Their study is the first to directly compare bacterial contamination of poultry products sold in US supermarkets from food producers who use antibiotics and from those who claim they do not.
The study focused on antibiotic resistance, specifically fluoroquinolone-resistance in Campylobacter, a pathogen responsible for 2.4 million cases of food-borne illness per year in the US, according to the Centers for Disease Control and Prevention, and was published online in the journal Environmental Health Perspectives."Our use of medically important classes of antibiotics in food-animal production creates a significant public health concern," said the study's lead author Lance Price, a doctoral candidate and fellow at the Bloomberg School of Public Health's Center for a Livable Future.
"Companies that use antibiotics foster the development of drug-resistant bacteria which can spread to the human population. Claims have been made that using antibiotics increases food safety by reducing pathogens on the meat.
"Interestingly, in addition to the results regarding fluoroquinolone-resistant Campylobacter, we also found that brands that do not use any antibiotics during production were no more likely to contain Campylobacter than those that do. In fact, the only brand with a significantly lower rate of Campylobacter contamination was actually an antibiotic-free brand."
Price's findings echo a recent American Academy of Microbiology report, The Role of Antibiotics in Agriculture, which suggested that intensive and extensive antibiotic use can lead to the establishment of a pool of antibiotic resistance genes in the environment.
The AAM report claims that both pathogenic bacteria and organisms that do not cause disease may become resistant to antibiotics, and bacteria of human and animal origin can serve as reservoirs for resistance genes.
Price explained that previous epidemiological studies have indicated that fresh poultry products are a major source of Campylobacter infections in humans. Exposure can occur from undercooked products or through cross-contamination during food preparation, when raw poultry is handled in the kitchen.
The danger of infection is heightened when this pathogen is resistant to antibiotics. Not only can the bacteria itself cause illnesses such as diarrhoea in humans, but fluoroquinolones are some of the most important drugs used to treat a variety of infections, including those caused by Campylobacter. The widespread presence of this drug-resistant form of the bacteria makes the antibiotic less effective in human medicine.
In 2000, the Food and Drug Administration proposed to withdraw approval of fluoroquinolone drugs for use in poultry production. That effort has since been stalled by legal objections from Bayer, one of the pharmaceutical companies manufacturing the drug.
In the meantime, two major US poultry producers, Tyson Food and Perdue Farms, separately announced in 2002 that they would immediately stop using fluoroquinolones to treat poultry flocks.
One year after the Tyson and Perdue announcements, Price and his team began a survey of Campylobacter isolates on uncooked chicken products from Tyson and Perdue and from two other producers, Eberly and Bell & Evans, who claim their production methods are completely antibiotic-free. Using both standard isolation methods and new methods modified to enhance detection of fluoroquinolone-resistant Campylobacter, they compared retail products purchased at grocery stores in Baltimore, US.
A high percentage of the products from the two conventional brands were contaminated with fluoroquinolone-resistant Campylobacter (96 per cent from Tyson and 43 per cent from Perdue) while significantly lower proportions of 'antibiotic-free' products were contaminated with fluoroquinolone-resistant Campylobacter (5 per cent from Eberly and 13 per cent from Bell & Evans).
"These results suggest that fluoroquinolone-resistance may persist in the food supply for a substantial period of time even after antibiotic use is discontinued," said Price. "Assuming that what we are observing are lingering resistant strains rather than the result of continued drug use, then one has to conclude that fluoroquinolone use in poultry production presents a long-term threat to people."
Antibiotics are often used in poultry production to promote growth and to prevent them from catching diseases. The global market for biological disease control and productivity enhancement products in poultry and pigs is already valued at over US$3 billion per annum, and is likely to continue to grow.
Lance's study was supported by the Johns Hopkins Center for a Livable Future and by the Heinz Family Foundation.