P2-117 Effect of Sanitizers on the Survival of Antibiotic-resistant Bacteria Applied to Raw Carrots through Contaminated Compost

Tuesday, August 2, 2016
America's Center - St. Louis
Natalie Pulido, Virginia Tech, Blacksburg, VA
Vaishali Dharmarha, Virginia Tech, Blacksburg, VA
Monica Ponder, Virginia Tech, Blacksburg, VA
Amy Pruden, Virginia Tech, Blacksburg, VA
Renee Boyer, Virginia Tech, Blacksburg, VA
Introduction: It has been established that raw vegetables are associated with outbreaks of human illness, however the potential for fresh vegetables to serve as a vehicle for antibiotic-resistant bacteria is poorly understood. Antibiotics and antibiotic-resistant bacteria have been shown to persist in manure of dosed animals, and in compost made from this manure, where they may be transferred to produce. 

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.