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.

Molecular typing of Leptospira interrogans serovar Hardjo isolates from leptospirosis outbreaks in Brazilian livestock

Background

Leptospirosis is caused by pathogenic spirochetes of the genus Leptospira spp. This zoonotic disease is distributed globally and affects domestic animals, including cattle. Leptospira interrogans serogroup Sejroe serovar Hardjo and Leptospira borgpetersenii serogroup Sejroe serovar Hardjo remain important species associated with this reproductive disease in livestock production. Previous studies on Brazilian livestock have reported that L. interrogans serovar Hardjo is the most prevalent leptospiral agent in this country and is related to clinical signs of leptospirosis, which lead to economic losses in production. Here, we described the isolation of three clinical strains (Norma, Lagoa and Bolivia) obtained from leptospirosis outbreaks that occurred in Minas Gerais state in 1994 and 2008.

Results

Serological and molecular typing using housekeeping (secY and 16SrRNA) and rfb locus (ORF22 and ORF36) genes were applied for the identification and comparative analysis of Leptospira spp. Our results identified the three isolates as L. interrogans serogroup Sejroe serovar Hardjo and confirmed the occurrence of this bacterial strain in Brazilian livestock. Genetic analysis using ORF22 and ORF36 grouped the Leptospira into serogroup Sejroe and subtype Hardjoprajitno. Genetic approaches were also applied to compare distinct serovars of L. interrogans strains by verifying the copy numbers of the IS1500 and IS1533 insertion sequences (ISs). The IS1500 copy number varied among the analyzed L. interrogans strains.

Conclusion

This study provides evidence that L. interrogans serogroup Sejroe serovar Hardjo subtype Hardjoprajitno causes bovine leptospirosis in Brazilian production. The molecular results suggested that rfb locus (ORF22 and ORF36) could improve epidemiological studies by allowing the identification of Leptospira spp. at the serogroup level. Additionally, the IS1500 and IS1533 IS copy number analysis suggested distinct genomic features among closely related leptospiral strains.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-017-1081-9) contains supplementary material, which is available to authorized users.

Molecular characterization by LSSP-PCR and DNA sequencing of a pathogenic isolate of Leptospira interrogans from Brazil

We report the initial characterization of a leptospiral isolate, Leptospira interrogans, serogroup Sejroe, serovar Hardjo, genotype Hardjoprajitno, strain Norma, and its relatedness with L. interrogans, serogroup Sejroe, serovar Hardjo, genotype Hardjoprajitno, strain Hardjo and Leptospira borgpetersenii, serogroup Sejroe, serovar Hardjo, genotype Hardjobovis, strain Sponselee. The Norma strain singled out during a leptospirosis outbreak in cattle immunized with antigens from the reference strain Hardjoprajitno (OMS). By applying a microscopic agglutination serological test (MAT) to cattle (n = 2966) with symptoms of leptospirosis between 2003 and 2007, more than 50% of sera were found positive for one of the following serotypes: Hardjoprajitno (31-21%), Hardjo Norma (46-40%), Hardjo hardjobovis (18-10%), Mini (8-4%) and Wolffi (7-4%). In immunization trials using six isolates plus Norma isolate, the remission of MAT in these isolates was observed following 6 months of the initial vaccination. To provide molecular ground for the high MAT Norma frequency found in these isolates, a DNA polymorphic analysis was conducted by comparing the Norma isolate with reference strains Hardjoprajitno and Sponselee. The polymorphic analysis in secY showed five base changes in Norma relative to Hardjoprajitno strain, corresponding to 98% identity, while Sponselee displayed 49 polymorphic sites relative to the Hardjoprajitno strain, representing 80% identity. The alignment of secY translated sequences shows no differences between Hardjoprajitno and Norma, and eight polymorphisms between genotype hardjoprajttno and strain Sponselee. Three-dimensional modelling located these variations within the loop region connecting helices 7 and 8 from secY which is less conserved. DNA sequencing of 23S ribosomal conserved fragment revealed a single polymorphism between Hardjoprajitno and Norma, and 13 polymorphisms between strains Sponselee, Hardjoprajitno and Norma. The differences between Hardjo and Norma were confirmed by low stringency single-specific primer polymerase chain reaction (LSSP-PCR) signature experiments with the primer G2, using as template the 285 bp fragment initially amplified with G1/G2 primers.