Mr Editor:
Antibiotics and microorganisms have coexisted for millions of years . A series of microorganisms produce antimicrobials which contribute to their biological success. These microorganisms developed a series of pathways to avoid self-killing, which may be considered as proto-mechanisms of antibiotic resistance. Similarly, other microorganisms developed specific adaptations to minimize the effect of these substances, allowing their survival while in the vicinity of antibiotic-producers. Nevertheless, prior to the introduction of antibacterial agents in therapeutic in the 20th century, the presence of antibacterial agents in the different environments was extremely low, with these low levels of antibiotic production having an insignificant impact, limited to the surroundings of producers, and only relevant in very specific circumstances . The introduction of either natural or nature-derived antibacterial substances or molecules of full-synthetic origin (hereafter collectively named “antimicrobial agents”) into clinical practice in the first half of the 20th century and the subsequent exponential uncontrolled use in humans, animals and agriculture unbalanced the situation , leading to the selection of antimicrobial-resistant microorganisms and favoring the spread of mobile structures carrying antimicrobial-resistant determinants; with these microorganisms and structures expanding to different environments, including those with a high and clear anthropic pressure , as well as previously unspoilt areas .