Purpose: To determine the survival of antibiotic-resistant bacteria on raw, peeled, carrots after washing with commonly used sanitizers.
Methods: Multi-drug resistant E. coli O157:H7 and Pseudomonas aeruginosa were spiked into a compost slurry in order to inoculate carrot surfaces with a background microbial flora containing defined antibiotic-resistant bacteria relevant to produce. Carrots (n=3, 25 g) were air-dried and stored at 4°C until washing with sodium hypochlorite (50 ppm free chlorine) or peroxyacetic acid (50 ppm free peracetic acid), according to manufacturer’s directions. Antibiotic-resistant bacteria were enumerated by serial dilution and plating onto antibiotic-supplemented R2A, R2A, Eosin Methylene Blue or Pseudomonas Isolation Agar.
Results: Reduction of E. coli O157:H7 and P. aeruginosa on carrots when washed with sodium hypochlorite were 3.27 log CFU/g and 1.65 log CFU/g, respectively, a significant difference in effectiveness between microbes (P<0.05). Washing with peroxyacetic acid reduced the numbers of E. coli O157:H7 and P. aeruginosa on carrots by 2.67 log CFU/g and 2.53 log CFU/g, respectively. Heterotrophic bacteria resistant to cefotaxime (10 µg/ml) reduced from 5.96 log CFU/g on cefotaxime-supplemented R2A plates (10 µg/ml) to 2.5 log CFU/g after washing with either sodium hypochlorite or peroxyacetic acid. A 4-log reduction in heterotrophic bacteria resistant to clindamycin (25 µg/ml) also occurred after sanitizer washes.
Significance: Results suggest that bacterial populations are altered by post-harvest washes and may select for antibiotic-resistant bacteria, indicating that vegetables may be an under-recognized source of antibiotic-resistant bacteria in the human diet.