The Collaborative International Dictionary of English v.0.48:

Spirobacteria \Spi`ro*bac*te"ri*a\, n. pl.; sing.
   Spirobacterium. [NL. See 4th Spire, and Bacterium.]
   (Biol.)
   See the Note under Microbacteria.
   [1913 Webster] Spirochaeta