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NEW ORLEANS -- Antibiotic-resistant bacteria were found in fresh produce and dairy products throughout California grocery stores, researchers found.

Samples from organic and conventional produce were the most resistant to cefotaxime, an antibiotic that treats a variety of bacterial infections. Moreover, numerous samples were positive for bacteria resistant to colistin, a "last resort" antibiotic, reported Bryan Sanchez, of California State University Northridge, and colleagues.

Dairy products were found to have lower levels of these antibiotic-resistant organisms as well, the authors noted.

They presented their results at a at the . The research did not connect the presence of such organisms on food products with actual human infections.

Sanchez and colleagues said that 80% of antibiotics in the U.S. are used by the food industry, and that there is no "kill" step, such as cooking, involved in most ready-to-eat foods such as vegetables and dairy. Antibiotic-resistant bacteria can then be consumed or contaminate kitchen surfaces, they said.

The authors described their study as "preliminary surveillance." They purchased organic and conventional produce and dairy products at grocery stores throughout the San Fernando Valley. They defined antibiotic concentrations as the minimal concentration to inhibit particular strains of both Escherichia coli and Staphylococcus aureus.

Researchers then measured antibiotic resistance against eight types of antibiotics:

• Ciprofloxacin
• Tetracycline
• Erythromycin
• Chloramphenicol
• Gentamicin
• Ampicillin
• Cefotaxime
• Colistin

Overall, fresh produce had levels of antibiotic-resistant bacteria that were 10,000 times higher than dairy products. Bacteria found in produce samples were the least resistant to tetracycline and the most resistant to cefotaxime. There were also numerous samples that were positive for colistin-resistant bacteria.

"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," said Sanchez. "However, we also identified a number of naturally sensitive bacteria that were found to be resistant to the different antibiotics tested."

However, dairy products had a lower average level of antibiotic-resistant bacteria compared to both conventional and organic produce. These products also had the highest resistance to cefotaxime, but were completely negative for resistance to chloramphenicol. A single yogurt sample was found to have colistin-resistant bacteria, as well.

The authors cited data that 2 million people become infected with antibiotic-resistant bacteria every year, which results in over $35 billion in additional healthcare costs.

But at an ASM expert media session, David Relman, MD, of Stanford University and vice-chair, ASM Microbe 2017, sounded a note of caution about such studies.

"Everything we put in our mouths has some degree of contamination," he said, which usually doesn't amount to problems because "the amount of organisms are low and the types of organisms are harmless."

"None of these are necessarily terrible pathogens and none are the one organism that you have to worry about – they're all capable of carrying these genes just as a part of doing business in the world, and they can share them," Relman said.

He added that humans pick up resistance from food and environmental sources, but not just from one resistant organism that's more important than the others.

"It's different organisms carrying these different genes and trading them like baseball cards," Relman noted.

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