PHYSIOLOGICAL AND ELECTRON-MICROSCOPIC INVESTIGATIONS ON SYNTROPHIC DICYANDIAMIDE DEGRADATION BY SOIL BACTERIA

Soil bacteria with high dicyandiamide (DCD) degrading capacity were is olated. In addition to pure cultures, two different consortia of soil bacteria were shown to degrade DCD under conditions of syntrophy. The single isolates of the consortia could not grow on minimal media or de grade DCD. These consortia turned out to be associations between Xanth omonas maltophilia and either Radiobacter sp. or Aureobacterium sp. Bo th types of consortia proliferate and degrade DCD rapidly. Supplementa tion experiments and microscopical investigations focusing on Xanthomo nas maltophilia and Aureobacterium sp. showed that Aureobacterium sp. is probably auxotrophic for a yet unknown factor. Supplementation of m inimal medium essential medium (MEM) vitamins enables this strain to g row with nitrate, but not with DCD or ammonia as single nitrogen sourc es. Xanthomonas maltophilia, however, is dependent on a substance whic h is not released into the medium by its companion. It cannot be provi ded either by the addition of vitamins or casamino acids or culture fi ltrates of proliferating consortia to the medium, but only by the pres ence of Aureobacterium sp. cells in the nutrient medium. Partially bas ed on novel preparation techniques, microscopical investigations const antly showed a close contact between Xanthomonas maltophilia cells and its companion in enrichment medium, while Aureobacterium sp. is also found as single cells. The close contact appears to be of paramount im portance to initiate growth and DCD degradation. The results demonstra te clearly the importance of investigations, including bacterial conso rtia in addition to pure culture studies when the degradability of agr ochemicals has to be assessed. (C) 1998 Elsevier Science Ltd. All righ ts reserved.