A new study from the laboratory of Dr. Anthony Maurelli (Department of Environmental and Global Health and University of Florida Emerging Pathogens Institute [UF-EPI]) has traced the origin of dangerous new strains of bacteria that present new threats to public health. Bacteria of the genus Shigella are the causative agents of mild to severe diarrhea commonly known as bacillary dysentery or shigellosis. Shigella infections account for a considerable burden of acute diarrheal diseases worldwide, notably in developing countries where shigellosis remains a major cause of childhood mortality. While all four species of Shigella (S. dysenteriae, S. flexneri, S. boydii, and S. sonnei) cause bacillary dysentery, only S. dysenteriae type 1 has been recognized as carrying the genes for Shiga toxin. This toxin has been linked to hemolytic uremic syndrome, a condition where the kidneys of infected individuals may become so impaired as to cause death. In collaboration with Dr. Keith Lampel of the U.S. Food and Drug Administration (FDA), Dr. Maurelli’s research team previously characterized 50 non-S. dysenteriae type 1 Shigella isolates that produce the Shiga toxin. They demonstrated a strong epidemiological link of many of these new strains to international travelers returning to their home countries from the island of Hispaniola, that is, Haiti and the Dominican Republic. Moreover, state-of-the-art whole genome sequence analysis by Lampel and Maurelli showed that the genes for Shiga toxin in every isolate tested were encoded on the same bacteriophage, a virus that infects and lives within bacteria.
In this new study, postdocs Marta Fogolari and Carla Mavian in the laboratory of Dr. Marco Salemi (UF-EPI) analyzed whole genome sequences of the Shiga toxin-encoding Shigella strains to assess bacteriophage genetic diversity and investigate the patterns of acquisition of toxin genes. Phylogenetic (i.e. how organisms are related to each other) analysis of the sequences provided by Dr. Lampel’s team showed that bacteriophage sequences encoded in all the Shigella strains from Hispaniola were almost identical but different enough to conclude that two distinct versions (clusters) emerged in Haiti and the Dominican Republic. Each cluster possibly originated from bacteriophages originally present in S. flexneri 2a that were subsequently released from that strain and went on to infect other strains of Shigella, carrying the genes for Shiga toxin with them. Thus, this form of horizontal gene transfer played a major role in the emergence of these new Shiga toxin-producing strains of Shigella strains in Hispaniola. The study also underscores the important role international travel plays in the spread of emerging bacterial pathogens and the potential risk to human health and effective treatment.
The paper, entitled “Distribution and characterization of Shiga toxin converting temperate phages carried by Shigella flexneri in Hispaniola”, appears in the journal Infection, Genetics and Evolution (https://www.sciencedirect.com/science/article/pii/S1567134818305586?via%3Dihub).