Investigating Antibiotic Resistance Levels of Salmonella Internalized in Lettuce Leaves

Contamination of food crops by the human pathogen Salmonella is a food safety threat worldwide. Though using treated wastewater for irrigation is a sustainable practice, it may introduce trace levels of Salmonella that may contaminate food crops. Salmonella could develop resistance to antibiotics present in wastewater. The overall goal of the project is to increase the understanding of the public health risk associated with the use of treated wastewater to irrigate food crops. The objective of this particular study is to determine the antibiotic resistance level of Salmonella internalized in lettuce leaves. In this experiment, thirty-six plants of the lettuce (Lactuca sativa) cultivar Green Salad Bowl were grown hydroponically in Hoagland growth medium and harvested at 21, 35, and 48 days. Five days before each harvest, solutions containing the antibiotic oxytetracycline (OTC) and the bacteria Salmonella enterica serovar Infantis were added to the hydroponic growth medium. At each harvest time, bacteria extracted from the lettuce leaves were quantified on XLD agar plates amended with 1, 2, 4, 8, 16, or 32 mg/L OTC. After an 18-20 hour incubation period, colonies that turned black were enumerated. By comparing the number of colonies grown on different plates, the antimicrobial resistance level of the internalized Salmonella was determined. Results show that the A n t i b i o t i c R e s i s t A n c e o f S a l m o n e l l a i n L e t t u c e L e A v e s 2 antibiotic resistance levels of Salmonella grown in LB media not receiving OTC, Salmonella internalized lettuce not receiving OTC, and Salmonella internalized in lettuce receiving OTC were all at 32 mg/L OTC. In addition, no significant differences were observed among different lettuce growth stages.