Use of antibiotics in humans and animals has created tremendous selective pressure to produce antibiotic-resistant pathogens. Antibiotic-resistant Salmonella enterica serovar Typhimurium, Escherichia coli (including E. coli O157:H7), Listeria monocytogenes, and Campylobacter jejuni have emerged en force. More than half of the antibiotics used in the United States are used in production agriculture, mainly to promote growth and disease resistance. Beef cattle, poultry, and swine consume the bulk of these antibiotics.
Scientists worry that sharing human antibiotics with livestock will result in development of resistant strains in livestock and render the drugs ineffective in humans. National and international organizations are now calling for banning the use of antibiotics as growth promoters in farm animals. These concerns are supported through research and data from the U.S. National Antibiotic Resistance Monitoring System.
The incidence of fluoroquinoline-resistant human Campylobacter isolates increased from 13% in 1997 to 18% in 1999. The increase coincided with the approval of fluoroquinolones for use in poultry. Approximately 60–90% of chickens in supermarkets carry Campylobacter species, the most common cause of foodborne illness in the U.S.
Salmonella is ubiquitous in the environment and a leading cause of foodborne illness. Multi-drug-resistant Salmonella have increased worldwide. In a study of retail ground meats, 20% were positive for Salmonella. Nearly 84% of isolates were resistant to at least one antibiotic and more than 50% were resistant to three antibiotics. Of perhaps greater concern, 16% of isolates were resistant to ceftriaxone, the antibiotic of choice for treating salmonellosis in children. There already has been a reported case of ceftriaxone-resistant Salmonella infection in a child.
The Food and Drug Administration has initiated a ban on two fluoroquinolones, sarafloxacin and enrofloxacin, used in poultry for treating respiratory infections. Unabated, use of fluoroquinolones increases the likelihood that fluoroquinolone-resistant Campylobacter will infect humans. Denmark banned the use of avoparcin, used for growth promotion in farm animals, in 1995. This step decreased the prevalence of resistant Enterococcus faecium in swine from 20% in 1995 to 6% by 2000. Similar action in the rest of the European Union has similar results. Changes in animal husbandry practices, improvements in sanitation, and development of alternative control strategies, including the use of probiotics, are essential for combating foodborne pathogens.
In a proactive action, FDA’s Center for Veterinary Medicine in October 2003 released a new guidance document for industry, “Evaluating the Safety of Antibiotic New Animal Drugs with Regard to their Microbiological Effects on Bacteria of Human Health Concern” (www.fda.gov/cvm/guidance/fguide152.doc). The risk-assessment process outlined in the guidance document aids the drug sponsor in determining that an antibiotic used to treat food-producing animals will not create a risk of antibiotic-resistant bacteria likely to lead to human health problems. If risks were considered significant, FDA could prevent the use of the drug in food animals.
Antibiotic usage in agriculture may not be the only cause of the increase in antibiotic-resistant pathogens. The food industry uses a range of antimicrobials, from food preservatives to sanitizers. Exposure to those compounds may confer resistance to antibiotics. Exposure of E. coli O157:H7, Salmonella, and L. monocytogenes to sanitizers, including chlorine and quaternary ammonium compounds, confers resistance to certain antibiotics. The overuse of antimicrobial agents in soaps, home cleaning compounds, and cutting boards contributes to the selection of antibiotic-resistant bacteria.
Antibiotic-resistant bacteria that find their way from animals into food may serve to spread resistance between different species and genera. The genes that make a bacterium resistant to one or more antibiotics may exit a cell and be taken in by a non-antibiotic-resistant cell, thereby conferring resistance. This type of transfer can occur between Gram-positive and Gram-negative bacteria. Potentially, normal flora and even starter cultures could acquire antibiotic resistance.
Regardless of why microbes develop resistance to antibiotics, resistant bacteria place human and animal health at risk, because the drugs we depend on to treat infection become ineffective. The measures taken nationally and internationally to reduce or in some instances ban the use of antibiotics in production agriculture will likely have a positive effect with respect to reducing the prevalence of antibiotic-resistant foodborne pathogens. Others factors such as the increase in demand for organic food may also have an impact, since the use of antibiotics as growth promoters is expressly prohibited for organic farmers.
by Karl R. Matthews is Associate Professor, Food Microbiology, Dept. of Food Science, Rutgers University, New Brunswick, NJ 08901.