A serious threat to health
Recently, our country was shocked by the dozens of deaths from a hospital-acquired bacterial infection caused by a contaminated medication. The bacterium Klebsiella pneumoniae was the primary biological cause of these deaths. This bacterium was discovered by German pathologist Carl Friedländer in 1882, who observed it in the lungs of patients who died from pneumonia. The introduction of penicillin in 1929 led to increasingly effective antibiotics, and bacterial infectious diseases began to decline. This decline was so severe that William H. Stewart, the U.S. Surgeon General , stated before Congress in 1969 that the era of infectious diseases "was coming to an end."
However, starting in the 1980s, bacteria resistant to the very antibiotics that had previously successfully combated them began to emerge. This is the case with Klebsiella pneumoniae. New cases of bacterial resistance to antibiotics are emerging and spreading across the globe every day, jeopardizing our ability to treat common infectious diseases in both humans and animals.
A growing number of infections, such as pneumonia, tuberculosis, sepsis, and foodborne illnesses, among others, are becoming more difficult and sometimes impossible to treat as antibiotics lose effectiveness against newly resistant populations. The World Health Organization places this resistance among the ten most serious health threats facing humanity. This bacterial resistance is primarily a consequence of a process that evolutionary biology calls "natural selection." Natural selection is a population-based, not an individual-based, phenomenon, as it is a population's response to the challenges posed by its environment. It is explained by four factors: 1) there is variation in some characteristics among individuals within a population; 2) this variation is heritable, meaning that offspring tend to resemble their parents more than individuals to whom they are not biologically related; 3) the mechanism of inheritance is information about the development of organisms that is contained in the genes (DNA) and that is passed from parents to offspring and 4) there is a different reproductive success (capacity to leave offspring in the next generation) between individuals that have different variants of a characteristic, depending on the environment in which they live.
In a given environment, one of the variants increases the survival and reproduction of individuals carrying it . Over time, that population will have a greater number of individuals with that variant, and all individuals with other variants less adapted to that particular environment will be reduced or even eliminated.
How does natural selection influence the emergence of bacterial resistance? Initially, antibiotics are effective against bacterial strains (populations). Within strains, a few resistant individuals may exist, or over time, individuals may emerge that become resistant due to changes in their genes (mutations). These resistant individuals are better adapted to the challenge posed by the antibiotic and, unlike non-resistant individuals, are capable of leaving offspring in the next generation. Through successive generations, resistant individuals become the majority . The changes in genes that cause resistance are not the result of antibiotic application, but rather existed before its application or appear randomly during treatment. One might think that this randomness makes the emergence of resistance unlikely. However, two facts increase this probability: 1) a bacterium produces a large number of generations (with potential random changes in the genes) in relatively short times, for example, under appropriate conditions, a single Escherichia coli bacterium can generate millions of individuals in a few hours, and (2) the human factor, generated by the excessive or unnecessary use of antibiotics or their misuse (inappropriate doses), which favors the selection of resistant microorganisms.
The agricultural use of antibiotics should not be forgotten, as they should be limited to animal welfare in care or production, rather than to developmental stimulation, as is sometimes the case in poultry, pig, and fish farming. Natural selection operates on the products of chance, but this operation involves not only chance but also rigorous and complex biological factors, such as the number of mutations generated, the probability of resistance emerging, the level of antibiotic resistance, the degree of resistance, and the strength of the selective forces. Therefore, it is impossible (at least for now) to predict the evolutionary trajectory of bacterial strains. Currently, bacterial resistance to antibiotics has only one correctable factor: humans, through prudent and appropriate use of antibiotics .
Professor Emeritus of the National University of La Plata, full member of the National Academy of Agronomy and Veterinary Medicine, and corresponding member of the National Academy of Sciences.
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