Some 160 mammalian species are infected with Leptospira bacteria, though in most cases the bacteria do not harm the host. Although several hundred serotypes have been identified, only a small number cause a human disease.
The family Leptospiraceae belongs to the order Spirochaetales and consists of mobile, flexible, helical, Gram-negative rods. Leptospira species are obligate, aerobic bacteria with optimum growth at 28 to 30° C.
The classic taxonomy of Leptospira was based on serotyping, and more than 260 different serovars have been described. The pathogenic serovars have been included in the species L. interrogans, while the nonpathogenic serovars were included in the species L. biflexa.
Source: CDC/NCID/ Rob Weyant
Pathogenic Leptospira species cause infections in both animals and humans. The bacteria can be transferred directly or indirectly from animals to humans and are a zoonosis. Animals are often asymptomatic carriers and humans are dead-end hosts. Some L. interrogans serovars have a host preference, such as Ballum in mice, Canicola in dogs, Icterohaemorrhagia in rats, and Pomona in pigs, whereas others do not seem to prefer specific hosts. Based on the available studies, Leptospira seems to be susceptible to most antimicrobial agents, including tetracyclins, Beta-lactams, fluoroquinolones, lincomycin, streptomycin, kanamycin, chloramphenicol, and quinupristin-dalfopristin, and highly susceptible to macrolides erythromycin and tylosin. Leptospira species are intrinsically resistant to metronidazole and vancomycin.