The Collaborative International Dictionary of English v.0.48:

Spirillum \Spi*ril"lum\, n. [NL., dim. of L. spira a coil.]
   1. (Biol.) A genus of common gram-negative motile
      microorganisms (family Spirillaceae) having the form of
      spiral-shaped filaments. They are obligately
      microaerophilic, and do not ferment carbohydrates.
      --Stedman.
      [1913 Webster]

   2. A species of bacteria (Borrelia recurrentis, also known
      as Obermeier's spirillum), once believed to belong to
      the genus Spirillum, which causes relapsing fever.
      [PJC]

WordNet (r) 3.0 (2006):

spirillum
    n 1: spirally twisted elongate rodlike bacteria usually living
         in stagnant water
    2: any flagellated aerobic bacteria having a spirally twisted
       rodlike form [syn: spirillum, spirilla]