Detection of sphingomonads and in situ identification in activated sludge using 16S rRNA-targeted oligonucleotide probes

The increasing significance of members of the genus Sphingomonas in biotech nological applications has led to an increased interest in the diversity, a bundance and ecophysiological potential of this group of Gram-negative bact eria. This general focus provides a challenge to improve means for identifi cation of sphingomonads; eg molecular genetic methods for rapid and specifi c detection could facilitate screening of new isolates. Here, fluorescently labeled oligonucleotide probes targeted against 16S rRNA were used to typi fy strains previously assigned to the genus, All 46 sphingomonads tested in cluding type strains of 21 Sphingomonas species could be detected with a pr obe originally designed for the genus and all but one with a probe designed for the alpha-4 subgroup of the Proteobacteria. The two probes are suitabl e for direct detection of sphingomonads in pure and mixed cultures as well as in environmental samples of unknown composition. The probes were used to identify sphingomonads in situ in activated sludge samples. Sphingomonads were rather abundant accounting for about 5-10% of the total cells in munic ipal sludges. Distinct patterns in aggregation of the cells suggest that th ese organisms could be involved in the formation process of sludge flocs.