Borrelia (Borreliella) burgdorferi is a bacterial species of the spirochete class in the genus Borrelia, and is one of the causative agents of Lyme disease in humans. Along with a few similar genospecies, some of which also cause Lyme disease, it makes up the species complex of Borrelia burgdorferi sensu lato. The complex currently comprises 20 accepted and 3 proposed genospecies. B. burgdorferi sensu stricto exists in North America and Eurasia and until 2016 was the only known cause of Lyme disease in North America. B. burgdorferi are often mistakenly described as Gram negative because of their two external membranes, but they lack lipopolysaccharide and possess many surface lipoproteins, unlike true Gram-negative bacteria.

Closest relatives