Thelen, Karin

Summary Twelve strains of filamentous bacteria morphologically identified as ‘ Microthrix parvicella ’ were isolated from industrial activated sludge wastewater treatment plants. 16S rRNA gene sequences analysis showed that these strains were all closely related to ‘ Candidatus Microthrix parvicella’. Six of them, however, had a 16S rRNA gene similarity of only 95.7% and 96.7% to ‘ Candidatus Microthrix parvicella’ suggesting the presence of a new species. The name ‘ Candidatus Microthrix calida’ is proposed for this new microorganism. The physiological properties of these six isolates supported the description of a new taxon. The ‘ Candidatus Microthrix calida’ strains produced thin filaments (0.3–0.7 µm diameter), they did not grow on the media supporting the growth of ‘ Candidatus Microthrix parvicella’ and could be cultivated at higher temperature (up to 36.5°C). Preliminary data on substrate uptake were obtained by microautoradiography on pure culture. Two new fluorescence in situ hybridization (FISH) probes, Mpa‐T1‐1260 specific for ‘ Candidatus Microthrix calida’ and Mpa‐all‐1410 targeting both Microthrix species, were designed. The presence of Microthrix spp. was investigated in 114 activated sludge plants. ‘ Microthrix parvicella ’ morphotype was detected in 23% of the analysed samples and FISH analysis revealed that ‘ Candidatus Microthrix calida’ was present in 5% of them. The remaining ‘ M. parvicella ’ filaments were positive with probe Mpa‐all‐1410 but could not all be identified as ‘ Candidatus Microthrix parvicella’ suggesting the presence of more hitherto undescribed biodiversity within this morphotype.