A sampling of grocery store meat in five US cities has shown a type of drug-resistant bacteria is contained in about one quarter of beef, chicken, pork and turkey for sale, according to a study. Staphylococcus aureus, a bacteria that can cause skin infections, pneumonia, sepsis or endocarditis in people with weak hearts, was found in 47 percent of samples, said the study in the journal Clinical Infectious Diseases, released on Friday. More than half -- 52 percent -- of the infected samples contained a tough strain of S. aureus that was resistant to at least three types of antibiotics. Most of the time, the bacteria would be killed off during cooking, but risks of contamination can come from handling raw meat in the kitchen and touching other utensils, or from eating meat that is not fully cooked. \"For the first time, we know how much of our meat and poultry is contaminated with antibiotic-resistant Staph, and it is substantial,\" said Lance Price of the Translational Genomics Research Institute in Phoenix, Arizona, and senior author of the study. \"The fact that drug-resistant S. aureus was so prevalent, and likely came from the food animals themselves, is troubling, and demands attention to how antibiotics are used in food-animal production today.\" S. aureus is not among the four bacteria routinely tested in meat by the US government: Salmonella, Campylobacter, E. coli, and Enterococcus. More than two million people in the United States are infected with these bacteria annually, and hundreds die. The young and the elderly, and people with weakened immune systems are most at risk. The 136 samples that were tested included 80 brands of meat and were taken from 26 retail grocery stores in five cities: Los Angeles; Chicago; Fort Lauderdale, Florida; Flagstaff, Arizona; and the US capital, Washington. The report said the bacteria was found inside the meat and therefore was not likely to have come from handling. Instead the likely culprit was \"densely-stocked industrial farms, where food animals are steadily fed low doses of antibiotics... ideal breeding grounds for drug-resistant bacteria that move from animals to humans,\" the study said. \"Antibiotics are the most important drugs that we have to treat Staph infections; but when Staph are resistant to three, four, five or even nine different antibiotics -- like we saw in this study -- that leaves physicians few options,\" Price said. The study did not assess the risk to the population posed by the resistant staph strain. \"Now we need to determine what this means in terms of risk to the consumer,\" said co-author Paul Keim, director of the Center for Microbial Genetics and Genomics at Northern Arizona University.