(HealthDay)— 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.