Evolution of multiple-antibiotic-resistance plasmids mediated by transposable plasmid deoxyribonucleic acid sequences

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Abstract
Two plasmid DNA sequences mediating multiple antibiotic resistance were transposed in vivo between co-existing plasmids in clinical [human] isolates of Serratia marcescens. This event resulted in evolution of a transferable multi-resistant plasmid. Both sequences, designated as Tn1699 and Tn1700, were flanked by inverted DNA repetitions and could transpose between replicons independently of the Escherichia coli recA gene function. Tn1699 and Tn1700 mediated ampicillin, carbenicillin, kanamycin and gentamicin resistance but differed in the type of gentamicin-acetyltransferase enzymes that they encoded. Structural genes for these enzymes share a great deal of polynucleotide sequence similarity despite phenotypic differences. Transposition of Tn1699 and Tn1700 to co-resident transferable plasmids contributed to dissemination of antibiotic resistance among other gram-negative bacteria. These organisms caused nosocomial infections in epidemic proportions.