Following characterisation by phenotypic tests and ampli®ed ribosomal DNA restriction analysis (ARDRA), 50 tetracycline-resistant (MIC > 16 mg=L) Acinetobacter strains from clinical (n 35) and aquatic (n 15) samples were analysed by PCR for tetracycline resistance (Tet) determinants 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 ®ve tetracycline resistance

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... rminants. The aquatic strains belonged to genomic species other than A. baumannii, and most (12 of 15) did not contain determinants Tet A±E. Strains negative for Tet A±E were also negative for Tet G and M; further analysis of two aquatic strains with speci®c primers for Tet O and Tet Y and degenerate primers for Tet M-S-O-P(B)-Q also showed negative results. Transfer of tetracycline resistance was tested for 20 strains with three aquatic Acinetobacter strains and Escherichia coli K-12 as recipients. Transfer of resistance was demonstrated between aquatic strains from distinct ecological niches, but not from clinical to aquatic strains, nor from any Acinetobacter strain to E. coli K-12. Most transconjugants acquired multiple relatively small plasmids (< 36 kb). Transfer did not occur when DNA from the donor strains was added to the recipient cultures and was not affected by deoxyribonuclease I, suggesting a conjugative mechanism. It is concluded that Tet A and B are widespread among tetracycline-resistant A. baumannii strains of clinical origin, but unknown genetic determinants are responsible for most tetracycline resistance among aquatic Acinetobacter spp. These differences, together with the inability of clinical strains to transfer tetracycline resistance in vitro to aquatic strains, contra-indicate any important¯ow of tetracycline resistance genes between clinical and aquatic acinetobacter populations.