CCL11 and allergic disease: It is therefore unlikely that the urban soil cohort is more at risk for allergy, and the increase in eotaxin expression is likely a natural aging phenomenon.38 Furthermore, unlike C. difficile which is common in hospital-delivered infants,39 the Clostridiaceae that colonized the urban soil cohort were not likely pathogenic, as there were no differences in major cytokine or chemokine expression between experimental groups and there was an increase in propionic acid with age, which is beneficial at appropriate levels.