Crop contamination evaluation by antimicrobial-resistant bacteria via livestock waste compost-fertilized field soil

Antimicrobial-resistant bacteria, selected by antimicrobial agent use in livestock, are emerging and their spread to crops from feces via composting represents a public health concern as they are ultimately transmitted to humans. In this study, we investigated Escherichia coli and other ampicillin (AMP)-resistant coliform spread conditions in field soil and dent corn, an agricultural crop, on a livestock-derived compost-applying farm. No AMP-resistant E. coli was detected in any samples of field soil and dent corn. In contrast, AMP-resistant and extended-spectrum β-lactam (ESBL) producing coliforms were consistently present in field soil and dent corn during the entire study period. In particular, extremely high AMP-resistant coliform levels were detected in dent corn stems and roots. AMP-resistant coliform detection in crops is pivotal and raises significant concerns regarding antimicrobial-resistant bacterial spread. Furthermore, AMP-resistant coliform isolate identification defined Enterobacter bugandensis and Enterobacter asburiae as the dominant species among AMP-resistant coliforms, both tested positive for ESBL production. This means that high concentrations of AMP-resistant coliforms are to be present on farms where crops are grown. However, we identified no common species among the AMP-resistant coliforms in the compost, field soil, and dent corn samples. Therefore, the initial hypothesis of the compost being the source of antimicrobial-resistant bacteria was not confirmed. Although their source remains unknown, a certain antimicrobial-resistant bacterial concentration could nonetheless be detected in the field.