A spirochaete is a member of a distinctive group of elongated, flexible bacteria with flagella housed in the periplasm, between the inner cellular body and outer cell membrane. These internal flagella contribute to characteristic bending or wave-like movement. A spiral-looking outline alone does not identify a spirochaete.
In Borrelia burgdorferi, disrupting the gene for a major flagellar protein produced nonmotile, rod-shaped cells in place of the normal flat-wave form. Thus the internal flagellar apparatus contributes both to motion and to body shape.[ref]
Borrelia burgdorferi periplasmic flagella have both skeletal and motility functions by M. A. Motaleb and others. 2000, abstract: targeted flaB mutation and changed morphology
Some spirochaetes cause disease, including Lyme disease, but the group is defined by evolutionary and cellular relationships, not by one clinical outcome. New to topics? Read the docs here!