Shigella Phages Isolated during a Dysentery Outbreak Reveal Uncommon Structures and Broad Species Diversity.

In 2016, Michigan experienced the largest outbreak of shigellosis, a type of bacillary dysentery caused by spp., since 1988. Following this outbreak, we isolated 16 novel -infecting bacteriophages (viruses that infect bacteria) from environmental water sources. Most well-known bacteriophages infect the common laboratory species and , and these phages have built the foundation of molecular and bacteriophage biology. Until now, comparatively few bacteriophages were known to infect spp., which are close relatives of We present a comprehensive analysis of these phages' host ranges, genomes, and structures, revealing genome sizes and capsid properties that are shared by very few previously described phages. After sequencing, a majority of the phages were found to have genomes of an uncommon size, shared by only 2% of all reported phage genomes. To investigate the structural implications of this unusual genome size, we used cryo-electron microscopy to resolve their capsid structures. We determined that these bacteriophage capsids have similarly uncommon geometry. Only two other viruses with this capsid structure have been described. Since most well-known bacteriophages infect or , our understanding of bacteriophages has been limited to a subset of well-described systems. Continuing to isolate phages using nontraditional strains of bacteria can fill gaps that currently exist in bacteriophage biology. In addition, the prevalence of phages during a shigellosis outbreak may suggest a potential impact of human health epidemics on local microbial communities. spp. bacteria are causative agents of dysentery and affect more than 164 million people worldwide every year. Despite the need to combat antibiotic-resistant strains, relatively few -infecting bacteriophages have been described. By specifically looking for -infecting phages, this work has identified new isolates that (i) may be useful to combat infections and (ii) fill gaps in our knowledge of bacteriophage biology. The rare qualities of these new isolates emphasize the importance of isolating phages on "nontraditional" laboratory strains of bacteria to more fully understand both the basic biology and diversity of bacteriophages.