Antibiotic-Resistant Bacteria Have Long Resided on Ready-to-Eat Foods

Investigators from California State University – Northridge presented an abstract at the American Society of Microbiology (ASM) Microbe 2017 meeting held in New Orleans, Louisiana, showing that human exposure to antibiotic-resistant bacteria (ARB) through food sources is substantially higher than previously thought.¹

Ready-to-eat foods are a potential source of infection with bacteria that may contribute to the 2 million infections documented annually in the United States.² Fresh produce that is consumed raw may carry bacteria that are not killed off by high cooking temperatures and can contaminate food preparation surfaces. Likewise, dairy products can rapidly spoil and foster bacteria.

The Cal State team therefore designed a study to quantify ARB concentrations on readily available foods and identify potential risks of infection. They purchased both organic and conventionally grown fresh produce and dairy products from multiple local grocery stores in the San Fernando Valley region of California. “We knew there was going to be antibiotic-resistant bacteria in various foods, particularly produce, but we didn’t know [at] what level they were going to be there,” explained Kerry Cooper, PhD, senior researcher on the team, in an interview with Infectious Disease Advisor.

“This is step 1 of a 3-step process to figure out what role this has in human health,” he continued. “It sets the playing field…how much is there, what is there, and what’s going to cause a problem?” Bacteria resistant to 8 commonly used antibiotics (ciprofloxacin, tetracycline, erythromycin, chloramphenicol, gentamicin, ampicillin, cefotaxime, and colistin) were more prevalent than expected, and concentrations of ARB in produce were much higher — 10,000 times higher — than those measured in dairy products.

The ARB load in dairy products in general was relatively low (18.3 colony-forming units [CFU]/g) when compared with both organic and conventionally grown produce (1.97 x 10⁵ CFU/g and 1.48 x 10⁶ CFU/g, respectively). The most common resistance was to the broad-spectrum antibiotic, cefotaxime. One sample of yogurt was found to have the highest ARB amount among dairy products, with a concentration of 6.6 x 10⁶ CFU/g of colistin-resistant bacteria.

Produce tested highest for resistance to cefotaxime (2.63 x 10⁶ CFU/g and 9.58 x 10⁶ CFU/g in organic and nonorganic produce, respectively) and lowest for resistance to tetracycline (4.9 x 10² CFU/g and 1.27 x 10⁴ CFU/g in organic and nonorganic produce, respectively). “The produce industry doesn’t use a lot of antibiotics,” Dr Cooper observed. “They use tetracycline on apples, and that was at the lowest end of what we found.”

of the majority antibiotic use in the United States occurs in agriculture, most often in food animals, setting the stage for widespread antibiotic resistance through soil seepage. Study co-investigator Bryan Sanchez explained in a press release that, “Since antibiotics are not commonly used by the produce industry, the fruits and vegetables are most likely contaminated with soil, a natural source of antibiotic-resistant bacteria.”³

The high quantity of ARB in produce and dairy contribute to human exposure but do not necessarily cause infection. “Many of the bacteria that were isolated from the foods are natural residents on plants/produce, are naturally resistant to these antibiotics, and are nonpathogenic to humans,” Dr Cooper said, adding that, “It’s been going on for millions of years in the soil — this is natural and not of immediate concern.” 

References

1. Sanchez B, Bayangos T, Rocha K, et al. Levels of antibiotic resistant bacteria in ready-to-eat foods. Presented at: ASM Microbe 2017; June 1-5, 2017; New Orleans, Louisiana; Poster 67.
2. Antibiotic resistance threats in the United States, 2013. Centers for Disease Control and Prevention (CDC) website. www.cdc.gov/drugresistance/threat-report-2013/. Updated April 10, 2017. Accessed July 7, 2017.
3. Antibiotic-resistant bacteria in ready-to-eat foods [press release]. Washington DC: American Society for Microbiology. June 4, 2017. https://www.asm.org/index.php/newsroom/item/6505-antibiotic-resistant-bacteria-in-ready-to-eat-foods. Accessed July 6, 2017.