Effect of Standard Post-harvest Interventions on the Survival and Regrowth of Antibiotic-Resistant Bacteria on Fresh Produce
Raw vegetables can sometimes be the source of 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 animals administered antibiotics, and in compost generated from this manure, where there is the potential for their transfer to produce. The purpose of this study was to determine the survival of antibiotic-resistant bacteria on raw, peeled, carrots after washing with commonly used chemical sanitizers. Multi-drug resistant E. coli O157:H7 and Pseudomonas aeruginosa were inoculated into a compost slurry of composted manure from dairy cattle, with and without prior administration of antibiotics, and used to inoculate carrot surfaces prior to the washing studies. This approach provided defined model antibiotic-resistant pathogens present within a background microbial community simulating potential carry over from manure-derived fertilizer. Carrots (n=3, 25g) were air-dried and stored at 4 °C until washing with tap water, XY-12 (sodium hypochlorite, 50 ppm free chlorine) or Tsunami 100 (peroxyacetic acid/hydrogen peroxide, 40 ppm free paracetic acid), according to manufacturer's directions. A second batch of carrots representing each inoculation x wash condition (n=3) were individually packaged for storage at 2 °C for 1,7, and 14 days, or 10 °C for 7 days and enumerated on those day intervals to recover bacteria from the surfaces of washed carrots. The resulting previously washed and stored carrots were subject to serial dilution and plated onto corresponding agar to enumerate total aerobic bacteria (R2A), aerobic bacteria tolerant or resistant to antibiotics (antibiotic-supplemented R2A), E. coli (Eosin Methylene Blue), and Pseudomonas spp. (Pseudomonas Isolation Agar). In addition, the tetA gene was quantified from the carrot samples as a measure of the effect of sanitizers and storage on an antibiotic resistance gene known to be carried by the inoculated bacteria.Inclusion of sanitizer in the wash water significantly reduced the absolute numbers of inoculated bacteria (E.coli and Pseudomonas) as well as populations of bacteria capable of growth on the R2A media containing cefotaxime (10μg/mL), sulfamethoxazole (100μg/mL), or tetracycline (3μg/mL). Comparable reductions in the inoculated P. aeruginosa resistant to tetracycline (PIA T, 4μg/mL), bacteria resistant to cefotaxime (10μg/mL) and tetracycline (3μg/mL) occurred after washing with XY-12 or Tsunami 100. The sanitizer effectiveness may be bacterial dependent, as evident by larger absolute reductions of the inoculated E. coli (EMB) and bacteria grown on sulfamethoxazole (100μg/mL)-amended plates after washing with Tsunami 100 compared to washing with tap water or XY-12. Re-growth of both the inoculated and native compost-associated bacteria was inhibited by storage at 2 °C, as there were no significant differences in the log CFU/g values on the various media (total aerobic bacteria, bacteria on antibiotic-amended plates, E. coli inoculum, P. aeruginosa inoculum) during the 14-day storage period. However, temperature abuse at 10 °C resulted in significant re-growth of native Pseudomonas, compared to storage at 2 °C. A sanitizer-associated interaction between re-growth and temperature was also observed for bacteria resistant to clindamycin (25μg/mL) and cefotaxime (10μg/mL), with substantial re-growth occurring only on carrots washed with Tsunami 100. There was no significant re-growth of the inoculated E. coli O157:H7 at either temperature. Results indicate that some bacterial populations are reduced by post-harvest washes and that temperature abuse of fresh produce may result in increases in antibiotic-resistant bacterial populations.