Phylogenetic, physiological and kinetic characterization of heterotrophic and lithoautotrophic Thiothrix strains isolated from activated sludge

The sheathed filamentous bacterium, strain CT3, isolated by micromanipulation from an activated sludge treatment plant in Italy, is a member of the genus Thiothrix in the Gammaproteobacteria according to 16S rDNA analysis. The closest phylogenetic neighbours of strain CT3 are strains I and QT comprising T. fructosivorans and isolated from activated sludge. These strains are, respectively, 99.2% and 99.4% identical to CT3 by 16S rDNA comparison. CT3 has 63 67% DNA:DNA hybridisation with strain I, the only currently viable strain of T. fructosivorans. CT3 is the second strain in Thiothrix shown to be capable of growing autotrophically with reduced sulfur compounds as the sole energy source; autotrophy was also confirmed in strain I. The first reported chemolithoautotrophic isolate of this genus was a strain of "Thiothrix ramosa", which was isolated from a hydrogen sulfide spring and is morphologically distinguishable from all the other described strains of Thiothrix, including CT3. CT3 is an aerobic organism, non fermentative not capable of denitrification, and is able to grow heterotrophically. Autotrophy in Thiothrix should be more fully investigated in order to better define the taxonomy of the genus