Environmental Pathways of Exposure to Antimicrobial-Resistant Bacteria: The Role of Poultry Litter Disposal Practices
Jay P. Graham, PhD, MBA                                                 Powerpoint Slides

This research focuses on characterizing potential environmental pathways of exposure to antimicrobial-resistant bacteria and resistance genes associated with waste from industrial poultry production. It provides evidence on the need for improved food animal waste management as it relates to public health.

More than 10 million metric tons of untreated poultry litter (i.e., excreta, feathers, spilled feed, bedding material, soil and dead birds) is applied to land every year in relatively small geographic regions of the United States and production trends indicate that this practice will continue to grow. To date, most research has focused on potential ecological impacts of land application of animal waste, associated with nutrient overloading of aquatic systems.  In addition to nutrients, however, this waste also contains animal feed constituents, such as antimicrobials, and antimicrobial-resistant strains of pathogenic and non-pathogenic bacteria.

The development of antimicrobial-resistant bacteria has been strongly associated with the use of antimicrobials for promoting growth, improving feed conversion efficiency and preventing disease in poultry flocks. Further, it is well documented that the use of antimicrobials in poultry production is associated with increased risks of food-borne infections, including infections by resistant pathogens.  There is a paucity of information, however, on the spread of antimicrobial-resistant bacteria, as well as genetic determinants conferring resistance in the environment through the extensive practice of land application of animal wastes, or additionally, dissemination of resistance by other means.

In my study, I assessed typical poultry litter handling methods and describe factors (e.g. pH, temperature, time and moisture) that affect the survival of drug-resistant enterococci and staphylococci. I also identified resistance genes in isolates from the litter stored over a period of 120 days. Although elevated temperatures greater than 60°C were observed in the core of the litter piles, both antimicrobial-resistant enterococci and staphylococci, as well as resistance genes, persisted throughout the 120-day study period. Resistance genes identified in the study include: erm(B), erm(A), msr(A/B), msr(C), erm(A), vat(E) and erm(C). This study indicates that typical storage practices of poultry litter are insufficient for reducing drug-resistant enterococci and staphylococci.

I then assessed antimicrobial-resistant bacteria and resistance genes isolated from flies in the poultry production environment, a potential pathway of human exposure to resistant bacteria. These data were used to show the correlation of resistance patterns and resistance genes found in enterococci and staphylococci from flies, to resistance patterns and genes found in enterococci and staphylococci from poultry litter. The observed patterns of resistance and resistance genes indicate that flies around poultry confinement operations likely transmit antibiotic-resistant enterococci and staphylococci to the surrounding community. Resistance genes, erm(B), msr(C), msr(A/B), and genetic mobile elements associated with Tn916, were found in isolates recovered from both poultry litter and flies. 

Jay Graham is a recent graduate of the Department of Environmental Health Sciences at Johns Hopkins Bloomberg School of Public Health. Prior to entering the PhD program, Jay worked on the U.S.-México border where he carried out environmental health research and outreach projects on water quality and sanitation infrastructure in low-income communities. He also has experience as an environmental consultant and has worked on environmental impact studies in Bolivia, Ecuador and Venezuela. Jay’s doctoral research focused on the development of antibiotic-resistant bacteria and resistance genes associated with the use of growth-promoting antibiotics in food animal production. His research is particularly focused on characterizing pathways of pathogen transfer that are intensified by current food animal production practices.