Horizontal transfer of the rfb cluster in Leptospira is a genetic determinant of serovar identity

Insights into host-pathogen interaction based on the comparison of genomes of leptospira interrogans isolated from dogs, humans, and a rodent in the same epidemiological context: A one health approach

Leptospirosis is a zoonotic infectious disease that significantly impacts animal and public health. Comparative genomics can aid in understanding poorly understood aspects of leptospirosis pathogenesis, including infection mechanisms, antimicrobial resistance, and host interactions across different epidemiological scenarios. This study aimed to compare the genomes of Leptospira interrogans serogroup Icterohaemorrhagiae strains isolated from three host species in a single epidemiological scenario. Four strains of L. interrogans serogroup Icterohaemorrhagiae from naturally infected and clinically symptomatic dogs (C20, C29, C51, and C82) were processed for whole genome sequencing (WGS). These results were compared against WGS data from two other rodent and human strains. Phylogenetic and genomic similarity analyses demonstrated high identity and synteny between the strains isolated from humans, canines, and rodents. Small regions of divergence were observed, especially in the genome obtained from a rodent sample. The presence of 23 genes potentially associated with biofilm formation was notable, with the identification of missense mutations in eight genes. Considering the need to better understand the molecular basis involved in biofilm formation, it is of fundamental importance to elucidate the effect of mutations on the expression of the phenotype (biofilm) among different strains. The present findings highlight the necessity of One Health-based collaborative interventions to address the complex dynamics of leptospirosis transmission, involving both common hosts such as rodents and dogs, as well as less-recognized hosts.

Navigating cross-reactivity and host species effects in a serological assay: A case study of the microscopic agglutination test for Leptospira serology

BACKGROUND

Serology (the detection of antibodies formed by the host against an infecting pathogen) is frequently used to assess current infections and past exposure to specific pathogens. However, the presence of cross-reactivity among host antibodies in serological data makes it challenging to interpret the patterns and draw reliable conclusions about the infecting pathogen or strain.

METHODOLOGY/PRINCIPAL FINDINGS

In our study, we use microscopic agglutination test (MAT) serological data from three host species [California sea lion (Zalophus californianus), island fox (Urocyon littoralis), and island spotted skunk (Spilogale gracilis)] with confirmed infections to assess differences in cross-reactivity by host species and diagnostic laboratory. All host species are known to be infected with the same serovar of Leptospira interrogans. We find that absolute and relative antibody titer magnitudes vary systematically across host species and diagnostic laboratories. Despite being infected by the same Leptospira serovar, three host species exhibit different cross-reactivity profiles to a 5-serovar diagnostic panel. We also observe that the cross-reactive antibody titer against a non-infecting serovar can remain detectable after the antibody titer against the infecting serovar declines below detectable levels.

CONCLUSIONS/SIGNIFICANCE

Cross-reactivity in serological data makes interpretation difficult and can lead to common pitfalls. Our results show that the highest antibody titer is not a reliable indicator of infecting serovar and highlight an intriguing role of host species in shaping reactivity patterns. On the other side, seronegativity against a given serovar does not rule out that serovar as the cause of infection. We show that titer magnitudes can be influenced by both host species and diagnostic laboratory, indicating that efforts to interpret absolute titers (e.g., as indicators of recent infection) must be calibrated to the system under study. Thus, we implore scientists and health officials using serological data for surveillance to interpret the data with caution.

Isolation of Virulent Leptospira Serogroup Australis Field Strains from Symptomatic Dogs for Canine Leptospiral Vaccine Development

Leptospirosis is a widespread zoonosis caused by spirochaetes belonging to the pathogenic species of Leptospira, which are classified into more than 25 serogroups and 250 serovars. Vaccination can prevent the disease in dogs but offers incomplete efficacy because of a lack of cross-protection between serogroups. The aim of this study was to validate a robust recruitment and sampling process, with the objectives of isolating and typing circulating Leptospira pathogenic strains and then selecting those of proven virulence and pathogenicity for vaccine development. Blood and urine samples from dogs with clinical syndromes compatible with acute leptospirosis were sterilely collected and transported to a reference laboratory for a micro-agglutination test (MAT), PCR, and bacterial isolation. Isolated strains underwent molecular typing using RNA16S, variable-number tandem repeat (VNTR), and pulsed-field gel electrophoresis (PFGE). Subtyping was performed using core genome multilocus sequence typing (CgMLST). Among 64 included dogs, 41 had MAT and/or PCR results compatible with Leptospira infection, and 14 Leptospira strains were isolated. Based on molecular typing, 11 isolates were classified as L. interrogans serogroup Australis, serovar Bratislava, and 3 as serogroup Icterohaemorrhagiae, serovar Icterohaemorrhagiae. CgMLST subtyping revealed a diversity of clonal groups (CGs) distributed in several regional clusters. Besides validating a robust recruitment and sampling process, this study outlines the value of combining PCR and serological testing when suspecting leptospirosis and the usefulness of implementing molecular typing methods to identify circulating field strains. It also confirms the epidemiological importance of the Australis serogroup and allows for the collection of different highly pathogenic strains for vaccine development.

Evolutionary insights into the emergence of virulent Leptospira spirochetes

Pathogenic Leptospira are spirochete bacteria which cause leptospirosis, a re-emerging zoonotic disease of global importance. Here, we use a recently described lineage of environmental-adapted leptospires, which are evolutionarily the closest relatives of the highly virulent Leptospira species, to explore the key phenotypic traits and genetic determinants of Leptospira virulence. Through a comprehensive approach integrating phylogenomic comparisons with in vitro and in vivo phenotyping studies, we show that the evolution towards pathogenicity is associated with both a decrease of the ability to survive in the environment and the acquisition of strategies that enable successful host colonization. This includes the evasion of the mammalian complement system and the adaptations to avoid activation of the innate immune cells by the highly-virulent Leptospira species (also called P1+ species), unlike other species belonging to the phylogenetically related P1- and P2 groups, as well as saprophytes. Moreover, our analysis reveals specific genetic determinants that have undergone positive selection during the course of evolution in Leptospira, contributing directly to virulence and host adaptation as demonstrated by gain-of-function and knock-down studies. Taken together, our findings define a new vision on Leptospira pathogenicity, identifying virulence attributes associated with clinically relevant species, and provide insights into the evolution and emergence of these life-threatening pathogens.

Genetic structure and diversity of the rfb locus of pathogenic species of the genus Leptospira

Leptospirosis is caused by pathogenic strains of the genus Leptospira and is considered the most widespread zoonotic bacterial disease. The genus is characterized by the large number of serology variants, which challenges developing effective serotyping methods and vaccines with a broad spectrum. Because knowledge on the genetic basis of the serological diversity among leptospires is still limited, we aimed to explore the genetic structure and patterns of the rfb locus, which is involved in the biosynthesis of lipopolysaccharides, the major surface antigen that defines the serovar in leptospires. Here, we used genomic data of 722 pathogenic samples and compared the gene composition of their rfb locus by hierarchical clustering. Clustering analysis showed that the rfb locus gene composition is species-independent and strongly associated with the serological classification. The samples were grouped into four well-defined classes, which cluster together samples either belonging to the same serogroup or from different serogroups but sharing serological affinity. Our findings can assist in the development of new strategies based on molecular methods, which can lead to better tools for serological identification in this zoonosis.

Global distribution of Leptospira serovar isolations and detections from animal host species: A systematic review and online database

OBJECTIVES

Leptospira, the spirochaete causing leptospirosis, can be classified into >250 antigenically distinct serovars. Although knowledge of the animal host species and geographic distribution of Leptospira serovars is critical to understand the human and animal epidemiology of leptospirosis, current data are fragmented. We aimed to systematically review, the literature on animal host species and geographic distribution of Leptospira serovars to examine associations between serovars with animal host species and regions and to identify geographic regions in need of study.

METHODS

Nine library databases were searched from inception through 9 March 2023 using keywords including Leptospira, animal, and a list of serovars. We sought reports of detection of Leptospira, from any animal, characterised by cross agglutinin absorption test, monoclonal antibody typing, serum factor analysis, or pulsed-field gel electrophoresis to identify the serovar.

RESULTS

We included 409 reports, published from 1927 through 2022, yielding data on 154 Leptospira serovars. The reports included data from 66 (26.5%) of 249 countries. Detections were from 144 animal host species including 135 (93.8%) from the class Mammalia, 5 (3.5%) from Amphibia, 3 (2.1%) from Reptilia, and 1 (0.7%) from Arachnida. Across the animal host species, Leptospira serovars that were detected in the largest number of animal species included Grippotyphosa (n = 39), Icterohaemorrhagiae (n = 29), Pomona (n = 28), Australis (n = 25), and Ballum (n = 25). Of serovars, 76 were detected in a single animal host species. We created an online database to identify animal host species for each serovar by country.

CONCLUSIONS

We found that many countries have few or no Leptospira serovars detected from animal host species and that many serovars were detected from a single animal species. Our study highlights the importance of efforts to identify animal host species of leptospirosis, especially in places with a high incidence of human leptospirosis. We provide an updated resource for leptospirosis researchers.

Levels of Cytokines in Leptospirosis Patients with Different Serovars and rfb Locus

Leptospirosis has a wide spectrum of clinical manifestations ranging from mild to severe disease. The cytokine response is considered one of the key drivers for this varying manifestation. The different cytokine response observed in patients with leptospirosis could be due to the variation of infecting serovars. Since the rfb locus codes for the lipopolysaccharide synthesis of the bacterial cell wall, which also determines the serovar, this locus may play a role in driving a specific cytokine response in the host. We investigated 12 commonly used cytokine profiles in serum samples of culture, microscopic agglutination test (MAT), or polymerase chain reaction (PCR)-positive patients with leptospirosis. The sequences of the rfb locus in culture-positive samples were generated from whole genome sequencing and serovar status was drawn from original data published. Isolated cultures were subjected to whole genome sequencing using the PacBio RS II system, and the resulting data were used to determine the species. The recovered genomic data were annotated with the Rapid Annotation using Subsystem Technology (RAST) subsystem, and the rfb locus was extracted. The cytokine analysis was carried out using the Qiagen human ELISA kit. Eighteen samples were found to be positive by culture, while the other 7 samples were positive by PCR or MAT. Infections from Leptospira interrogans serovar Autumnalis (5), Pyrogens (3), Icterohaemorrhagiae (1) Leptospira borgpetersenii (all 7 samples clustered in same clonal group with serovar status not determined), Leptospira weilii (1 with serovar status not determined), and Leptospira kirschneri serovar Grippotyphosa (1) were included in the analysis. Three patients [infected with Leptospira interrogansserovar Autumnalis (2) and Pyrogens (1)] and 2 MAT-positive patients (highest titer against serovar Bratislava of L.interrognas) were reported to have severe clinical manifestations, while the rest had mild to moderate symptoms. Although the serum cytokine concentration of patients with severe clinical manifestation was comparatively higher, a statistically significant difference was observed only for interleukin (IL)-1β (P < 0.05). IL-10/tumor necrosis factor-alpha (TNF-α) ratio was high in patients with severe complications. In general, patients infected with L. interrogans showed higher concentration of cytokines compared to L. borgpetersenii.

Complete genome sequences of twelve strains of Leptospira interrogans isolated from humans in Sri Lanka

Leptospirosis, a major zoonotic disease caused by pathogenic Leptospira spp. is recognized globally as an emerging zoonotic disease. Whole-genome sequencing reveals hidden messages about Leptospira's pathogenesis. We used Single Molecule Real-Time (SMRT) sequencing to obtain complete genome sequences of twelve L. interrogans isolates from febrile patients from Sri Lanka for a comparative whole genome sequencing study. The sequence data generated 12 genomes with a coverage greater than X600 with sizes ranging from 4.62 Mb to 5.16 Mb, and a G + C content ranging from 35.00% to 35.42%. The total number of coding sequences predicted by the NCBI (National Center for Biotechnology Information) genome assembly platform ranged from 3845 to 4621 for the twelve strains. Leptospira serogroup with similar-sized LPS biosynthetic loci that belonged to the same clade had a close relationship in the phylogenetic analysis. Nonetheless, variations in the genes encoding sugar biosynthesis were found in the serovar determinant region (rfb locus). Type I and Type III CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) systems were found in all of the strains. Genome BLAST Distance Phylogeny of these sequences allowed for detailed genomic strain typing. These findings may help us better understand the pathogenesis, develop a tools for early diagnosis, comparative genomic analysis and evolution of Leptospira.

Lipid A structural diversity among members of the genus Leptospira

Lipid A is the hydrophobic component of bacterial lipopolysaccharide and an activator of the host immune system. Bacteria modify their lipid A structure to adapt to the surrounding environment and, in some cases, to evade recognition by host immune cells. In this study, lipid A structural diversity within the Leptospira genus was explored. The individual Leptospira species have dramatically different pathogenic potential that ranges from non-infectious to life-threatening disease (leptospirosis). Ten distinct lipid A profiles, denoted L1-L10, were discovered across 31 Leptospira reference species, laying a foundation for lipid A-based molecular typing. Tandem MS analysis revealed structural features of Leptospira membrane lipids that might alter recognition of its lipid A by the host innate immune receptors. Results of this study will aid development of strategies to improve diagnosis and surveillance of leptospirosis, as well as guide functional studies on Leptospira lipid A activity.