Leptospira interrogans biofilm formation in Rattus norvegicus (Norway rats) natural reservoirs

Author summaryLeptospirosis is an infectious disease caused by pathogenic Leptospira bacteria. The main reservoir hosts of Leptospira are the brown rats (Rattus norvegicus), which are chronically colonized in the kidneys. Leptospires form biofilms, which are communities of microorganisms embedded in an extracellular polysaccharidic matrix. Leptospira pathogenesis in reservoir hosts is poorly understood. We captured 87 brown rats from an impoverished urban community that is endemic for leptospirosis. To investigate the biofilm in the rats' kidneys, we co-localized leptospires and saccharides of the biofilm extracellular matrix in the renal tubules, using immunohistochemistry anti-Leptospira and carbohydrate staining, respectively. We quantified Leptospira using molecular tools and characterized the biofilm using electron microscopy. We analysed demographic data to identify variables correlated with renal carriage. We found that Leptospira infected 78 rats. From those, 65 were positive for immunohistochemistry in the kidneys and 24 (37%) were biofilm-positive. We found significant positive correlation between the intensity of colonization and the presence of biofilm in the kidneys. The intensity of colonization was also associated with the rats' gender and age. Biofilm formation by Leptospira in the kidneys of natural reservoir rats fills a gap into the knowledge of leptospirosis pathogenesis. Rattus norvegicus (Norway rat) is the main reservoir host of pathogenic Leptospira, the causative agent of leptospirosis, in urban environments. Pathogenic Leptospira forms biofilms in the environment, possibly contributing for bacterial survival and maintenance. Nonetheless, biofilms have not yet been studied in natural animal reservoirs presenting leptospiral renal carriage. Here, we described biofilm formation by pathogenic Leptospira inside the renal tubules of R. norvegicus naturally infected and captured in an urban slum endemic for leptospirosis. From the 65 rats carrying Leptospira in their kidneys, 24 (37%) presented biofilms inside the renal tubules. The intensity of leptospiral colonization in the renal tubules (OR: 1.00; 95% CI 1.05-1.1) and the type of occlusion pattern of the colonized renal tubules (OR: 3.46; 95% CI 1.20-9.98) were independently associated with the presence of Leptospira biofilm. Our data showed that Leptospira interrogans produce biofilms during renal chronic colonization in rat reservoirs, suggesting a possible role for leptospiral biofilms in the pathogenesis of leptospirosis and bacterial carriage in host reservoirs.