[en] We studied the prevalence of resistance to antibiotic belonging to the MLSB group, resistance phenotypes and genetic determinants involved, in viridans streptococci isolated from healthy individuals, who had not undergone antibiotic treatment recently. The prevalence of erythromycin resistance was 68.9%. The most frequent phenotype was the M phenotype (80,65%), while MLSB phenotype was 19.35%. All the isolates showing M phenotype harboured mef genes, mefE in most cases (94%). For all the isolates showing MLSB phenotype, this phenotype was constitutive. All of them harboured ermB genes. 91.7% of isolates showing MLSB phenotype harbored mefE genes too, though this fact did not modified their phenotype. All the isolates harbouring a mefA gene, had it included into a Tn 1207.1 trasposon, as is usual in other streptococci such as S. pyogenes. mefE genes were included in a genetic element similar, but not identical, to the MEGA element previously described in S. pneumoniae. This element has been called MEGA-like. Moreover, we detected mef genes in 65% of susceptible isolates, what had not been described previously in other species. All the macrolide-resistant isolates harboured the MEGA element variant that has unmodified the intergenic fragment between mef and mel, while all the susceptible isolates harboured a MEGA-like variant that, as happens to one of the variants found in MEGA, shows a 99-bp deletion in the previously referred integenic fragment. Nevertheless, transformation of DNA belonging to these viridans streptococci into Streptococcus pneumoniae R6, led always to the emergence of macrolide resistace with a M phenotype, even when DNA was transformed from macrolide-susceptible, mef-positive isolates.

[es] Se llev? a cabo un estudio sobre la prevalencia de la resistencia a antimicrobianos del grupo MLSB en estreptococos viridans procedentes de individuos sanos no sometidos a tratamiento antibi?tico recientemente, los fenotipos de resistencia y los determinantes gen?ticos implicados en la misma. La prevalencia de resistencia a eritromicina fue del 68,9%. Predomin? el fenotipo M (80,65%) frente al fenotipo MLSB (19,35%). Todas las cepas con fenotipo M portaban genes mef, en la mayor parte de los casos mefE (94%). Todas las cepas con fenotipo MLSB mostraron este fenotipo de forma constitutiva, y portaban en todos los casos en gen ermB. Un 91.7% de ?stas portaban adem?s genes mefE, si bien ?ste no influ?a en su fenotipo. Todas las cepas portadoras de gen mefA, inclu?an ?ste dentro de un traspos?n Tn 1207.1, como es habitual en otros estreptococos como S. pyogenes. Los genes mefE se encontraban inclu?dos en un elemento gen?tico similar, aunque no id?ntico, al elemento MEGA descrito previamente en S. pneumoniae, que se ha denominado MEGA-like. Adem?s, en este estudio se han detectado genes mefE en un 65% de las cepas sensibles, circunstancia no descrita previamente en especie alguna. De las dos variedades de elementos MEGA descritos en neumococo, todas las cepas de estreptococos viridans resistentes a macr?lidos portaban la que conserva ?ntegro el fragmento interg?nico entre los genes mef y mel, mientras que todas las sensibles eran portadoras de un elemento MEGA-like que, al igual que una de las variantes del elemento MEGA, muestra una deleci?n de 99 pares de bases en el mencionado fragmento interg?nico. No obstante, la transformaci?n del DNA de estos estreptococos viridans en Streptococcus pneumoniae R6 condicion?, en todos los casos, la aparici?n de resistencia a macr?lidos con fenotipo M, incluso cuando el DNA proced?a de cepas sensibles, pero portadoras de mef.