A strong link between manure and antibiotic-resistant bacteria in soil

What are antibiotics?

Antibiotics are the drugs that are used to treat bacterial infections in humans as well as animals. These are widely used in livestock as a solution to halt any disease in cattle. Using antibiotics as preventatives is called “therapeutic use”. However, farmers, cattle owners or ranch owners use antibiotics for non-therapeutic purposes as well. For example, antibiotics are added as food supplements to enhance the growth of cattle or to suppress any existing bacterial growth in animals which might be subject to crowded unhygienic conditions. One of the important effect as a result of such indiscriminate use of antibiotics is that the bacteria residing inside these cattle develop resistance to the antibiotics. And this, in return could also lead to the spread of antibiotic resistance via cattle dung, which is often applied to agricultural fields as fertilizer. So, would that mean that we can prevent the spread of antibiotic resistance by checking the use of these drugs?



Why be bothered by antibiotics and antibiotics resistance?

Scientists carried out a particular study to understand the mechanism of the spread of drug-resistance from the agricultural use of these drugs. The purpose of this study was to  develop an understanding of the status of the antibiotic-resistant bacteria in the soil after application of manures from untreated (i.e. without antibiotics) cows.  For this  investigation, the researchers worked solely with the (beta)β-lactam drugs. The beta lactam drugs represent a class of antibiotics that halts the formation of the cell surface (specifically the cell wall) of disease causing bacteria of cows. Thus beta-lactams are used as therapeutic agents to treat common infections like mastitis (inflammation of the udder) in cows. Although use of antibiotics is prohibited in organic farming, the use of cattle dung is an essential element in the practice of organic farming and this in turn, may foster unintended growth of the resident bacteria of soil having antibiotic resistance. Manure from antibiotic treated animals definitely selects for antibiotic resistant bacteria while addition of manure from untreated animals still encourages of these bacteria.  Hence getting an idea of antibiotic resistance and its spread through such practices is very important. Researchers carried out an experiment in order to examine the presence and absence of the resistance in soil bacteria by using manure from cows with no prior antibiotics exposure.


What did the researchers do and what did they find?

The researchers used manure from the cows which were never treated with antibiotics and applied them in experimental farm plots. Additionally, they also used the commonly used inorganic fertilizers i.e. NPK to apply in the other farm plots. Each of the two treatments was done in triplicate. The researchers collected small amounts of soil before the application of the two types of fertilizers as well as after the application of these two fertilizers. After application they collected the soil samples at varied time intervals over the next 103 days. Using these soil samples they tried to culture the bacteria in the lab while exposing the bacteria to β-lactam drug Cephalothin to detect the presence of antibiotic resistant bacteria. The researchers also used lab techniques to quantify the abundance of bacteria with antibiotics resistance in the soil before and after fertilizer incorporation.

Their results:

They found that the bacterial population having antibiotic resistance, in the manure treated soil sample was greater than in the soil treated with the inorganic fertilizer. Also, these bacteria increased in population on application of the manure which was not the case while applying the inorganic fertilizer. And over time the bacteria from manure decreased in population in the soil. So, this brings us to the question: what could be the factors in the manure which are fostering the growth of the resistant bacteria in soil?



Further possibilities

Source: http://www.geograph.org.uk/photo/999357

Agricultural use of antibiotics is still a viable option for treating the diseased cattle and trying to keep up with the consumer demands. At present the antibiotic resistance in the animals is not the only problem; rather associated with it is another herculean task of delving deeper into the concepts of sources and mechanism of antibiotic resistance that link animals, soil,  plants and humans intricately. The above study familiarizes us about an additional mechanism that aids in the bloom of antibiotic-resistant bacteria despite using the cow manure devoid of antibiotics. This also brings the case of organic farming into forefront where the antibiotic resistant bacteria is not directly introduced, rather a sudden bloom of the antibiotic-resistant bacteria in soil is witnessed. Therefore, beside identifying the sources of antibiotic spread, their patterns of spread the need of the moment is to look for the possibility of biochemical interactions in the soil by taking various antibiotic resistant bacteria and other microorganisms as well as the ones from the manure.


Source: Udikovic-Kolic, N., Wichmann, F., Broderick, N. A., & Handelsman, J. (2014). Bloom of resident antibiotic-resistant bacteria in soil following manure fertilization. Proceedings of the National Academy of Sciences111(42), 15202-15207. http://www.pnas.org/content/111/42/15202.long

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