BOSTON – One in five samples of supermarket ground meat and poultry bought for a study was contaminated with salmonella, and most of the strains were resistant to antibiotics.

The findings – which are generally in line with what the Food and Drug Administration has seen in previous surveys of the food-poisoning bacteria – spurred calls for stronger restrictions on the use of antibiotics in livestock.

Overuse of antibiotics in humans and farm animals contributes to the rise of drug-resistant strains of bacteria.

The study and two others on livestock and antibiotic resistance were published in Thursday’s New England Journal of Medicine.

For decades, farmers have given antibiotics to animals raised for food. The Animal Health Institute, which represents makers of animal drugs, says more than 20 million pounds of antibiotics are used yearly in animals, mostly to treat or prevent disease.

The Union of Concerned Scientists estimates that as little as 2 million pounds go to sick animals, while the rest is meant largely to shield animals from disease and promote growth. By contrast, humans take an estimated 3 million pounds.

Scientists worry that powerful animal germs can sicken people through their food and transfer antibiotic resistance to humans.

In the salmonella study, researchers at the University of Maryland and the FDA collected 200 samples of ground beef, ground chicken, ground turkey and ground pork from three supermarkets around Washington, D.C.

Forty-one samples, or 20 percent, were contaminated with salmonella, a bacterium blamed for about 1.4 million cases of food poisoning each year in the United States. Of the strains isolated, 84 percent were resistant to at least one antibiotic, and 53 percent to three or more.

Four samples carried an unusually powerful strain known as DT104. “It’s very alarming because this organism is resistant to at least five different antibiotics and has been the cause of outbreaks,” said Jianghong Meng, a University of Maryland microbiologist.

Biologist Margaret Mellon, director of food programs at the Union of Concerned Scientists, blamed heavy use of antibiotics in livestock. “We need to take action now to reduce the unnecessary use in animals,” she said.

In one of the other studies, researchers at the Centers for Disease Control and Prevention found that more than half of 407 supermarket chickens brought from 26 stores in four states – Georgia, Maryland, Minnesota and Oregon – carried the sometimes fatal germ Enterococcus faecium in a form resistant to Synercid, one of the few drugs of last resort against the infection. The drug was approved for humans only two years ago, but a similar one has been given to livestock since the 1970s.

In the study, 1 percent of human stool samples also carried the resistant germ. But the researchers warned of future increases and suggested that use of the livestock drug virginiamycin may need to be limited.

The FDA is considering such a ban, according to Stephen F. Sundlof, director of the agency’s Center for Veterinary Medicine.

Sundlof said the salmonella findings appear to be in line with other FDA samplings for the bacteria in recent years. Those surveys, however, looked at meat in slaughterhouses, before it had undergone processing that can introduce more contamination.

Ron Phillips, a spokesman for the Animal Health Institute, said antibiotics generally help keep animals well and food safe.

“There is a concerted effort among industry and the government to come up with measures that truly address the resistance problem, but I don’t think that outright banning of these products is the best way,” he said.

Dr. Sherwood Gorbach, an infectious-disease specialist at Tufts University, said in an accompanying editorial that especially important human drugs should be banned for any use in farm animals. He said other antibiotics should be used only in sick animals, not to boost growth or protect healthy livestock. The European Union has enacted such a ban.

Critics of drug use in farm animals say better ventilation, cleaning and other handling changes could improve livestock health with less use of antibiotics. They say a ban would raise the cost of meat only slightly.