Antimicrobial growth promoter use in livestock: a requirement to understand their modes of action to develop effective alternatives

•Antimicrobial growth promoters (AGPs) enhance animal health and improve agricultural productivity.•Mechanisms of AGP action are poorly understood; research has diverged into host- and bacteria-centric hypotheses.•Due to risks of antimicrobial resistance, AGPs are being phased out of livestock production.•Understanding the mechanisms of AGP action is critical in developing effective non-antibiotic alternatives.