and mycolic acids of three MACB species: Tsukamurella pulmonis, Rhodococcuserythropolis, and Rhodococcus opacus. Culturing tests verified that S. lividansdoes not respond to the intact dead cells of three MACB. Observation of combined-cultureby scanning electron microscopy (SEM) indicated that adhesion of live MACB to S. lividansmycelia 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 attachmentby live MACB cells is proposed as one of the possible factors that causesStreptomyces to alter its specialized metabolism in combined-culture.