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Resistance genes in our food supply

May 23, 2007

Could the food we eat be contributing to the continuing rise of antibiotic-resistant infections? Harmless and even beneficial bacteria that exist in our food supply may also be carrying genes that code for antibiotic resistance. Once in our bodies, could they transmit the resistance genes to disease-causing bacteria?

"The data indicate that food could be an important avenue for antibiotic-resistant bacterial evolution and dissemination. The role of commensals, especially food-borne microbes, in transmitting resistance genes are becoming a concern to the scientific community," says Hua Wang of the Ohio State University, presenting May 23, 2007 at the 107th General Meeting of the American Society for Microbiology (ASM) in Toronto.

The culprit is a process known as horizontal gene transfer, in which bacteria in close proximity to each other can share genetic information, including genes that code for antibiotic resistance. Horizontal gene transfer between disease-causing bacteria in the hospital setting has already been recognized as an important avenue for the exchange of antibiotic-resistance genes among pathogens.

Research has also already demonstrated that pathogenic bacteria have the ability to engage in horizontal gene transfer with various commensal bacteria and even beneficial bacteria, including those from the food chain. What concerns scientists is that the size and diversity of the gene pool represented by commensal bacteria increases the likelihood of gene transfer and some commensals possess high frequency gene transfer mechanisms.

"We have demonstrated not only that organisms carrying such intrinsic mechanisms have the potential to become an important reservoir for antibiotic resistance genes but, more importantly, that these intermediate organisms can disseminate antibiotic resistance genes in subsequent events much more effectively than the parental donor strain," says Hua.

"Once we no longer limit ourselves to foodborne pathogens and look at commensal bacteria, we will find that the magnitude of antibiotic-resistant bacterial contamination in the food chain is tremendous," says Hua.

In a study published last year, she and her colleagues tested a variety of ready-to-eat food samples including seafood, meats, dairy, deli items and fresh produce purchased from several grocery chain stores. With the exception of processed cheese and yogurt, antibiotic-resistance gene-carrying bacteria were found in many food samples examined.,

"Despite the fact that this study only screened for a limited number of resistance markers, it illustrated the prevalence of antibiotic-resistant commensals and antibiotic-resistance genes in retail foods," says Hua. "While further research is needed to establish the direct correlation between the antibiotic-resistant microbes from foods and the antibiotic-resistant population in host ecosystems, it is evident that a constant supply of antibiotic-resistant bacteria, partnered with occasional colonization and horizontal gene transfer, are at least partially responsible for the increased antibiotic resistance profiles seen in humans."

Antibiotic resistant infections are an increasing public health problem, says Marilyn Roberts of the University of Washington. Depending on the disease and the patient, an antibiotic-resistant infection could triple a hospital stay. A methicillin-resistant Staphylococcus aureus infection in a hospital patient can cost thousands of dollars more to treat. In some cases, such as the new extensively resistant tuberculosis, antibiotics are no longer effective, forcing doctors to take extreme measures like removing an infected lung.

The problem is not just confined to the food supply. Recent studies have shown antibiotic resistance genes in bacteria in the digestive tract of young infants. Since these children were still breast- or formula-feeding and had not eaten solid food yet, they must have acquired these genes somewhere other than the food supply. This suggests that resistance genes from the environment might have played an important role, says Hua.

"Antibiotics and the contamination of the environment is a medical problem, an agricultural problem and a human problem. Everybody plays a role in it. They also have a stake in it," says Roberts.

But there are things that can be done to minimize resistance genes in our food. Hua is currently working on characterizing the optimum conditions and processing parameters to minimize the emergence of these genes in fermented products. In time, and with a little help, she hopes to expand this research to other food industries as well.

"Given the proper investment of money, effort and time we can identify the steps that need to be taken at the processing level to minimize the emergence of antibiotic resistance genes in our food supply," says Hua.

American Society for Microbiology