Abstract
Catellibacterium nectariphilum was recently proposed as a new genus and species within the `Rhodobacter group' in the α-3 subgroup of Proteobacteria. This species did not show significant growth on a nutrient medium, and required unknown growth factor(s) for vigorous growth. The growth factor(s) would be diffused in the supernatant of the enrichment culture of the activated sludge from which C. nectariphilum was isolated. We have searched for microorganisms that produce growth factor(s) for C. nectariphilum, and obtained three bacterial strains, designated GF9, GF20 and GF22, from the same sludge. A phylogenetic analysis based on the 16S rRNA gene sequence indicated that these strains were closely related to each other, and all belonged to the genus Sphingomonas. Addition of a supernatant prepared from a culture of strain GF9 clearly increased the cell yield and growth rate of C. nectariphilum. Growth-promoting activity in other species of the family Sphingomonadaceae was also measured. The results indicated that various Sphingomonas species and their relatives had a similar ability and it was widely distributed within the family Sphingomonadaceae. The growth factor(s) produced by Sphingomonas species have not been identified, but were heat-stable, non-peptides and low-molecular weight (below 1,000 Da) compound(s).
