Species belonging in the Phylum Spirochetes are classified under Domain Bacteria. They are generally long, helically coiled organisms which are chemoheterotrophic in nature. However, there are certain distinctions regarding their physical characteristics.
Their characteristics which made them belong in the Domain Bacteria are: Their cell membrane contains ester bonds with a peptidoglycan-based cell wall. They only have one RNA polymerase. One familiar membrane of the Phylum Spirochetes is the disease-causing Leptospira.
The distinctive characteristic of the Phylum Spirochete are: They are diderm or double-membrane bacteria. Their peptidoglycan layer is organized together with the cytoplasmic membrane rather than the outer membrane. They also differ in the location of their flagella.