Submitted to: CSA News
Publication Type: Popular Publication
Publication Acceptance Date: 8/18/2014
Publication Date: 9/5/2014
Citation: Cook, K.L. 2014. Detection of pathogens, indicators, and antibiotic resistance genes following land application of poultry litter. CSA News. 59:39.
Technical Abstract: The United States (U.S.) is the world’s largest producer of poultry with over eight billion broilers produced yearly. Poultry litter (PL) is a mixture of manure, bedding, feathers, and spilled feed that is a by-product of broiler production. In 2009, the U.S. produced more than 50 million tons of PL most of which was applied to agricultural land where it serves as a valuable soil conditioner and nutrient source for crop production. However, soil with applied PL may also be a source of microbial contaminants including pathogens (i.e., Campylobacter and Salmonella) and antibiotic resistant bacteria. Despite the importance of these manure-associated bacteria to public health, however, the state of knowledge about the fate and survival of these populations following land application is limited. In an article just published in the Journal of Environmental Quality, researchers with the USDA Agricultural Research Service (ARS) and Western Kentucky University in Bowling Green, KY characterized the fate of PL associated bacteria following application to soils under conventional (CT) or no till (NT) management. PL was surface applied in the spring during two field seasons and soil cores (0 to 15 cm) were taken at twenty four different sampling times. Concentrations of naturally occurring pathogens, fecal indicator bacteria (FIB) and bacteria containing antibiotic resistance genes (ARG) were monitored by conventional microbiological culture and using quantitative, real-time PCR (qPCR). Concentrations of Campylobacter in PL from the broiler facility were high (5.4 ± 3.2 X 106 cells per gram PL) while Salmonella sp. was only detected following enrichment. However, in soils with applied PL, Campylobacter was only detected in 15% of samples while Salmonella sp. was detected in between 45% and 70% of soil samples. Either Campylobacter populations quickly decline under the harsh environmental conditions encountered in the soil or the organism enters a “resting” or viable but non-culturable state that makes it difficult to detect, but permits survival of some minimal portion of the population until it enters a suitable host. Low initial concentrations of Salmonella sp. in PL added to the soil and the high background in untreated control plots (up to 27% positive) suggest that either Salmonella came from outside sources (wildlife, or cross-contamination from surrounding farm areas) or that it grew in response to precipitation and/or nutrients added with the PL. Pathogens are present in much lower concentrations than are FIB used to signify fecal contamination. In soils with applied PL, no E. coli was ever detected. However, enterococci were present in high concentrations in the PL applied to soil and they were the only group consistently detected in PL amended soil. These results suggest that enterococci may be better FIB for field applied PL. A “one size fits all” approach to fecal monitoring (i.e., using E. coli as an indicator or monitoring for pathogens present in high concentrations in initial manure) is not feasible with materials containing populations as diverse as those in PL or in environments as complex as soils. In addition to enterococci, PL had high concentrations of bacteria with ARG for sulfonamide, streptomycin, and tetracycline. Application of PL to the soil increased concentrations of these bacteria orders of magnitude above background. In most cases concentrations decreased to near background levels by the end of each field season. Furthermore, the response to PL application in the second field season was significantly reduced compared to the first year; peak concentrations of resistance genes were reached sooner, concentrations were significantly lower, and there were fewer days above background. These findings suggest that the soil microbial community is resilient in the face of repeated manure applications. Further studi