Perspectiva histórica de la resistencia a antibióticos 105 ARTÍCULO DE REVISIÓN Perspectiva histórica del origen evolutivo de la resistencia a antibióticos Evolutionary origin of antibiotic resistance, a historical perspective Resumen La resistencia a antimicrobianos representa un aspecto natural de evolución bacteriana, que puede resultar de mutaciones o por adquisición de genes foráneos. Hay diferentes posturas sobre el origen de ésta resistencia que explican la habilidad de estos microorganismos de adquirir nuevas características. Las teorías de la evolución de Lamarck y Darwin, han dado pie a experimentos diseñados para explorar el origen de la variación bacteriana y surgimiento de nuevas características. Estos estudios muestran que la resistencia está relacionada con mutaciones en genes cromosomales y/o la transferencia de elementos genéticos extracromosomales, que se expresan según la presión antibiótica ejercida. Está revisión recopila los principales experimentos y las conclusiones derivadas para explicar el fenómeno de resistencia a antibióticos.Palabras clave: resistencia a antibióticos, evolución, adaptación, mutación, transferencia horizontal. AbstractAntimicrobial resistance is a natural aspect of bacterial evolution that can result from mutations or acquisition of foreign genes. Various views on the origin of this resistance explain the ability of these organisms to acquire new features. Lamarck and Darwin's theories of evolution have led to experiments designed to explore the origin of bacterial variation and the emergence of new features. These experiments show that antimicrobial resistance is related to mutations in chromosomal genes and/or transfer of extrachromosomal genetic elements that can be expressed based on the antibiotic pressure exerted. The main experiments and findings that seek to explain the phenomenon of antibiotic resistance are reviewed here in.

Tetracycline resistance (TetR) has been evidenced as a good phenotypic marker for detection of livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) isolates of the clonal complex CC398. The aim of this study was to characterise a collection of 95 TetR-MRSA isolates, not belonging to the lineage CC398, that were obtained in a previous multicentre study, to detect other MRSA clonal complexes that could be associated with this phenotypic TetR marker. The TetR-MRSA isolates were recovered from 20 Spanish hospitals during 2016 and they were characterised to determine their antimicrobial resistance and virulence phenotypes/genotypes as well as the presence of the immune evasion cluster (IEC). A high proportion of isolates belonging to the CC1 lineage (46%) were observed, as well as to the CC5, CC8 and CC45 lineages (11% each one). Thirty-two different spa-types were identified, being predominantly CC1-t127 (40%) and CC45-t1081 (11%). The IEC system (with the gene scn as marker) was present in 73% of isolates and 16% produced the Panton Valentine leucocidin (PVL). A high proportion of MRSA-CC1 isolates were scn-negative (38.6%) and 52.9% of them were blaZ-negative. A multidrug resistance (MDR) phenotype was identified in 86% of MRSA isolates. The knowledge of other TetR-MRSA genetic lineages, in addition to CC398, is highly relevant, since most of them were MDR and some of them presented important virulence factors. Strains potentially associated with livestock (as the subpopulation CC1-t127-scn-negative) or with humans (as the CC45 lineage or the subpopulation CC1-scn-positive) have been found in this study. The use of tetracycline-resistance for detection, not only of CC398 but also of other LA-MRSA lineages should be tracked in the future.