Purpose: To evaluate the presence, persistence, and transfer of AMR Enterobacteriaceae bacteria from organic dairy manure to organically grown spinach plants.
Methods: The presence of 3rd generation cephalosporin-resistant Enterobacteriaceae was assessed from organic dairy cow manure, soil, spinach plants, and soil-manure-spinach mesocosms. Enterobacteriaceae, Escherichia coli, and Shiga toxin producing E. coli (STEC) were extracted and plated on selective and differential media supplemented with four different ceftiofur concentrations (0 µg/ml (control), 4 µg/ml (susceptible), 8 µg/ml (intermediate), and 16 µg/ml (resistant)). Bacterial recovery was performed at 0, 7, 14, and 30 days from six independent replicates each of 150 g. A subset of 65 Enterobacteriaceae, E. coli, and STEC bacteria were further screened for multidrug resistance (MDR) to a panel of 15 antimicrobials using the broth microdilution method.
Results: Significant differences (P<0.05) in the population of ceftiofur susceptible and resistant Enterobacteriaceae were observed in organic dairy cow manure (4.2 and 1.2 log CFU/g, respectively). The entire STEC population showed ceftiofur resistance, while no differences were observed between antibiotic concentrations. In soil-manure-spinach mesocosms, no significant differences (P<0.05) in the Enterobacteriaceae population were observed over 30 days, while for Eschericha coli no detectable resistant populations were determined after 14 days of initiating each experiment. Further screening of MDR from ceftiofur-resistant and nonresistant Enterobacteriaceae and STEC, indicated a high incidence of antibiotic resistance to sulfisoxazole (72%), streptomycin (55%), and tetracycline (52%).
Significance: Under the selected conditions (15-year-old organic system) there was wide spread distribution of MDR Enterobacteriaceae, which could pose a threat to human health.