Assessing the potential impacts of California Senate Bill 27 (SB27) on the antimicrobial susceptibility of Escherichia coli from raw meat

Background
Antimicrobial use in food-animal production selects for antimicrobial-resistant Escherichia coli (E. coli) that can be transmitted to humans via contaminated meat products. California Senate Bill 27 (SB27), which took effect on January 1, 2018, restricts the use of medically important antimicrobials in California food-animal production. Over time, SB27 could reduce the prevalence of antimicrobial-resistant E. coli on meat produced in California.

Objectives
We aimed to assess whether the implementation of SB27 was associated with significant decreases in resistance to medically important antimicrobials among E. coli strains contaminating raw chicken produced in California.

Methods
We purchased raw chicken products in Southern California, including those produced in and outside of California, from 2017 to 2021 and cultured them for E. coli. Susceptibility to 19 antimicrobials was determined using the disk diffusion method. Changes in antimicrobial susceptibility over the course of the study were evaluated using the Mann-Kendall test.

Results
We observed significant decreases in resistance to multiple classes of antimicrobials in E. coli isolated from retail chicken meat from 2017 to 2021. Resistance to penicillins had a relative decrease of 14-18% annually in E. coli from chicken raised in California but not from chicken raised outside California, potentially indicating that SB27 was effective. Resistance to multiple other classes of antimicrobials saw an absolute decrease of up to 8% in chicken produced both inside and outside California.

Discussion
Our findings suggest that the downward trends in antimicrobial resistance among E. coli populations from California-produced chicken products reflect national trends. It is possible that the California SB27 legislation helped motivate industry-wide decreases in antimicrobial use among broiler chicken producers. Alternatively, the changes observed in California may have been driven by industry-wide trends independent of SB27. The lack of publicly available data regarding actual antimicrobial use in California and non-California broiler chicken production limits our ability to make stronger conclusions about our observations.