IMPACT OF GROWTH PROMOTERS ON MICROBIAL COMMUNITY COMPOSITION AND ANTIBIOTIC GENE ABUNDANCE IN POULTRY LITTER

For several decades antibiotics have been added to animal feed as growth promoters. Although banned in Europe due to concerns that this practice is contributing to antibiotic resistance, feeding livestock and poultry with antibiotics is still extensively practiced in Israel and other parts of the world. This practice is believed to enhance animal health and advocates claim it doesn’t promote resistance because added antibiotics are not used in human medicine. Conversely, there is evidence that treatment with growth-promoters co-selects for resistance to antibiotics used for treatment of humans. Furthermore, the use of antibiotics in feed may impact the composition of animal-associated microbiomes and thereby impact animal health. We evaluated microbial community composition and antibiotic resistance in the litter of poultry fed feed amended with four different antibiotic growth promoters (bacitracin, flavomycin, virginiamycin and avilamycin) and three organic acid mixtures, relative to non-amended feed, at the end of a six-week poultry growth period. Collectively, Corynebacteriaceae, Leuconostocaceae, Staphylococcaceae, Lactobacillaceae and Dermabacteraceae were the dominant families detected in the litter samples from all treatments; and no significant treatment-associated impact was observed in microbial community composition. In contrast, the microbiome of most of the litter samples of chickens fed with antibiotic-amended feed was less diverse than the non-amended samples. In tandem, we evaluated the relative abundance of selected antibiotic resistance genes (tetA, ermC, mecA and vanA) and the class 1 integron integrase gene intI1 to determine whether growth-promoting agents impact the ”resistome” of the chicken litter; and found initial indications of co-selection for some of the tested antibiotic treatments. This study provides important data on the ecological and epidemiological impacts of growth promoters in poultry feed.