Adhesivity and motility of a zoonotic spirochete: Implications in host-dependent pathogenicity

Bacterial pathogens can infect particular hosts or have a broad range of host species. Mechanisms of host preferences involve complicated host-pathogen associations such as bacterial adherence, gene regulation, and the host’s immune responses1. Here, we show the role of adhesivity and motility of the zoonotic spirochete Leptospira in host-dependent pathogenicity. Leptospira spp. are classified into more than 300 serovars and whereas infection of susceptible mammals can cause clinical symptoms, it can also result in an asymptomatic colonization in some cases, thereby generating maintenance hosts2–5. Although the severity of leptospirosis is generally dependent on a combination of Leptospira serovars and host species, the scenarios leading to such diverse consequences of infection remain unclear. To investigate the underlying mechanisms of the different outcomes of Leptospira infection, we infected kidney cells with various prepared pairs of Leptospira serovars and analyzed the Leptospira dynamics on the host cells. Quantitatively monitoring the behavior of individual Leptospira cells showed that adherence and movement on the host cell surface, called “crawling” correlated with the severity of infection in the host. In the most severe cases, a large fraction of leptospires attached to the host cells and persistently traveled long distances using the crawling mechanism. The biophysical implications of the kinetics and kinematic features of these bacterial pathogens were found to be critical factors for disease severity in their hosts.