The day-care centers that millions of parents entrust their children to harbor a hidden danger. According to research presented this weekend at a conference organized by the American Society for Microbiology, such facilities foster the development and transmission of antibiotic-resistant bacteria.

In a two-year study of eight day-care centers, 240 toddlers and 46 of their siblings, Israeli scientists sampled Streptococcus pneumoniae, or strep, bacteria from the throats and noses of day-care kids and found that antibiotic treatment of the bacteria spurred the development of resistance traits. Furthermore, samples from the childrens siblings revealed the same resistant strains, demonstrating that the antiobiotic-resistant bacteria can spread out from a day-care center and into the community.

Day-care facilities are associated with a high rate of respiratory infections. As a result, antibiotic use rate is high. That combined with toddlers underdeveloped hygiene standards, team member Ron Dagan says, provides bacteria with the ideal conditions for developing such resistance.