PLASMID-MEDIATED RESISTANCE TO TRIMETHOPRIM IN ENTEROBACTERIACEAE ISOLATED IN GRAN-CANARIA, CANARY-ISLANDS (SPAIN)

During a 2 month period, 145 Enterobacteriaceae isolated from clinical specimens, at the Hospital Nuestra Senora del Pino (Gran Canaria), we re tested for their susceptibility to trimethoprim (Tp) by an agar dif fusion method. Seven-two (49.6%) strains (including E. coli, Proteus s pp., E. cloacae, Citrobacter spp., Serratia spp., Klebsiella spp. and S. enteritidis) were considered as resistant to Tp, and 28.1% of the r esistant strains could transfer the Tp-resistance marker into E. coli by conjugation. Tp-resistance was encoded by plasmids of different mol ecular weights, ranging from 52.5 to 165 kb (mean 107.5). These plasmi ds encoded a high level resistance to Tp (MIC greater-than-or-equal-to 1000 mug/ml). The other resistance markers co-transferred with the Tp were ampicillin (80.9%), sulfamethoxazole (61.9%), streptomycin (57.1 %), kanamycin (47.6%), tetracycline (42.8%), tobramycin, gentamicin an d chloramphenicol (28.5%, respectively). These data suggest that: (I) plasmid-mediated resistance to Tp represents a serious problem in the Canary Islands; (II) in at least 42.9% of the strains, resistance to T p is not due to dissemination of Tn7, encoding joint-resistance to Tp and streptomycin; (III) selection of Tp-resistant strains may be achie ved by unrelated antibiotics such ampicillin, whose resistance is enco ded by the same plasmid.