spirochetes

Spirochetes are a phylum of spiral-shaped, Gram-negative bacteria distinguished by their slender, helical bodies and by endoflagella, or axial filaments, that run within the periplasm between the inner and outer membranes. These internal flagella produce a distinctive corkscrew-like motility that enables movement through viscous media and host tissues.

Morphology and motility

Spirochetes typically measure a few micrometers in width but can be several tens of micrometers long. Their axial filaments cause both rotation and flexing motions, allowing efficient propulsion in mucous, connective tissue, and other complex environments. Their unique envelope structure and motility contribute to their ability to invade hosts and persist in challenging niches.

Taxonomy and notable genera

The group comprises several genera, including Treponema (e.g., Treponema pallidum, the syphilis agent), Borrelia (e.g., Borrelia burgdorferi, the Lyme disease agent), Leptospira (leptospirosis), and Spirochaeta among others. They are broadly placed in the phylum Spirochaetes (also called Spirochaetota). Some species are free-living in aquatic or soil habitats, while others are obligate or opportunistic pathogens of humans and animals.

Ecology and transmission

Spirochetes inhabit diverse environments, including freshwater, soil, and host-associated niches. Transmission routes vary: some are vector-borne (such as ticks transmitting Borrelia), others are acquired through exposure to contaminated water or tissue fluids. Pathogenic species often establish infection in tissues such as the skin, nervous system, or kidneys.

Medical relevance and diagnosis

Several spirochetes are medically important, notably Treponema pallidum (syphilis), Borrelia species (Lyme disease and relapsing fever), and Leptospira (leptospirosis). Diagnosis relies on clinical presentation supplemented by serology, PCR, and, where possible, culture, though culture can be challenging and is not available for all species. Treatment typically involves antibiotics such as penicillin or doxycycline, with regimens chosen by the specific infection and stage.