( Health.com ) -- Almost half of the meat and poultry sold at U.S. supermarkets and grocery stores contains a type of bacteria that is potentially harmful to humans, a new study estimates. Researchers tested 136 packages of chicken, turkey, pork, and ground beef purchased at 26 grocery stores in five cities around the country, and found that 47 percent contained Staphylococcus aureus (S. aureus), a common cause of infection in people. What's more, roughly half of the contaminated samples contained strains of the bacteria that were resistant to at least three antibiotics, such as penicillin and tetracycline. Some strains were resistant to a half dozen or more. Although the high contamination rates may sound alarming, the threat these bacteria pose to humans is still unclear. "We know that nearly half of our food supply's meat and poultry are contaminated with S. aureus, and more than half of those are multidrug resistant," says Lance B. Price, Ph.D., the senior author of the study, which was published Friday in the journal Clinical Infectious Diseases. "What we don't know [is] how often these transfer to people. We need more studies to quantify the public health impact." S. aureus, and drug-resistant strains in particular, can cause serious infections and even death in humans. However, simple precautions including cooking meat thoroughly, washing hands after handling meat, and keeping raw meat separate from other foods to prevent cross-contamination are believed to neutralize the risk of infection, according to experts not involved in the research. "Numerous