THURSDAY, Oct. 3, 2019 (HealthDay News) -- In a finding that suggests organic is best, a new study indicates that chickens raised without antibiotics may have fewer types of antibiotic-resistant salmonella than animals raised at factory farms.

Salmonella is a common infection among poultry, so some large farms feed their chickens antibiotics to prevent the birds from getting sick, and to help them gain weight faster. But this practice can make salmonella resistant to the antibiotics usually used to treat it, the researchers said.

"Chicken and poultry meat samples that were labeled antibiotic-free or organic were half as likely to contain multidrug-resistant salmonella as conventionally raised poultry," said researcher Nkuchia M'ikanatha. He is lead epidemiologist for antimicrobial resistance response at the Pennsylvania Department of Health, in Harrisburg.

A related study found that almost one-third of meat and poultry were contaminated with antibiotic-resistant forms of the bug, M'ikanatha said.

Genes that make bacteria resistant to recommended antibiotics are a problem because they undermine treatment of severe infections, he added.

"One specific gene found in salmonella isolated from meat and patients makes the bug resistant to the only drug, ceftriaxone, recommended for treating severe salmonellosis in children," M'ikanatha said.

For the study, M'ikanatha and his colleagues tested 2,500 samples of poultry, ground beef and pork chops purchased from 2015 to 2017 at randomly selected markets in Pennsylvania.

The investigators found that up to 30% of the salmonella they found in the meat samples was resistant to three to five classes of antibiotics. And, over the study period, resistance grew to the antibiotics ceftriaxone, amoxicillin and extended-spectrum cephalosporins.

The researchers also found that meat samples were contaminated by the same bacteria recovered from patients, M'ikanatha said. "Those bugs had genes that make salmonella resistant to recommended antibiotics," he said.

The studies were funded by the Pennsylvania Department of Health and the U.S. Food and Drug Administration.

Overuse or misuse of antibiotics in humans and animals can lead to antibiotic resistance, increasing the risk that antibiotics will not be effective when needed, M'ikanatha explained.