Caused by the bacterial genus Leptospira, leptospirosis is a disease that affects both humans and animals, such as cattle, cats, dogs, and rodents. Sources of infection are the urine or urine-contaminated media (e.g., food, water, and soil) of infected animals.
Leptospires can be present in fresh water, which is contaminated by the urine of an infected animal in warmer climates. Most of the people infected by the bacteria experience mild flu-like symptoms. Approximately 10% of cases may progress to the more severe presentation of the condition called Weil’s disease, which is characterized by internal hemorrhaging and multiple organ dysfunction and/ or failure.
Leptospires are small, coil-shaped bacteria with pointed ends that have a distinctive hook. They are gram-negative, aerobic, motile bacteria with paired flagella. The pathogenic Leptospira interrogans has over 200 serovars that are associated with different animals. The structure of their flagellar proteins is very complex and they have two distinct forms of movement, namely, translational and non-translational. Moreover, they possess lipopolysaccharide (LPS), like other gram negative bacteria; however, it has lower endotoxic activity.
Earlier, the Leptospira genus was divided into two species. All pathogenic strains were classified under L. interrogans, while those that contained saprophytic strains were classified under L. biflexa. Distinction was made between the two on the basis of the latter failing to form spherical cells in the presence of 1 M NaCl and its growth at 13°C. They both possess different serovars, which have been defined by agglutination after a process of cross-absorption with antigen of a homologous strain.
Culture and Molecular Biology
Serum or albumin with polysorbate medium has been described in the growth of leptospires. The oleic acid albumin medium is generally the most widely used and is commercially available from many manufacturers. Some strains of leptospires may require rabbit serum or pyruvate, because they are more fastidious in nature. In addition to this, 5-fluorouracil may be used to inhibit the growth of contaminating species. Antibiotics may also be used. Growth is often slow with primary isolates and cultures may be retained for up to 13 weeks prior to being discarded. Pure cultures, on the other hand, usually grow within 14 days in liquid media.
The genome of leptospires is approximately 5000 kb. Due to the lack of a transformation system, the study of the microorganism’s genetics has been retarded. A shuttle vector has been just recently developed using a bacteriophage from L. biflexa and this is great in terms of advancing the molecular understanding of the Leptospira genus. Nonetheless, a number of genes from leptospires have been analyzed and cloned.
Evasion of Immune Defenses
There are several mechanisms that leptospires use to evade the immune defenses. Virulence factors include LPS, outer-membrane proteins, and bacterial attachment and motility. The pathogenic bacteria are able to move through monolayers of cells at a quicker rate than their non-pathogenic counterparts. This quick movement allows for quicker access to the bloodstream as well as rapid dissemination to the various organ systems. In the renal system, for instance, leptospires can enter fibroblasts to induce apoptosis.
Reviewed by Susha Cheriyedath, MSc