Plasmids encoding trimethoprim resistance in bacterial isolates from man and pigs

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Abstract
Trimethoprim (Tp) resistant gram-negative bacteria were isolated from humans and pigs in Perth, Western Australia. The bacterial hosts were characterized by their resistance pattern and biotype. The presence of transferable Tp plasmids was demonstrated in 86% of 59 porcine isolates and 37% of 49 human isolates. The Tp R plasmids carried a diversity of resistance determinants such as tetracycline, chloramphenicol, spectinomycin, streptomycin and sulfafurazole. Incompatibility tests distinguished 2 major groups, Inc FIV and Inc N. Of 99 Tp R plasmids isolated from pigs, 30 were grouped as Inc FIV and 3 as Inc N. Of 26 Tp R plasmids isolated from humans, 11 were grouped as Inc FIV and 8 as Inc N. The results of MW determination of Tp R plasmids performed by agarose gel electrophoresis were consistent with the existence of 2 groups: larger R plasmids (76-104 megadaltons [Md]) belonging to Inc FIV and lower MW R plasmids (25-36 Md) belonging to Inc N. Thus, the Tp R plasmids isolated in Perth evidently have evolved independently from those described in Europe and the UK. There is also evidence for their local spread between Escherichia, Klebsiella, Enterobacter, Citrobacter and Acinetobacter from man and animals.