Distribution and in-vitro transfer of tetracycline resistance determinantsin clinical and aquatic Acinetobacter strains

Following characterisation by phenotypic tests and amplified ribosomal DNA restriction analysis (ARDRA), 50 tetracycline-resistant (MIC greater than o r equal to 16mg/L) Acinetobacter strains from clinical (n=35) and aquatic ( n=15) samples were analysed by PCR for tetracycline resistance (Tet) determ inants of classes A-E, All the clinical strains were A. baumannii; most (33 of 35) had Tet A (n = 16) or B (n = 17) determinants, and only two did not yield amplicons with primers for any of the five tetracycline resistance d eterminants. The aquatic strains belonged to genomic species other than A. baumannii, and most (12 of 15) did not contain determinants Tet A-E, Strain s negative for Tet A-E were also negative for Tet G and M; further analysis of two aquatic strains with specific primers for Tet O and Tet Y and degen erate primers for Tet M-S-O-P(B)-Q also showed negative results. Transfer o f tetracycline resistance was tested for 20 strains with three aquatic Acin etobacter strains and Escherichia coli K-12 as recipients. Transfer of resi stance was demonstrated between aquatic strains from distinct ecological ni ches, but not from clinical to aquatic strains, nor from any Acinetobacter strain to E. coli K-12, Most transconjugants acquired multiple relatively s mall plasmids (<36 kb). Transfer did not occur when DNA from the donor stra ins was added to the recipient cultures and was not affected by deoxyribonu clease I, suggesting a conjugative mechanism. It is concluded that Tet A an d B are widespread among tetracycline-resistant A. baumannii strains of cli nical origin, but unknown genetic determinants are responsible for most tet racycline resistance among aquatic Acinetobacter spp, These differences, to gether with the inability of clinical strains to transfer tetracycline resi stance in vitro to aquatic strains, contra-indicate any important flow of t etracycline resistance genes between clinical and aquatic acinetobacter pop ulations.