Purpose: Quantify sul1 ARGs and bacterial numbers on plates containing sulfamethoxazole associated with lettuce grown in soils amended with manure from cattle with different antibiotic administration histories.
Methods: Beef cattle were administered chlortetracycline, sulfamethazine, and tylosin while dairy cattle were administered cephapirin and pirlimycin. Manure from antibiotic-treated or control cattle were then mixed with soil before transplanting lettuce in the greenhouse (n=3). Fertilizer was applied to controls. At maturity, bacteria were removed from outer leaves and enumerated on R2A media and R2A containing sulfamethoxazole. Real-time PCR was used to quantify total microbes (16S rRNA genes) and sul1 ARGs in DNA from the same samples used for plating.
Results: Despite the difference in soil amendment, the R2A plate counts from lettuce surfaces were not significantly different (P>0.05, Wilcoxon). However, bacterial counts on R2A containing sulfamethoxazole were lower on lettuce grown in manure-amended soil compared to the fertilizer control (P< 0.043, Wilcoxon). There were no significant differences in sul1 numbers in relation to 16S rRNA numbers on lettuce surfaces among the manure types amended, regardless of antibiotic administration.
Significance: None of the manure treatments seemed to affect the sul1 numbers of the harvested lettuce leaves, regardless of the histories of antibiotic administration of the cattle from which the manure originated. This highlights that sulfonamide resistance may not only be selected for by sulfonamide administration, but other factors could be involved in the transfer of ARGs onto vegetables.