Abstract
Streptococcus agalactiae is a major pathogen and a hindrance on tilapia farming worldwide. The aims of this work were to analyze the genomic evolution of Brazilian strains of S. agalactiae and to establish spatial and temporal relations between strains isolated from different outbreaks of streptococcosis. A total of 39 strains were obtained from outbreaks and their whole genomes were sequenced and annotated for comparative analysis of multilocus sequence typing, genomic similarity and whole genome multilocus sequence typing (wgMLST). The Brazilian strains presented two sequence types, including a newly described ST, and a non-typeable lineage. The use of wgMLST could differentiate each strain in a single clone and was used to establish temporal and geographical correlations among strains. Bayesian phylogenomic analysis suggests that the studied Brazilian population was co-introduced in the country with their host, approximately 60 years ago. Brazilian strains of S. agalactiae were shown to be heterogeneous in their genome sequences and were distributed in different regions of the country according to their genotype, which allowed the use of wgMLST analysis to track each outbreak event individually.
Highlights
Streptococcus agalactiae is a major pathogen and a hindrance on tilapia farming worldwide
The main epidemiological tool applied in studies of GBS diseases is the MLST11,15, which can discriminate strains in lineages and combine them based on genetic proximity in clonal complexes (CC)
multilocus sequence typing (MLST) is the main epidemiological tool for studying human and animal diseases caused by S. agalactiae, and it has been widely used to study evolutionary relationships between strains from different epidemiological sources, such as hosts species, geographical distances, and periods of isolation[4,9,13,30,31,32]
Summary
Streptococcus agalactiae is a major pathogen and a hindrance on tilapia farming worldwide. Streptococcus agalactiae (Lancefield’s group B Streptococcus, GBS) is a Gram-positive coccus that causes septicaemia and meningoencephalitis in many species of marine and freshwater fish worldwide[1,2,3,4,5] This bacterium may cause septicaemia and meningitis in human new-borns[6] and has already been reported in other animals, including guinea pigs, camels, cats, dolphins, horses and frogs[7]. The main epidemiological tool applied in studies of GBS diseases is the MLST11,15, which can discriminate strains in lineages (sequence type, ST) and combine them based on genetic proximity in clonal complexes (CC) This method has proven efficient for understanding evolutionary stories between lineages and has so far discriminated fish GBS isolates worldwide in several CCs, including the fish-specific CC5524,16. Examples of this are several strains from human, bovine, feline and rodent
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