Structure-activity relationship of mycoloyl glycolipids derived from Rhodococcus sp. 4306.

Novel mycoloyl glycolipids with short carbon chains were isolated and purified from Rhodococcus sp. 4306, a soil origin of Actinomycetales. Their chemical structures were identified as trehalose 6,6'-dimycolate (TDM), trehalose 6-monomycolate, glucose 6-monomycolate, mannose 6-monomycolate and fructose 6-monomycolate. The length of carbon chains and number of double bonds of mycolic acids were C(34), C(36)and C(38)saturated, monoenoic and dienoic molecular species, which were much shorter than those of Mycobacterium tuberculosis (C(78-88)monoenoic and dienoic). Among them, only TDM could induce prominent granulomatous inflammation of the lung and spleen in mice. By contrast, other mycoloyl glycolipids induced mild lesions. The small-sized TDM of Rhodococcus possessed granulomatogenic activity, however, the toxicity was much lower than that of M. tuberculosis. Rhodococcal TDM was composed of mycolic acid with the shortest carbon chains, when compared to granulomatogenic TDM of Mycobacterium, Nocardia and Rhodococcus reported previously. Our results imply that rhodococcal TDM is a pathogenetic factor similar to that of M. tuberculosis, although rhodococcal TDM exhibits low toxicity.