The Case of MRSA
Methicillin is an antibiotic that was developed to treat infections by the bacterium Staphylococcus aureus, which had begun to develop resistance to many other antibiotics. At a Boston hospital in 1968, there was an outbreak of Staphylococcus infections that couldn't be cured by methicillin or several other antibiotics the first outbreak of MRSA in the U.S. MRSA has since proven to be an extremely troubling source of infections, first in hospital patients and later in communities.
There are only a limited number of antibiotics that can be given to humans to treat infections. And with disease organisms becoming resistant to multiple antibiotics, this number was rapidly dwindling. Antibiotic resistance was no longer a theoretical topic for scientists to discuss. It was now an extremely serious health problem.
A 2007 study published by the CDC found that 20 percent of all human MRSA in the Netherlands is of animal origin, from pigs and likely from cows. And the researchers strongly doubt that this problem is confined to the Netherlands. In other words, antibiotic-resistant bacteria from farm animals have been linked to antibiotic-resistant infections in humans. (5)
The issue of whether antibiotic-resistant bacteria from farm animals have already caused infections in humans is really moot. The ease with which resistance spreads from one species of bacteria to another means that the pool of drug-resistant bacteria created on animal farms is bound to spread to humans sooner or later. Unless of course, these resistant bacteria can be reduced or eliminated.
This was the idea behind the Maryland School of Public Health study. Could drug-resistant bacteria be discouraged by a reduced reliance on antibiotics?
Short-Circuiting Antibiotic Resistance
Bacteria don't choose to become antibiotic resistant. When they are growing in the presence of an antibiotic that would normally kill them, those that are able to resist the antibiotic survive and breed, while those that don't die. It's a survival trait, pure and simple.
But what happens when that antibiotic disappears? Antibiotic resistance, whether caused by a protein that inactivates the antibiotic or a pump that transports it out of the cell, comes at an energy cost to the bacteria.
When antibiotics are removed from feed and no antibiotic is present, the energy spent on antibiotic resistance is no longer a survival trait, it's a liability. Non-resistant bacteria now are more energy efficient and should be at an evolutionary advantage and grow better, eventually replacing their resistant cousins. At least that's the theory. The Maryland researchers decided to put this theory to the test.
The Maryland Study Design
The researchers compared bacterial antibiotic resistance in five large conventional Mid-Atlantic poultry farms to that of five large Mid-Atlantic farms that had recently converted to organic growth practices and been certified as organic. Among the many requirements for a farm to be certified as organic, poultry cannot be fed any antibiotics or growth hormones.
They looked at the resistance of two species of Enterococcus, Streptococcus-like bacteria found in the intestine and colon of both poultry and humans. These bacteria normally cause no harm but can cause infection when they spread to other parts of the body. Because Enterococcus species are a major source of hospital-acquired infections in humans and because they can spread resistance to other bacterial species through genetic transfer, they're medically important bacteria.
The researchers collected bacterial samples from poultry litter, feed, and water and tested their resistance to 14 antibiotics. They looked at the two most common species of Enterococcus, E. faecalis and E. faecium. These bacteria spread from farm animals, chiefly through animal waste, into air, soil, and water.
A Large and Immediate Decrease in Resistance
Perhaps the most important study finding was the marked drop in bacteria that were multi-antibiotic resistant on the newly organic farms, since multi-drug resistant bacteria create the greatest problems in human infections. While 84 percent of the E. faecium on conventional farms were multi-drug resistant (resistant to three or more antibiotics), only 17 percent were on the organic farms. Multi-drug resistance in E. faecalis showed a similar pattern, 42 percent on conventional farms and only 10 percent on the organic farms.
Resistance to the vast majority of antibiotics tested was lower on the newly organic farms. For example, 81 percent of the E.faecium on conventional farms were resistant to tetracycline, compared to only 12 percent on the newly organic farms. And while all the declines seen weren't this spectacular, the researchers emphasize that all this occurred in the very first flock grown organically. The decreases are only expected to grow larger in the next year or two.
The results of the study are summarized in the table below.