Large piles of aging chicken manure to be used as fertilizer on farm crops can house bacteria that are resistant to antibiotics, finds a study from Johns Hopkins University, reports Environmental Health News.

The results raise concern that typical storage conditions may fail to keep the microbes from reaching people through contaminated food or drinking water. Poultry manure is not required to be treated before it is applied to farm fields.

Poultry producers commonly use antibiotics to promote growth of the chickens. This can lead to bacteria in the chickens' digestive system becoming resistant to antibiotics. The antibiotic-resistant bacteria are excreted and wind up in the manure – or poultry litter.

The poultry industry in the United States produces an estimated 13 to 26 million metric tons of manure each year. Much of the litter is used as a fertiliser. It is stored in huge piles until it is ready to be spread onto farm fields. Rich in nitrogen, it is also fed to beef cattle and farmed fish.

The study's researchers examined the survival and the antibiotic-resistance pattern of two different types of bacteria, staphylococci and enterococci, in chicken litter. These bacteria are found in the digestive systems of both chickens and people.

Although the bacteria numbers initially declined in the manure piles, some survived and increased in numbers again over the course of the four-month survey.

Antibiotic-resistant bacteria were found throughout the entire four months of monitoring. The resistant bacteria ranged from 0 to 69 percent of the total bacteria, depending on the strain and the type of antibiotic against which the bacteria were tested for resistance.

Composting may be a better choice than mere storage, suggest the authors. Composting more effectively kills bacteria by controlling the storage environment so that high temperatures occur throughout – not just in the middle – of the entire pile.

Further studies will be needed to determine if the resistant bacteria in manure used as fertiliser can wind up in people or if improved storage/treatment methods are necessary, according to the article in Environmental Health News.

Reference

Graham, J.P., S.L. Evans, L.B. Price, and E.K. Silbergeld. 2009. Fate of antimicrobial-resistant enterococci and staphylococci and resistance determinants in stored poultry litter. Environmental Research. 109:682-689.

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