Spirochaetes

Spirochaetes. They are a distinctive phylum of Gram-negative bacteria characterized by their long, helically coiled (spiral-shaped) cells. These organisms are motile through a unique internal structure called the axial filament, which is located within the periplasmic space between the cell membrane and the outer membrane. This phylum includes both free-living, symbiotic species and several notorious pathogens responsible for significant diseases in humans and animals.

Description and morphology

The defining morphological feature is the helical or corkscrew-shaped cell, which can range from 5 to 250 µm in length. Motility is provided by endoflagella, also known as axial filaments, which are anchored at both ends of the protoplasmic cell cylinder and wrap around it within the periplasmic space. This arrangement, lying between the cytoplasmic membrane and a flexible peptidoglycan layer, allows the cell to rotate and flex, producing characteristic crawling, boring, or spinning movements. The outer membrane, which encloses the entire structure, is often loosely associated, contributing to the cell's flexibility. This unique ultrastructure is visible via advanced imaging techniques like cryo-electron tomography and was first described in detail by scientists studying Treponema pallidum.

Phylogeny and taxonomy

Based on 16S ribosomal RNA gene sequencing, the phylum is divided into several classes and orders. The class Spirochaetia contains the largest order, Spirochaetales, which encompasses families like Spirochaetaceae and Leptospiraceae. Historically, many spirochaetes were difficult to culture, leading to their initial classification based on morphology and disease association, such as those causing syphilis and Lyme disease. Modern polyphasic taxonomy has clarified relationships, placing genera like Borrelia, Treponema, and Leptospira within distinct families. The order Brachyspirales contains the family Brachyspiraceae, including the intestinal pathogen Brachyspira hyodysenteriae. The order Brevinematales is a smaller, distinct lineage.

Metabolism and growth

Spirochaetes exhibit diverse metabolic strategies, largely corresponding to their ecological niches. Many, like those in the genus Leptospira, are aerobic or microaerophilic and can utilize a wide range of carbon sources, including long-chain fatty acids and amino acids. Others, particularly pathogenic Treponema species, are often obligate anaerobes with fastidious growth requirements, fermenting carbohydrates and requiring complex media supplements; Treponema pallidum subspecies pallidum cannot be continuously cultured *in vitro*. Members of the genus Borrelia are microaerophilic and utilize glucose as a primary carbon source via glycolysis. Growth rates vary widely, from the rapid division of some free-living species to the slow, persistent growth characteristic of pathogens within host tissues.

Pathogenicity and clinical significance

Several genera contain species of major medical and veterinary importance. Treponema pallidum is the causative agent of syphilis, a sexually transmitted infection with a complex clinical history studied by figures like Paul Ehrlich. Borrelia burgdorferi, transmitted by Ixodes ticks, is the primary cause of Lyme disease in North America and Europe. Other Borrelia species cause relapsing fevers, such as Borrelia recurrentis transmitted by the human body louse. Leptospira interrogans causes leptospirosis, a zoonotic disease often associated with exposure to water contaminated by the urine of rodents like Rattus norvegicus. In animals, Brachyspira hyodysenteriae causes swine dysentery, and Leptospira borgpetersenii serovar Hardjo affects cattle.

Ecological roles and habitats

Beyond pathogenesis, spirochaetes occupy vital and diverse ecological niches. Many are free-living in aquatic environments, including marine sediments, hydrothermal vents, and freshwater ponds, where they participate in the degradation of complex organic matter. A significant number form symbiotic or commensal relationships; for instance, spirochaetes are abundant in the hindguts of termites like Reticulitermes flavipes and wood-eating cockroaches of the genus Cryptocercus, where they assist in the digestion of cellulose and hemicellulose. Others are found in the oral cavity of mammals, including humans, as part of the normal microbiota, and in the rumen of animals like Bos taurus. Their presence in extreme environments, such as the anoxic muds of Lake Okeechobee, highlights their metabolic versatility.

Category:Bacteria