| Summary: | Among oral microbiota methanogens, <i>Methanobrevibacter massiliense</i> (<i>M. massiliense</i>) has remained less studied than the well-characterised and cultivated methanogens <i>Methanobrevibacter oralis</i> and <i>Methanobrevibacter smithii</i>. <i>M. massiliense</i> has been associated with different oral pathologies and was co-isolated with the <i>Synergistetes</i> bacterium <i>Pyramidobacter piscolens</i> (<i>P. piscolens</i>) in one case of severe periodontitis. Here, reporting on two additional necrotic pulp cases yielded the opportunity to characterise two co-cultivated <i>M. massiliense</i> isolates, both with <i>P. piscolens</i>, as non-motile, 1–2-µm-long and 0.6–0.8-µm-wide Gram-positive coccobacilli which were autofluorescent at 420 nm. The two whole genome sequences featured a 31.3% GC content, gapless 1,834,388-base-pair chromosome exhibiting an 85.9% coding ratio, encoding a formate dehydrogenase promoting <i>M. massiliense</i> growth without hydrogen in GG medium. These data pave the way to understanding a symbiotic, transkingdom association with <i>P. piscolens</i> and its role in oral pathologies.
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