Food Scientist Shelly McKee's team wasn't looking for antibiotic resistant bacteria. But when routine tests hinted at resistance, it had to investigate.

The University of Nebraska researchers were studying the prevalence of Salmonella and Campylobacter – two major foodborne illness culprits – when they found bacteria growing where it shouldn't survive. When more tests revealed antibiotic resistance, the team expanded its research to assess resistance to eight antibiotics in three major classes typically used in human and veterinary medicine.

The Institute of Agriculture and Natural Resources study measured resistance in Salmonella and Campylobacter strains on chicken from two U.S. processing plants. All Salmonella and 95 percent of Campylobacter samples proved resistant to at least one antibiotic. The organisms showed varying degrees of resistance to several antibiotics. Preliminary findings indicate antibiotic use in livestock and poultry isn't the only factor in resistance development, McKee said.

"Everyone has assumed that antibiotic use is the only cause of resistance," she said. "We're saying that this is one factor but our research indicates there are other contributing factors."

About 58 percent of the Campylobacter samples were resistant to ciprofloxacin, or cipro, a powerful human antibiotic, but not to a similar drug used to treat animals.

"Cipro isn't used in veterinary medicine," McKee said. "That begs the question: Who is giving resistance to whom and how is that resistance being turned on? What's causing it?"

Salmonella was a different story. Researchers found widespread resistance to antibiotics used to treat respiratory infections in chickens. "This was directly related to antibiotics used in feed," McKee said.

Poultry producers in the study halted non-emergency antibiotic use in their operations after learning these findings, she said.

"Antibiotic resistance is a concern in both animal and human medicine," McKee said. "We all need to use antibiotics prudently and find ways to keep resistance from developing."

She hopes her research eventually yields specific steps poultry producers and processors can take to prevent resistance development.

This IANR research aims to pinpoint environmental, production and processing factors from farm through processing plant that might encourage resistant organisms to thrive.

Understanding stressors is key because anything that stresses bacteria could foster resistance development, McKee said. Stresses needn't come from antibiotics. Anything that creates hostile conditions that threaten survival can contribute.

"When bacteria are stressed, they turn on survival genes. Some of these genes can trigger antibiotic resistance," she explained. "Or some organisms may be able to pick up DNA from a different kind of bacteria to survive. If that borrowed DNA comes from a resistant bacteria, it can literally be passed from one type of bacteria to another."

Understanding specific stressors has practical as well as scientific application.

"Once we identify all these factors, we can work on ways to help processors manage them," McKee said.

For consumers worried about harmful bacteria in food, McKee said, the solution is simple: handle and cook meat and poultry properly.

"Antibiotic resistant or not, proper cooking kills them," she said.

A USDA grant helped fund this research.

– Vicki Miller

Food Scientist Shelly McKee and graduate assistant Marcos Sanchez examine a petri dish that contains strips of different antibiotics and Salmonella or Campylobacter bacteria samples. Top: A dish right after preparation, top left, and after bacteria grows for several days. The bigger the clear zone around the strip, the more susceptible the bacteria is to that antibiotic. Each strip contains graduated concentrations of antibiotic to help quantify bacterial resistance or susceptibility.