Emerging antimicrobial resistance among strains in bloodstream infections in Toronto, 2006-2016: a retrospective cohort study.

Background: Enterobacteriaceae that produce extended-spectrum β-lactamase (ESBL) have emerged as a serious threat, with variable rates depending on geographic region. We determined the prevalence of ESBL-producing , , and in bloodstream infections in Toronto from 2006 through 2016.

Methods: All patients with , , and isolated from blood in a tertiary care microbiology laboratory in Toronto between 2006 and 2016 (1 isolate per species per patient per year) were included in this retrospective cohort study. Organisms were identified by conventional methods, and susceptibility testing was performed according to Clinical and Laboratory Standards Institute standards. Screening for ESBL and phenotypic confirmatory testing were done with a modified Clinical and Laboratory Standards Institute method. ST131 clonal type was determined by means of an established protocol.

Results: The proportion of ESBL-producing isolates increased significantly between 2006 and 2016, from 6.4% (19/296) to 17.3% (89/513) ( < 0.001). This trend was seen in both intensive care units and emergency departments. Concurrently, the proportion of ST131 among ESBL-producing also increased significantly, from 31.6% (6/19) in 2006 to 73.0% (65/89) in 2016 ( = 0.03). Among ESBL-producing significant resistance was noted to multiple antimicrobial classes. Comparable increases in the proportion of ESBL-producing , and were not noted.

Interpretation: We observed a significant increase in the proportion of ESBL-producing in bloodstream infections in Toronto temporally correlated with an increase in the ST131 clonal type. Recognition of this dramatic rise is important to inform empiric antibiotic treatment.