Manure from antibiotic-free cows can cause antibiotic-resistant bacteria in soil to bloom, Yale University researchers report (Proc. Natl. Acad. Sci. USA 2014, DOI: 10.1073/pnas.1409836111). Organic farmers commonly substitute manure for inorganic nitrogen and phosphorus fertilizers. Studies of soils fertilized with manure from pigs treated with sulfonamide antibiotics have shown transient increases in antibiotic-resistance genes of soil bacteria. Microbiologists are concerned that these genes could transfer through contaminated crops or groundwater to human pathogens. In the new work, a team led by Yale’s Jo Handelsman found that manure from cows not treated with antibiotics caused population shifts in soil microbes so that bacteria resistant to β-lactam compounds such as cephalosporins became more abundant. Soil treated with an inorganic fertilizer did not show the same population shift. More research is needed to pinpoint the manure component responsible for promoting the antibiotic resistance, such as a nutrient, metal, or toxin to which nonresistant bacteria are vulnerable.