This is the second installment in a series. You can visit the first installment here.
By: James Peacock
Welcome back to our series on antibiotic resistance. Last time, we discussed the basics of antibiotics, how they came about, and what they do. We also touched briefly on the development of antibiotic resistance. In this post, we are taking a look at the development of antibiotic resistance, and what affects resistance is having on the way health officials deal with outbreaks. So, without any further ado, let’s get started.
Bacterial Exposure to Antibiotics
Bacteria have been developing antibiotic resistance for as long as antibiotics have been in use. If you recall from part 1, bacteria can develop resistance to any chemical that it is exposed to over the course of generations. Evolution moves at a very slow pace, but the relatively short lives of bacteria make it much easier for newly acquired traits to become prevalent. When we consider evolution for human beings, it usually takes thousands of years for evolutionary changes to be noticeable. This is because humans live for a long time and reproduce at a slower rate than most of the animal kingdom and microorganisms. When scientists and researchers track evolutionary changes in bacteria, though, the amount of years it takes before a difference is noticeable is far less. For instance, penicillin, which was first mass produced in 1945, is hardly as useful today than it was at that time. Most bacterial infections are handled by other antibiotics, because many infections have resistance to penicillin.
Again, exposure to these chemicals is what necessitates the development of resistance. By limiting exposure, it is likely that bacteria will take longer to develop resistance to antibiotics. Unfortunately, bacteria are exposed to antibiotics quite regularly in our society. Doctors are quick to prescribe antibiotics, so exposure is quite common. The CDC estimates that there were more than 266 million courses of antibiotics prescribed in the United States in 2014 alone. Five prescriptions were written for every 6 people in the United States. It is an important distinction that these are courses of antibiotics being used, not individual pills. This puts the amount of antibiotic pills being distributed at much higher than 266 million .The CDC estimates that at least 30% of prescribed antibiotics are unnecessary, meaning that the illness present in the patient did not require antibiotics. When the number of incorrect antibiotics selections are included, as many as half of all prescriptions will have little to no effect on the illness. This allows more exposure to antibiotics for bacteria, which is potentially harmful to humans in the long run. The CDC also reports that the most commonly prescribed antibiotics are amoxicillin and Azithromycin.
It’s not just through the medical world that bacteria gain exposure to these antibiotics. Antibiotics are used in almost every kind of food production in order to prevent contamination. While this preemptive use of antibiotics keeps bacteria away, they are largely unnecessary. By constantly feeding cattle and pigs antibiotics, we are offering more opportunities for bacteria to develop resistance. Antibiotics are also used in growing produce, leading to further issues. With exposure taking place in every facet of society, it is hardly a surprise that antibiotic resistance has become a global problem.
Back in November of 2015, a study done in China showed that antibiotic resistance had truly become an issue that needed to be dealt with immediately. Colistin, a cyclic polypeptide class antibiotic, was deemed to be a last resort antibiotic by the World Health Organization (WHO). This Chinese study confirmed that bacteria, specifically E. coli, had begun showing resistance to this last resort antibiotic The mechanism for resistance to colistin, called the MCR-1 gene, was detected in quite a few samples taken from livestock in the country. These findings would not be the end of MCR-1, though, as less than one month later the MCR-1 gene was found in a food poisoning patient in Denmark. More and more sightings of the gene took place, eventually leading to the CDC beginning to track the gene. The first time MCR-1 was detected in the United States was in 2016. Since the CDC began tracking the gene, it has been found 14 times. These findings are split into groups based on whether the sample was taken from a human or an animal. California, Maryland, New Jersey, Pennsylvania, Tennessee, and Virginia have all reported 1 human isolated MCR-1 gene. Connecticut, Michigan, and New York have reported 2 cases. Both South Carolina and Illinois have reported one case of MCR-1 found in an animal sample.
The MCR-1 gene is dangerous for two reasons. First, it is a direct counter to our last resort antibiotic, colistin. Second, the gene is considered to be a plasmid. A plasmid is made up of DNA just like any gene, but it is physically separate from the normal chromosomal DNA found in a bacteria. This means that not only can the gene replicate itself rather than wait on the normal DNA replication process, but it stays in the cell even if not in use. Plasmids, because they are not connected to the bacteria’s DNA, can leave the cell and easily be picked up by other bacteria. This allows the MCR-1 gene to be passed from bacteria to bacteria as long as they are geographically close together, independent of the replication process. Thus, a specific bacterium may be able to gain resistance over the course of its life rather than inheriting the resistance gene. Because of this ease of transfer, the MCR-1 gene has spread much more quickly than other methods of resistance. In fact, the MCR-1 gene was recently found in a very common from of the bacteria Salmonella. Previously limited to E. coli, the MCR-1 gene has shown an ability to jump from bacteria to bacteria. While the MCR-1 gene is far less common in Salmonella than in E. coli, it should increase in the rate of incidence as time goes on.
Bacteria are exposed to antibiotics on a daily basis. This exposure can lead to the development of resistance mechanisms, like the MCR-1 gene. This new gene has recently made waves because it is the first to offer resistance against our last resort antibiotic, known as colistin. What makes this gene dangerous is also its ability to pass from bacteria to bacteria almost effortlessly. The emergence of MCR-1 has led to the CDC, as well as health officials around the world, tracking its incidence rates. As the gene grows more common, as it is on track to do, it will become more common for bacterial infections to be resistant to most antibiotics. However, simple tracking is not the only method of studying the growing issue of antibiotic resistance. In the next post of our series, we will take a look at other methods health officials use to track and combat the antibiotic resistance problem.