Nearly half the meat and poultry samples purchased in five U.S. cities were found to be contaminated with the bacterium Staphylococcus aureus. And over half of these bacteria were resistant to three or more classes of antibiotics.

Most food animals are raised on densely stocked industrial farms and given low doses of antibiotics because they make them grow faster. These are ideal breeding conditions for drug resistant bacteria.

The public health implications of these findings are unclear. Although the bacteria should all be killed by cooking, cross-contamination of other uncooked food can occur, for instance when a utensil or plate used for raw meat comes in contact with other food.

The emergence of antibiotic resistant bacteria like MRSA has created enormous problems in treating the infections that these bacteria cause. Though the staph bacteria in this case is different from MRSA, having another reservoir of antibiotic resistant bacteria in the nation's meat supply can't possibly help the situation.

The U.S. government routinely surveys meat and poultry for four different types of drug resistant bacteria, but S. aureus is not one of them. This study is the first national assessment of the prevalence of S. aureus in the food supply.

DNA testing suggests that the bacteria did not come from contamination of the meat. Rather, they came from the food animals themselves. Most food animals are raised on densely stocked industrial farms and given low doses of antibiotics because they make them grow faster. These are ideal breeding conditions for drug resistant bacteria.

Over half of the bacteria isolated from chicken were resistant to the fluoroquinolone antibiotic ciprofloxacin (Cipro); virtually none of the bacteria isolated from other meats were. The researchers suspect that this resistance stems from fluoroquinolones routinely being fed to broilers from 1995-2005. The FDA banned fluoroquinolone use in poultry effective September 2005.

The study was performed by the Translational Genomics Research Institute. Researchers collected 136 samples of beef, chicken, pork and turkey from 26 supermarkets in five U.S. cities: Chicago, Flagstaff, Fort Lauderdale, Los Angeles and Washington, D.C. The samples were of 80 different commercial brands.

Of the 136 samples, 64 (47%) were contaminated with S. aureus. Some samples contained multiple strains of the bacteria. In all, the researchers isolated 79 unique strains of S. aureus and tested them all for susceptibility to 17 different antibiotics. Fully 41 of these strains (52%) were resistant to three or more classes of antibiotic.

Though they are uncertain of the consequences, the researchers find the presence of antibiotic resistant S. aureus in roughly one-quarter of all meat and poultry samples disturbing. They say that this finding demands focusing more attention on how antibiotics are currently used in food animal production.

An article on the study was published online by Clinical Infectious Diseases on April 15, 2011. It will also appear in a future print edition of the journal.