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.