A newly discovered species of Clostridium would still be an anaerobic , spore‑forming, Gram‑positive rod that fits within the broader definition of the genus, even if some species can appear Gram‑variable on stain.

Core point

  • Clostridium species are classically defined as strictly anaerobic, Gram‑positive, spore‑forming bacilli.
  • Modern phylogenetic work has shown the old genus is not truly monophyletic, which is why many former Clostridium species have been reclassified into new genera, but newly recognized members that remain in Clostridium must still meet these phenotypic and genetic criteria.
  • As of 2018, more than 200 Clostridium species were recognized, illustrating that new species continue to be added as they are isolated and characterized.