Jan 11, 2002 (CIDRAP News) – A recent analysis of ground pork in grocery stores in five states showed that 4% of the samples contained enterococci with high-level resistance to gentamicin, an antibiotic used to treat enterococcal infections in humans. In addition, most Enterococcus faecium isolates were resistant to quinupristin-dalfopristin (Synercid), a streptogramin antibiotic used to treat infections caused by vancomycin-resistant E faecium.
The study implies a risk that hospital patients will be exposed to the resistant pathogens and contract hard-to-treat infections, according to senior author Fred Angulo, MD, of the National Antimicrobial Resistance Monitoring System (NARMS) at the Centers for Disease Control and Prevention (CDC) in Atlanta. The study was reported at the 41st Interscience Conference on Antimicrobial Agents and Chemotherapy, held Dec 16-19, 2001, in Chicago.
"Clearly these [resistant pathogens] are very common on meat in grocery stores," Angulo told CIDRAP News. "It's reasonable to expect that someone's going to carry this [resistant bacteria] into a hospital, and to expect that this resistance is going to be transferred to a bacteria [in hospital patients] that needs it because it's under selective pressure. The bacteria is fighting to survive and it's clearly going to find this."
Enterococci—primarily E faecalis and E faecium—are leading causes of nosocomial bloodstream infections, surgical site infections, and urinary tract infections, according to a 1998 report in the CDC journal Emerging Infectious Diseases. Recent NARMS reports say that gentamicin-resistant enterococci are increasingly common causes of nosocomial infections in the United States.
The pork study was conducted by a team led by Jennifer McClellan, MPH, a NARMS surveillance epidemiologist in Atlanta. The group arranged for laboratories in Georgia, Maryland, Michigan, Minnesota, and Oregon to buy and test 10 packages of ground pork a month for 12 months. Enterococci isolates were sent to the CDC for species identification and antimicrobial susceptibility testing.
Ninety-nine percent of the pork packages yielded enterococci. Of 54 isolates that were identified by species, 42 (78%) were E faecalis, 8 (15%) were E faecium, and 2 (4%) were E gallinarum. The team tested 436 (74%) of the enterococci isolates for antimicrobial susceptibility; 18 (4%) of these had high-level gentamicin resistance, defined as a minimum inhibitory concentration (MIC) of 500 mcg/mL or higher. Samples with high-level gentamicin resistance came from all five states.
Seven of the 8 E faecium isolates (87%) had resistance to quinupristin/dalfopristin, defined as an MIC of at least 4 mcg/mL. All 7 of these isolates came from Michigan.
The authors' report notes that gentamicin is injected into pigs to treat infections, and virginiamycin, an analog of quinupristin/dalfopristin, is commonly used in pig feed to promote growth. The report adds that there is cross-resistance between virginiamycin and quinupristin-dalfopristin.
Angulo said the prevalence of resistant enterococci seen in this study is lower than that found in studies of enterococci in retail chicken. However, the resistance seen here is "remarkably" higher than in studies of pork in Denmark, where antibiotic use in pigs is much lower than in the United States, he said.
Angula and McClellan said their study was a pilot for a broader, ongoing study of antimicrobial resistance in retail meat and poultry products. The study, launched this month, involves sampling of pork, chicken, ground beef, and turkey in Minnesota, Georgia, Connecticut, Maryland, and Tennessee. The project is being conducted by the CDC's Foodborne Diseases Active Surveillance Network (FoodNet) in cooperation with the Food and Drug Administration. McClellan said investigators will test for Salmonella and Campylobacter species in all five states and for Escherichia coli and enterococci in Georgia, Maryland, and Tennessee.
McClellan J, Joyce K, Rossiter S, et al. High-level gentamicin-resistant enterococci and quinupristin/dalfopristin-resistant E. faecium from ground pork purchased from grocery stores. Presented at the 41st Interscience Conference on Antimicrobial Agents and Chemotherapy, Chicago, Dec 16-19, 2001