Bacteria are divided into two groups the Eubacteria and the Archeobacteria. All medically important bacteria are Eubacteria. The first step in classifying members of the Eubacteria is based on their shape cocci are round, bacilli are rod shaped, Vibrios look like commas and spirilli are spiral shaped.
Spirilli are either flexible or rigid and the flexible spirilli are called spirochetes. All spirilli are gram-stain negative and motile. The spiral shape may be a tightly bound corkscrew or a lose spiral depending on the genus of the bacterium
Spirilli are the only bacteria to have internal flagella. Also known as axial filaments, internal flagella are only found in spirochetes. These filaments run from the ends of the bacterium meeting and sometimes crossing each other in the middle. They are located between the bacteria’s cell wall and outer membrane. The theory of spirochete motions is that the axial filaments rotate causing the outer membrane to rotate. This sets up the corkscrew motion that propels the bacterium. Spirochetes may have anything from two to hundreds of axial filaments depending on species
Other spirilli have external flagella, which propel the bacteria by flexing in a whip-like manner.
There a number of medically important spirilli.
Treponema pallidum is the causative organism of syphilis. This is a very slender bacterium and cannot be seen in normal light microscopy. Under a dark-ground microscope, the organisms’ tightly bound spiral and corkscrew motion is easily visible. Treponemes are spirochetes with axial filaments and a flexible cell wall. Other diseases caused by treponemes include pinta and yaws.
Lyme disease is another disease caused by spirochetes in this case Borrelia burgdorferi is the cause of infection. Borrelia species also cause tick borne relapsing fever and louse born relapsing fever.
Leptospira interogans is the spirochete, which causes Leptospirosis. This bacterium is carried by rats and contaminates natural water sources. Ingestion of the bacterium is not required for infection as it can bore through intact skin.
Helicobacter pylori causes gastric infections and ulcers. This is another spirochete. Other Helicobacter species such as H. felis cause infections in animals.
Spirillum minus has a rigid cell wall and external flagella. This organism causes rat bite fever and is the only Spirillum species of medical importance.
Not all spirilli cause infections a number form part of the normal gastrointestinal flora in man. Many are environmental bacteria and some play a part in the nitrogen cycle by fixing nitrogen in the roots of plants.
Reference sources:
Western Kentucky University on line teaching notes.
Medical Microbiology, fourth edition. Edited by Samuel Baron MD.
Community College of Baltimore County on Line course notes.
Isolation of Acetylene-reducing Spirilla from the Roots of Potamogeton filiformis from Loch Leven (Kinross). Rosemary Sylvester-Bradley Journal of General Microbiology (1976) 97, 129-132.
