The food chain is a critical avenue for the introduction of antibiotic resistance through daily food consumption, said Dr. Hua H. Wang, professor, food science and microbiology, Ohio State University. Cooking our food before we eat it reduces the risk of transmitting antibiotic-resistant pathogens into our bodies. Cooking is such an effective biocide that Wang said, “In terms of transmitting antibiotic resistance due to foodborne pathogens to humans, that risk is relatively limited.” Speaking at the International Association for Food Protection annual meeting, she said that, in spite of cooking, food can still be a major source of antibiotic resistance genes.

Antibiotic resistance and cheese

Wang cited the example of how the consumption of cheese could increase the prevalence of antibiotic-resistant bacteria in the guts of humans to demonstrate how our food can impact the antibiotic resistance status of our gut flora. Wang found that the transmission of a drug resistance-encoding plasmid is facilitated up to 10,000 times by the presence of Lactococcus lactis, which is used as a starter culture in cheese making. A few years ago, a survey of retail ready-to-eat deli and restaurant foods found that many of these foods carry antibiotic resistant bacteria. She said that up to billions of copies of the antibiotic resistance gene were found per gram of cheese and that another study found high levels in ready-to-eat shrimp.

Researchers and dairy producers worked together to identify critical control points for antibiotic resistance in cheese making. Some problematic strains of starter cultures were identified and eliminated. The overall control measures have been successful; she said that the antibiotic resistance gene pool in microorganisms found on cheese today have basically been brought down to the level of detection.

Gut impact

Wang said that her research has shown that there is rapid development in antibiotic resistance in infant fecal biota. These infants have not been fed conventional food and have not been treated with antibiotics. “Non-pathogenic bacteria might not just be a key reservoir for antibiotic resistance gene development, dissemination and persistence, but potential facilitators of horizontal gene transmission.”

She explained that probiotics, starter cultures, spoilage and other commensal bacteria make up most of the microorganisms in our food and in our gastrointestinal tracts. This is a large volume and broad spectrum of organisms. 

The gastrointestinal system plays a key role in the amplified circulation of antibiotic-resistant bacteria in the ecosystem. Antibiotic-resistant bacteria can persist in the food system, the environment and in the host without the presence of antibiotics, Wang reported.

Oral versus injection

Research with mice has shown that there is a link between means of treatment and development of antibiotic resistant bacteria. There are more antibiotic-resistant bacteria found in the gut if the antibiotics were given to the mice orally than by injection.

Wang said that the emergence of the three main antibiotic-resistant bacteria that are human health concerns roughly coincides with the switch in the primary means of administering antibiotics to human patients from injections to orally. She said that this also might explain why antibiotic-resistant bacteria are more prevalent in poultry and swine than in cattle since cattle tend to be treated by injection and swine and poultry tend to receive antibiotics orally.

“Antibiotic resistance is still a puzzle. Antibiotic use is still a risk factor, but there are many other factors playing a role, and at this time the list of risk factors is still growing,” Wang said. “Finally, it is important to recognize that prudent use of antibiotics does not mean a ban, but also what, when and how to use the antibiotics.”

Animal agriculture

Wang said that antibiotic-resistant strains of probiotic cultures used for humans have been cleaned up, but she suggested that the same might need to be done for animal probiotics. The work her lab has done with mice and the example of antibiotic-resistant strains of bacteria becoming more prevalent as human medicine transitioned from injected to orally given antibiotics raises treatment questions for poultry, swine and cattle. Are there methods that can be developed that allow for efficient non-oral mass treatment?

Human medicine, according to Wang, might need to rethink the strategy at times, not using prophylactic antibiotic treatment. She said that planktonic bacteria are easier to kill than are bacteria that have formed a biofilm. For postoperative patients, she suggested that it might be better to treat prophylactically rather than wait until an infection has developed before treating. There might be a lesson in here for animal agriculture as well.