What are Leptospira?

Definition

Leptospira is a genus of gram-negative, motile, obligate aerobic spirochetes that use only long-chain fatty acids as an energy source. Both free-living and parasitic forms of Leptospira exist.

Natural Habitat and Features

The name Leptospira was derived from the Greek words leptos and spira, meaning “thin coil.” Leptospires are 10 to 20 micrometers (µm) long and 0.1 µm thick and are tightly coiled in a right-handed manner around a central cylinder. They are highly motile, due to two periplasmic flagella that, as in other spirochetes, are attached to opposite ends of the protoplasmic cylinder, unattached at the other end, and extend about two-thirds of the way along the cylinder. The membrane cell wall of the cylinder is rigid, while the complex outer sheath is flexible. When the flagella rotate in the space between the sheath and the cylinder, the entire organism rotates in the opposite direction, allowing for motility.

Because Leptospira are so thin, they are difficult to visualize under a normal light microscope; most can be seen using dark-field or phase-contrast microscopy. All strains grow best at a pH level of 7.2 to 7.6. Pathogenic strains do best at a temperature between 82.4 and 86 degrees Fahrenheit (28–30 degrees Celsius), but some can grow at temperatures as low as 55.4 degrees Fahrenheit (13 degrees Celsius).

Visible growth on agar often takes four to seven days. Nonpathogenic strains, which are saprobic, do best at slightly lower temperatures, and many have minimal growth temperatures in the 41–50 degrees Fahrenheit (5–10 degrees Celsius) range. Visible growth on agar is usually apparent after two to three days. All species require long-chain fatty acids, which they break down through beta-oxidation, as their energy source. These fatty acids are usually supplied in vitro by the Ellinghausen-McCullough-Johnson-Harris medium.

No leptospires are able to use sugars as energy sources, but they can build needed sugars through Krebs cycle intermediates. Most pathogenic strains also require vitamins B1 and B12, and all strains require iron. The genome size is usually about four million base pairs, forming more than four thousand genes. The genome of L. interrogans consists of a larger 4.3-megabase chromosome and a smaller 359-kilobase chromosome. The Leptospira genome is larger than those of most other pathogenic spirochetes.

Taxonomic separation for many years has been by serotype, and more than 240 serotypes of Leptospira have been discovered. Genotypic DNA analysis has shown that strains that share the same serotype may be genetically distant, while closely related strains may show different serotypes. Modern Leptospira taxonomy uses genotyping and has placed most pathogenic serotypes in L. interrogans and most saprobic serotypes in L. biflexa. However, fifteen to twenty other less common, genetically distinct species have been postulated by genotyping.

Nonpathogenic strains can be found in many aquatic and damp habitats throughout the world, excluding polar regions. Pathogenic strains are parasitic or commensal in many animal species, including humans, but rodents, especially mice and rats, seem to serve as primary reservoirs for many pathogenic strains. In commensal or parasitized animals, the bacteria usually reside in the kidneys and are introduced into the environment through urine. The leptospires can remain alive for several weeks outside their host as long as they remain damp and warm. Infections occur year round in the tropics, but mainly in the summer, when the weather is warmer. In more temperate regions, infections occur mainly in winter, when rodents are more likely to enter homes for shelter. It has been postulated that human infections in temperate regions may increase with global warming.

Pathogenicity and Clinical Significance

Leptospirosis, caused by infection with Leptospira, is considered a zoonotic disease that affects a variety of animals, including mammals, birds, reptiles, and insects. Humans are only occasionally infected, mostly in the tropics and mostly by contact with dogs or small rodents. The most severe form of human leptospirosis is also known as Weil’s disease, named for Adolph Weil, who first described it in the late nineteenth century. The bacteria usually enter a host when contaminated water comes in contact with abraded skin or with mucous membranes.

Early symptoms of leptospirosis in humans include fever, chills, and headache, which are often mistaken for flu. During this time, bacteria can be isolated from the blood. After a brief asymptomatic phase, bacteria become ensconced in the endothelium of internal organs, such as the liver, the nervous system, the lungs, the heart, and especially the kidneys. This can lead to liver damage, which leads to jaundice, meningitis, pulmonary hemorrhage, renal failure, and, occasionally, cardiovascular problems and delirium. Mortality is common, especially in untreated persons, and is usually caused by pulmonary problems or renal failure.

Drug Susceptibility

Early in the infection, oral doxycycline is the drug of choice. In acute infections, hospitalization and intravenous penicillin G are the preferred treatments. For persons allergic to penicillins, erythromycin is an effective alternative. Third-generation cephalosporins, such as cefotaxime and ceftriaxone, have also been shown to be effective treatments, but dosage must be monitored carefully if the infected person is in renal failure, because these drugs can build up to toxic levels if not cleared properly by the kidneys.

Immunizations for humans are not routine in the United States because the immunizations are serotype specific, and there are many different serotypes. Outside the United States, however, some at-risk workers are immunized against locally endemic serotypes. Immunization of pets, especially dogs, against the more common serotypes can reduce the chance that dogs will become infected and then pass the bacteria to humans.

Bibliography

Madigan, Michael T., et al. Brock Biology of Microorganisms. 14th ed. San Francisco: Benjamin, 2015. Print.

World Health Organization and International Leptospirosis Society. Human Leptospirosis: Guidance for Diagnosis, Surveillance and Control. Geneva: WHO, 2003. World Health Organization. Web. 29 Dec. 2015.

Zuerner, Richard L. “Genus I. Leptospira.” Bergey’s Manual of Systematic Bacteriology. Ed. Noel R. Krieg et al. 2nd ed. Vol. 4. New York: Springer, 2010. 546–56. Print.