Recent Outbreaks of Shigellosis in California Caused by Two Distinct Populations of Shigella sonnei with either Increased Virulence or Fluoroquinolone Resistance

Shigella sonnei has caused unusually large outbreaks of shigellosis in California in 2014 and 2015. Preliminary data indicated the involvement of two distinct bacterial populations, one from San Diego and San Joaquin (SDi/SJo) and one from the San Francisco (SFr) Bay area. Whole-genome analysis and antibiotic susceptibility testing of 68 outbreak and archival isolates of S. sonnei were performed to investigate the microbiological factors related to these outbreaks. Both SDi/SJo and SFr populations, as well as almost all of the archival S. sonnei isolates belonged to sequence type 152 (ST152). Genome-wide single nucleotide polymorphism (SNP) analysis clustered the majority of California (CA) isolates to an earlier described lineage III. Isolates in the SDi/SJo population had a novel lambdoid bacteriophage carrying genes encoding Shiga toxin (STX) that were most closely related to that found in Escherichia coli O104:H4. However, the STX genes (stx1A and stx1B) from this novel phage had sequences most similar to the phages from Shigella flexneri and S. dysenteriae. The isolates in the SFr population were resistant to ciprofloxacin due to point mutations in gyrA and parC genes and were related to the fluoroquinolone-resistant S. sonnei clade within lineage III that originated in South Asia. The emergence of a highly virulent S. sonnei strain and introduction of a fluoroquinolone-resistant strain reflect the changing traits of this pathogen in California. An enhanced monitoring is advocated for early detection of future outbreaks caused by such strains. IMPORTANCE Shigellosis is an acute diarrheal disease causing nearly half a million infections, 6,000 hospitalizations, and 70 deaths annually in the United States. S. sonnei caused two unusually large outbreaks in 2014 and 2015 in California. We used whole-genome sequencing to understand the pathogenic potential of bacteria involved in these outbreaks. Our results suggest the persistence of a local S. sonnei SDi/SJo clone in California since at least 2008. Recently, a derivative of the original clone acquired the ability to produce Shiga toxin (STX) via exchanges of bacteriophages with other bacteria. STX production is connected with more severe disease, including bloody diarrhea. A second population of S. sonnei that caused an outbreak in the San Francisco area was resistant to fluoroquinolones and showed evidence of connection to a fluoroquinolone-resistant lineage from South Asia. These emerging trends in S. sonnei populations in California must be monitored for future risks of the spread of increasingly virulent and resistant clones.