Detailed restriction enzyme map of crown gall-suppressive IncW plasmid pSa, showing ends of deletion causing chloramphenicol sensitivity

Different pathways to acquiring resistance genes illustrated by the recent evolution of IncW plasmids

DNA sequence analysis of five IncW plasmids (R388, pSa, R7K, pIE321, and pIE522) demonstrated that they share a considerable portion of their genomes and allowed us to define the IncW backbone. Among these plasmids, the backbone is stable and seems to have diverged recently, since the overall identity among its members is higher than 95%. The only gene in which significant variation was observed was trwA; the changes in the coding sequence correlated with parallel changes in the corresponding TrwA binding sites at oriT, suggesting a functional connection between both sets of changes. The present IncW plasmid diversity is shaped by the acquisition of antibiotic resistance genes as a consequence of the pressure exerted by antibiotic usage. Sequence comparisons pinpointed the insertion events that differentiated the five plasmids analyzed. Of greatest interest is that a single acquisition of a class I integron platform, into which different gene cassettes were later incorporated, gave rise to plasmids R388, pIE522, and pSa, while plasmids R7K and pIE321 do not contain the integron platform and arose in the antibiotic world because of the insertion of several antibiotic resistance transposons.

In34, a complex In5 family class 1 integron containing orf513 and dfrA10

A complex class 1 integron, In34, found in a conjugative plasmid from a multidrug-resistant Klebsiella pneumoniae strain isolated in 1997 at a hospital in Sydney, Australia, was shown to have a backbone related to that of In2, which belongs to the In5 family. In In34, the aadB gene cassette replaces the aadA1a cassette in In2, and two additional resistance genes, dfrA10 and aphA1, that are not part of a gene cassette are present. The aphA1 gene is in a Tn4352-like transposon that is located in the tniA gene. The dfrA10 gene lies adjacent to a 2,154-bp DNA segment, known as the common region, that contains an open reading frame predicting a product of 513 amino acids (Orf513). Orf513 is 66 and 55% identical to the products of two further open reading frames that, like the common region, are found adjacent to antibiotic resistance genes. A 27-bp conserved sequence was found at one end of each type of common region. The loss of dfrA10 due to homologous recombination between flanking direct repeats and incorporation of the excised circle by homologous recombination were demonstrated. Part of In34 is identical to the sequenced portion of In7, which is from a multidrug-resistant Escherichia coli strain that had been isolated 19 years earlier in the same hospital. In34 and In7 are in plasmids that contain the same six resistance genes conferring resistance to ampicillin, chloramphenicol, gentamicin, kanamycin, neomycin, tobramycin, trimethoprim, and sulfonamides, but the plasmid backbones appear to be unrelated, suggesting that translocation of a multiple-drug-resistance-determining region as well as horizontal transfer may have occurred.

Behavior of the hybrid plasmid pNSW301 in Zymomonas mobilis grown in continuous culture

The stability of the plasmid pNSW301 which was formed by cointegration of the Inc W R plasmid Sa and the 14.5-kb pNSW1 plasmid of Zymomonas mobilis ZM6100 was investigated in ZM6100(pNSW301) grown in continuous culture without antibiotic selection. The cointegrate plasmid, pNSW301, was found to be structurally unstable and a total reduction in the size of pNSW301 of approximately 21 kb occurred during growth in continuous culture. Following a systematic study, a number of deletion derivatives of pNSW301 were isolated and used to transform Escherichia coli HB101, with the exception of pNSW312. The plasmid pNSW312 was 100% stable in ZM6100(pNSW312) in continuous culture but was unable to replicate in E. coli.

Convenience of plasmid R6K higher-copy-number deletion derivative for cloning of larger DNA fragments

Vector properties of plasmid pNH602, a higher-copy-number deletion mutant of plasmid R6K, were tested by cloning the 6.5 Mg/mol BamHI pSa fragment carrying determinants of resistance to four antibiotics in the unique BamHI site of pNH602. The resulting in vitro constructed recombinant plasmid pNH606 was found to be stable, conjugative, multicopy (20 copies of pNH606 per E. coli chromosome were estimated) and to ensure the increased expression of different genes responsible for the antibiotic resistance. The pSa fragment inserted in the BamHI site of plasmid pNH602 (located in Tn2660) was proved to be transposable to other replicons. Recombinant plasmid pNH606 was analyzed using restriction enzymes BamHI and EcoRI and its physical and genetic map was constructed.

Behavior of the IncW plasmid Sa in Zymomonas mobilis

The stability of the broad-host-range IncW R plasmid Sa in Zymomonas mobilis ZM6100(Sa) was monitored using three antibiotic resistance markers carried by Sa. When grown in batch culture without selection, ZM6100(Sa) rapidly lost the Sa plasmid. When grown with selection for either kanamycin or spectinomycin resistance, the three Sa markers were retained in at least 90% of the population, with spontaneous loss of chloramphenicol resistance being observed in the rest of the population. When ZM6100(Sa) was grown with selection for chloramphenicol resistance, the Sa plasmid either formed a cointegrate plasmid, designated pNSW301, with a native ZM6100 plasmid or was lost except for the chloramphenicol resistance marker. Possible transportation of the chloramphenicol resistance region of Sa to the Z. mobilis chromosome was indicated by hybridization data. The cointegrate plasmid, pNSW301, was maintained stably, except for spontaneous loss of chloramphenicol resistance, in Z. mobilis without selection for 300 generations. pNSW301 was able to replicate in Escherichia coli and was capable of conjugal transfer.

Restriction enzyme map of cryptic plasmid accompanying pR711b and pR409; identity of pR409 and pR388

A reference strain containing the IncW plasmid, pR409, was found to contain also a cryptic, conjugative plasmid, here named pJR15. After the separation of the two plasmids into different strains of bacteria, pR409 was found to have an identical restriction enzyme map to the IncW plasmid, pR388. Both pR409 and pR388 were isolated from the same hospital in London and confer sulphonamide and trimethoprim resistance. Originally pR409 was reported to confer resistance to tetracycline, but this was not confirmed by the Plasmid Reference Center. pR409 appears to represent another isolate of pR388 in a different host background. The restriction enzyme map of the cryptic plasmid, pJR15 (39 kb), has been determined. pJR15 was found compatible with plasmids from 15 different incompatibility groups and has been found also in the reference strain of the IncD plasmid, pR711b. The HindIII and BglII digests of pR711b are shown. The possible presence of the conjugative plasmid, pJR15, should be examined for studies on the sex pili or chromosomal mobilization properties of pR711b.

One-kilobase direct repeats of plasmid pSa

One-kilobase, direct repeats were found on either side of the chloramphenicol resistance gene of plasmid pSa. The right repeat corresponded to the region coding for sulfanilamide resistance. The repeats were not identical as judged by distances between restriction enzyme sites, hybridization, and by the ability to confer resistance to sulfanilamide.

Transferable plasmid-linked chloramphenicol acetyltransferase conferring high-level resistance in Bacteroides uniformis

Bacteroides uniformis RYC3373 resistant to 64 micrograms of chloramphenicol per ml was isolated from a peritoneal pelvic abscess of a patient not previously treated with this drug. Chloramphenicol resistance was transferable at low frequency to a suitable Bacteroides fragilis recipient. The acquisition of resistance was linked to the presence of a 39.5-kilobase plasmid (pRYC3373), which was subsequently transferred to a secondary recipient. The transfer of Cm resistance occurred by a conjugation-like process. Donor and transconjugant strains produced chloramphenicol acetyltransferase constitutively. The Km for chloramphenicol was 40 microM, and its inactivation by 5-5'-dithiobis(2-nitrobenzoic acid) suggested its similarity to the type II enterobacterial enzymes encoded by different conjugative plasmids and also to a previously described enzyme of B. fragilis F47 and F48. The specific activity and the resistance level in pRYC3373-bearing strains were more than 10-fold higher than in the case of the enzyme from B. fragilis strains F47 and F48. The genetic basis of chloramphenicol acetyltransferase synthesis in Bacteroides spp. had not been previously established.

Characterization of Tn3926, a new mercury-resistance transposon from Yersinia enterocolitica

A new transposon coding for mercury resistance (HgR), Tn3926, has been found in a strain of Yersinia enterocolitica, YE138A14. The element has a size of 7.8 kb and transposes to conjugative plasmids belonging to different incompatibility groups. A restriction map has been established. DNA-DNA hybridization indicates that Tn3926 displays homology with both Tn501 and Tn21; the greatest homology is shown with the regions of these transposons that encode HgR. Weaker homology is observed between Tn3926 sequences and those regions of Tn501 and Tn21 that encode transposition functions. Complementation experiments indicate that the Tn3926 transposase mediates transposition of Tn21, albeit somewhat inefficiently, but not of Tn501, while the resolvase mediates resolution of transposition cointegrates formed via Tn21, Tn501, or Tn1721.