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

Shigella sonnei has caused unusually large outbreaks of shigellosis in California in 2014 - 2015. Preliminary data indicated the involvement of two distinct yet related bacterial populations, one from San Diego and San Joaquin (SD/SJ) and one from the San Francisco (SF) Bay area. Whole genome sequencing of sixty-eight outbreak and archival isolates of S. sonnei was performed to investigate the microbiological factors related to these outbreaks. Both SD/SJ and SF populations, as well as almost all of the archival S. sonnei isolates belonged to sequence type 152 (ST152). Genome-wide SNP analysis clustered the majority of California (CA) isolates to an earlier described global Lineage III, which has persisted in CA since 1986. Isolates in the SD/SJ population had a novel Shiga-toxin (STX)-encoding lambdoid bacteriophage, most closely related to that found in an Escherichia coli O104:H4 strain responsible for a large outbreak. However, the STX genes (stxla and stxlb) from this novel phage had sequences most similar to the phages from S. flexneri and S. dysenteriae. The isolates in the SF population yielded evidence of fluoroquinolone resistance acquired via the accumulation of point mutations in gyrA and parC genes. Thus, the CA S. sonnei lineage continues to evolve by the acquisition of increased virulence and antibiotic resistance, and enhanced monitoring is advocated for its early detection in future outbreaks.