In order to study the resistance mechanisms to aminoglycosides, tetracyclines and erythromycin, we investigated the genetic determinants on 85 Streptococcus spp., Staphylococcus spp., and Enterococcus spp. isolates collected from 46 hospitals of Argentina over a two-year period. Methodology: The MICs to amikacin, gentamicin, kanamycin, and streptomycin, tetracycline and erythromycin were determined by the standard broth dilution method according to CLSI recommendations. Detection of resistance

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... enes to the antibiotic tested was assessed by the PCR standard technique whereas the clonal relationships of each species was performed by PFGE. Results: Major heterogeneity was detected in aminoglycoside and erythromycin resistances. Indeed, 37.6% of the isolates harbored the aac(6')-aph(2") genes; 27% harbored the aph(3')-IIIa and ant(6)-Ia genes along with the aac(6')-aph(2") gene; 7% carried the ant(4')-Ia gene; and 71% harbored one or more of the erm(A), erm(B), erm(TR), mef(A), mef(E) and msr(A) genes. The tetracycline resistance was determined by the tet(M) gene and was found in 23 isolates that were resistant to this antibiotic. Spreading of tet(M) by the Tn916-like transposon was not a frequent event since the integrase of this element was detected only in 3 Streptococcus spp. isolates. Instead, a 370 bp fragment was detected that corresponded to a region of the CW459-like element integrase in 10 of 11 methicillin-resistant Staphylococcus aureus and in 3 group G Streptococcus isolates, a finding that implies a novel mechanism for tetracycline resistance spreading. Conclusion: This study demonstrates the wide spreading of resistance mechanisms in our nosocomial cocci population and underscores the importance of continuous and efficient epidemiological surveillance.