The 64 508 bp IncP-1β antibiotic multiresistance plasmid pB10 isolated from a waste-water treatment plant provides evidence for recombination between members of different branches of the IncP-1β group

The complete 64 508 bp nucleotide sequence of the IncP-1 β antibiotic-resistance plasmid pB10, which was isolated from a waste-water treatment plant in Germany and mediates resistance against the antimicrobial agents amoxicillin, streptomycin, sulfonamides and tetracycline and against mercury ions, was determined and analysed. A typical class 1 integron with completely conserved 5′ and 3′ segments is inserted between the tra and trb regions. The two mobile gene cassettes of this integron encode a β -lactamase of the oxacillin-hydrolysing type (Oxa-2) and a gene product of unknown function (OrfE-like), respectively. The pB10-specific gene load present between the replication module ( trfA1 ) and the origin of vegetative replication ( oriV ) is composed of four class II (Tn 3 family) transposable elements: (i) a Tn 501 -like mercury-resistance ( mer ) transposon downstream of the trfA1 gene, (ii) a truncated derivative of the widespread streptomycin-resistance transposon Tn 5393c , (iii) the insertion sequence element IS 1071 and (iv) a Tn 1721 -like transposon that contains the tetracycline-resistance genes tetA and tetR . A very similar Tn 501 -like mer transposon is present in the same target site of the IncP-1 β degradative plasmid pJP4 and the IncP-1 β resistance plasmid R906, suggesting that pB10, R906 and pJP4 are derivatives of a common ancestor. Interestingly, large parts of the predicted pB10 restriction map, except for the tetracycline-resistance determinant, are identical to that of R906. It thus appears that plasmid pB10 acquired as many as five resistance genes via three transposons and one integron, which it may rapidly spread among bacterial populations given its high promiscuity. Comparison of the pB10 backbone DNA sequences with those of other sequenced IncP-1 β plasmids reveals a mosaic structure. While the conjugative transfer modules ( trb and tra regions) and the replication module are very closely related to the corresponding segments of the IncP-1 β resistance plasmid R751 and even more similar to the IncP-1 β degradative plasmids pTSA and pADP-1, the stable inheritance operons klcAB – korC and kleAEF are most similar to those of the IncP-1 β resistance plasmid pB4, and clearly less similar to the other IncP-1 β plasmids. This suggests that IncP-1 β plasmids can undergo recombination in the environment, which may enhance plasmid diversity and bacterial adaptability.