This was the conclusion of a US team of researchers led by environmental engineer Pedro Alvarez of Rice University in Houston, Texas, who report their study in a paper published online on 5 February in Environmental Science and Technology.
Antibiotic Resistance Is a Serious Environmental ProblemAlvarez and colleagues are looking for ways to control resistant genes in the environment.
In a statement, he says the spread of antibiotic resistance is not just about the huge medical and public health problem that is grabbing the headlines, but also a serious environmental one too, something that not many people are aware of.
He believes one way antibiotic-resistant genes may find their way into the ground, and then eventually into the water and food supply, is because of confined animal feeding operations (CAFOs).
"A lot of the antibiotic-resistant bacteria originate in animal agriculture, where there is overuse, misuse and abuse of antibiotics," says Alvarez.
"Microbes Don't Like to Carry Excess Baggage"Alvarez goes on to explain how he and his team started with the idea that "microbes don't like to carry excess baggage".
In other words, they "drop genes they're not using because there is a metabolic burden, a high energy cost, to keeping them", he adds.
In their study report they describe how over 120 generations, the "starved" Pseudomonas aeruginosa bacteria opted to keep hold of their precious energy rather than sacrifice it to help pass on to future generations the piece of DNA (the"plasmid") that gave them the ability to resist tetracycline.
Plasmids are self-contained DNA molecules that are capable of multiplying on their own. They can cross over into other species through a process called horizontal gene transfer.
One example of a plasmid that gives bacteria the ability to resist nearly all types of antibiotic is NDM-1.
Tested How Two "Model" Bacteria Shed their Antibiotic ResistanceFor their study, the researchers decided to test their idea that microbes don't like to carry excess baggage on two strains of bacteria: P. aeruginosa, which is found in soil, and could serve as a reservoir for antibiotic-resistant genes in the environment, and a "model resistant strain" of E. coli that is excreted by farm animals.
They chose those particular two bacteria so they could compare the results and "gain insight into response variability".
They grew colonies of the bacteria under starvation conditions so they did not have enough oxygen or nutrients, and also without any tetracycline in their environment.
The results showed that after 120 generations under starvation conditions in the absence of tetracycline, P. aeruginosa completely jettisoned its tetracycline resistance plasmid, while E. coli, even after 500 generations, still retained some.
Under the same starvation conditions, but in the presence of tetracycline, both bacteria retained a level of resistance.
The researchers conclude that for the two model strains, it appears that "conditions that ease the metabolic burden of plasmid reproduction", such as availability of oxygen and nutrients, enhance retention of the resistance plasmid.
"... such conditions (in the presence of residual antibiotics) may be conducive to the establishment and preservation of ARG [antibiotic resistance gene] reservoirs in the environment," they note.
Potential for Practical Application: Mulches in Drainage Channels Near SourceA driver of antibiotic resistance is what biologists term "selective pressure". In any mix of microbes, whether in humans, animals or in the environment, selective pressure means the weaker ones die off and the resistant ones survive and multiply.
The researchers say this means, in order to wipe out the resistance plasmid from the DNA of bacteria in the environment, you have to tackle the problem as close to the source as possible.
Alvarez says, "there are practical implications to what we did," and offers an example:
"If we can put an anaerobic barrier at the point where a lagoon drains into the environment, we will essentially exert selective pressure for the loss of antibiotic-resistant genes and mitigate the propagation of these factors."
A cheap way to create an anaerobic (oxygenless) barrier is to put mulch in the drainage channel, he says.
"If you have a CAFO draining through a channel, then put an anaerobic barrier in that channel. A mulch barrier will do," he explains.
Alvarez and his environmental engineering team believe in the philosophy that "an ounce of prevention is worth more than a pound of remediation".
In other, words, don't let the genes amplify in the environment in the first place:
"Stop them before they're released. And one easy way is to put up an anaerobic barrier," he urges.