Previously unidentified antibiotic resistance genes have been discovered in the bacteria of dairy cows' guts. Commonly used as a farm soil fertilizer, cow manure could aid in transferring this bacteria to soil where food is grown for human consumption. But will the bacteria take the antibiotic resistance genes with them?
The vast majority of these antibiotic resistance (AR) genes do not currently present any threat to humans, as the bacteria they are present in is harmless.
But it is possible that these genes could appear in harmful bacteria - causing food-borne illnesses or hospital infections.
Researchers from Yale University in New Haven, CT, wanted to investigate whether the use of cow manure as a fertilizer, therefore, would create an environment conducive to these AR genes moving into the human ecosystem. They publish their findings in the journal mBio.
"Since there is a connection between AR genes found in environmental bacteria and bacteria in hospitals, we wanted to know what kind of bacteria are released into the environment via this route of manure fertilization," explains Fabienne Wichmann, lead study author and former postdoctoral researcher at Yale.
Cow manure hosts 80 antibiotic resistance genes
To answer this, the researchers used a "powerful screening-plus-sequencing approach" to identify which AR genes can be found in cow manure. They discovered 80 of these genes in just five samples of manure.
When the genes were added to a laboratory strain of Escherichia coli bacteria, the E. coli became resistant to antibiotics including beta-lactams (such as penicillin), aminoglycosides, tetracycline and chloramphenicol.
When added to E. coli, the AR genes from cow manure made the bacteria resistant to antibiotics.
When the researchers sequenced these 80 AR genes in cow manure, they found that the majority of the new genes - about 75% - were only distantly related to previously identified AR genes.
"The diversity of genes we found is remarkable in itself considering the small set of five manure samples," says Jo Handelsman, senior study author and microbiologist at Yale. "But also, these are evolutionarily distant from the genes we already have in the genetic databases, which largely represent AR genes we see in the clinic."
Although the new AR genes are only distantly related to the AR genes that are known to occur in humans, the researchers do not know yet whether this means they are unable to transfer to humans.
How could these genes potentially enter the human ecosystem?
There are two processes by which AR genes can be introduced to the human ecosystem.
The bacteria containing the genes could colonize humans - this has been shown to occur in farmers who receive bacteria from their animals. If some of the bacteria in cow manure is shown to be harmful to humans, and it acquires resistance to antibiotics, then this could cause public health problems.
Another colonization process is called "horizontal gene transfer." This is when genes are able to "jump" between unrelated mico-organisms. In this process, bacteria that is benign and does not cause a problem for humans may still transfer the AR genes to pathogens at some point. This transference could occur in manure, soil, food or within the human body.
Having identified 80 AR genes in manure, the researchers are now planning a series of studies to plot exactly how these genes may be able to transfer to other environments.
"This is just the first in a sequence of studies - starting in the barn, moving to the soil and food on the table and then ending up in the clinic - to find out whether these genes have the potential to move in that direction," says Handelsman.
She adds that:
"We're hoping this study will open up a larger field of surveillance, to start looking at new types of resistance before they show up in the clinic."