Effects of Organic Soil Amendments on Antimicrobial-Resistant Bacteria in Urban Agriculture Environments

Biological soil amendments of animal origin (BSAAOs) are widely used in urban agriculture to improve soil quality. Although BSAAO use is regulated due to risks for introducing foodborne pathogens, effects on antimicrobial-resistant (AMR) bacteria are not well established. Here, we aimed to explore the impacts of BSAAOs on levels of resident AMR bacteria in leafy vegetable production environments (i.e., kale, lettuce, chard, cabbage) across urban farms and community gardens in the greater Washington D.C. area (n = 7 sites). Leaf tissue (LT), root zone soil (RZS; amended soil in crop beds), and bulk soil (BS; site perimeter) were collected and analyzed for concentrations of total heterotrophic bacteria (THB), ampicillin (Amp) or tetracycline (Tet) resistant THB, and coliforms. As expected, amended plots harbored significantly higher concentrations of THB than bulk soil (P < 0.001). The increases in total bacteria associated with reduced fractions of Tet-resistant bacteria (P = 0.008), as well as case-specific trends for reduced fractions of Amp-resistant bacteria and coliforms. Site-to-site variation in concentrations of AMR bacteria in soil and vegetable samples reflected differences in land history and crop management, while within-site variation was associated with specific amendment sources, as well as vegetable type and cultivar. Representative isolates of the AMR bacteria and coliforms were further screened for multidrug resistance (MDR) phenotypes, and a high frequency was observed for the former. In amended soils, as the soil pH (range 6.56-7.80) positively correlated with the fraction of Tet-resistant bacteria (rho = 0.529; P < 0.001), crop management strategies targeting pH may have applications to control related risks. Overall, our findings demonstrate that soil amendments promote soil bacteria concentrations and have important implications for limiting the spread of AMR bacteria, at least in the urban landscape.