The Collaborative International Dictionary of English v.0.48:

clostridium \clostridium\ n. clostridia
   1. spindle-shaped bacterial cell esp. one swollen at the
      center by an endospore.

   Syn: clostridia.
        [WordNet 1.5]

   2. [capitalized] a genus of anaerobic, spore-forming motile
      bacteria of the family Bacillaceae, including several
      pathogenic species, such as the causative agents of gas
      gangrene (Clostridium perfringens), botulism
      (Clostridium botulinum), and tetanus (Clostridium
      tetani).
      [PJC]

WordNet (r) 3.0 (2006):

clostridium
    n 1: spindle-shaped bacterial cell especially one swollen at the
         center by an endospore [syn: clostridium, clostridia]