Poultry manure is not in short supply, with 20 million tons produced in the US annually. Typically, poultry are housed on bedding, such as wood shavings, and the resulting poultry litter is a prized organic fertilizer. However, poultry litter may also have a darker side, in the form of antibiotic-resistant bacteria. These superbugs can cause dangerous infections that are difficult to treat. Alarmingly, some superbugs can also pass on genes that confer antibiotic resistance to other bacteria in their environment. This phenomenon begs the question: “Can poultry litter spread antibiotic resistance throughout the environment?” Read More
To find out, Professor John Maurer of Virginia Tech and colleagues examined the potential of microbes within poultry litter to share antibiotic-resistance genes. If so, applying poultry litter to the soil as a fertilizer could spread antibiotic resistance to soil bacteria and other microbes in the environment. These resistance genes are typically housed on a loop of DNA called a plasmid, and bacterial cell-cell contact can result in plasmid transfer.
Maurer and his colleagues examined levels of antibiotic-resistance in poultry litter samples. Up to 35% of the bacteria were resistant to an antibiotic called sulfonamide, whereas streptomycin and tetracycline resistance were present in about 4% and 5% of the bacterial population, respectively.
Next, the researchers investigated whether antibiotic-resistance genes found in the poultry litter bacteria could be transferred to E. Coli bacteria. They incubated the litter-derived bacteria together with E. Coli on a piece of filter paper, which would bring the bacterial cells into close contact, potentially allowing plasmid transfer to occur.
Interestingly, no plasmid transfer was observed under these conditions. The researchers then tried changing various conditions, including temperature, the fluid that the filter paper was incubated in, and the numbers of bacteria. They still did not observe any plasmid transfer.
The team also included a type of bacteria called E. Coli R100 that will readily transfer its plasmid, but they only observed plasmid transfer at 37 degrees Celsius, which is not reflective of the chicken house environment.
The researchers hypothesize that poultry litter may contain factors that interfere with plasmid transfer or bacterial growth and survival. Future research should investigate whether the type of poultry litter or different approaches to its management could affect this.
Nevertheless, under the conditions investigated here, poultry litter does not appear to readily spread antibiotic-resistance plasmids to other bacteria, which is great news for organic farmers. It still contains significant levels of antibiotic-resistant bacteria, however, and should be treated with caution.
