Spiral bacteria
Spiral bacteria, bacteria of spiral shape, form the third major morphological category of prokaryotes along with the rod-shaped bacilli and round cocci. Spiral bacteria can be subclassified by the number of twists per cell, cell thickness, cell flexibility, and motility. The two types of spiral cells are spirillum and spirochete, with spirillum being rigid with external flagella, and spirochetes being flexible with internal flagella.Spirillum
A spirillum is a rigid spiral bacterium that is Gram-negative and frequently has external amphitrichous or lophotrichous flagella. Examples include:
A spirochete is a very thin, elongate, flexible, spiral bacteria that is motile via internal periplasmic flagella inside the outer membrane. They comprise the phylum Spirochaetes. Owing to their morphological properties, spirochetes are difficult to Gram-stain but may be visualized using dark field microscopy or Warthin–Starry stain. Examples include:
