Seasonal variation in antimicrobial resistance rates of community-acquired Escherichia coli bloodstream isolates.

Seasonal variation in community antimicrobial consumption has been demonstrated, with the lowest utilisation rates during summer months. This retrospective cohort study examined seasonality in antimicrobial resistance (AMR) rates of community-acquired Escherichia coli bloodstream isolates. Escherichia coli bloodstream isolates (2010-2015) were identified through the central Palmetto Health microbiology laboratory database. Multivariate logistic regression was used to examine seasonal variation in AMR. Poisson regression was used to evaluate the association between proportion of multidrug-resistant (MDR) isolates and bimonthly ambulatory antimicrobial prescription rates. Among 339 unique patients with community-acquired E. coli bloodstream infection [median age 65 years; 205 (60.5%) female], AMR rates were lower during summer (June-September) than the rest of the year for amoxicillin/clavulanic acid (17% vs. 29%; aOR = 0.53, 95% CI 0.30-0.92; P = 0.02), cefazolin (6% vs. 19%; aOR = 0.26, 95% CI 0.10-0.58; P < 0.001), ceftriaxone (2% vs. 6%; aOR = 0.25, 95% CI 0.04-0.93; P = 0.04) and trimethoprim/sulfamethoxazole (9% vs. 27%; aOR = 0.27, 95% CI 0.13-0.53; P < 0.001). The proportion of MDR E. coli declined from 31-36% during peak antimicrobial prescription to 11-14% in summer months; a 6.8% decline per interval decrease in antimicrobial prescription rates of 10/100 person-years (P = 0.01). There is significant seasonal variation in AMR rates of E. coli bloodstream isolates to four agents from frequently utilised antimicrobial classes in the community. Examination of seasonal variation in dominant serotypes of community-acquired E. coli bloodstream isolates in future will be valuable.