High pathogenic potential of Shigella flexneri strains descending from a common ancestor isolated over 34 years in Brazil.

Shigellosis is a serious public health problem worldwide caused by Shigella spp. The aims of this study were to molecularly and phenotypically characterize 50 S. flexneri strains isolated from human feces between 1983 and 2017 in different States of Brazil. The pathogenic potential of the strains studied was verified by the ability of the strains to invade human gut epithelial cells (Caco-2), as well as to survive in human macrophages (U-937). The virulence of eight S. flexneri strains, representative of the decades analyzed, was evaluated using the Galleria mellonella infection model. The phylogeny of the 50 strains was verified by Multilocus Sequence Typing (MLST). The percentage of invasion in Caco-2 cells was ≥ 40% and survival in U-937 was ≥ 60% for all 50 S. flexneri strains. Seven of the eight S. flexneri strains caused the death of more than 50% of the Galleria mellonella larvae. All 50 strains were typed as ST245 by MLST. In conclusion, the percentage of cell invasion, macrophage survival and G. mellonella mortality by the strains studied highlighted their pathogenic potential and virulence. The MLST result suggested the presence of phylogenetically related S. flexneri strains circulating in Brazil.