Interaction between an R factor and an element conferring resistance to mercuric ions in Pseudomonas aeruginosa

Genetic Characteristics of Novel IncpSE5381-aadB Plasmids, Integrative and Mobilizable Elements, and Integrative and Conjugative Elements in Pseudomonas aeruginosa

Purpose

Pseudomonas aeruginosa is a common causative bacteria in nosocomial infections. This study aims to describe the structure and evolutionary characteristics of mobile genetic elements (MGEs) carrying antibiotic resistance genes (ARGs) from P. aeruginosa and to conduct bioinformatics and comparative genomic analysis to provide a deeper understanding of the genetic characteristics and diversity of MGEs in P. aeruginosa.

Methods

Fifteen clinical isolates of P. aeruginosa from China were collected and sequenced in this study, and 15 novel MGEs were identified. Together with four MGEs from GenBank, a total of 19 MGEs were used to perform detailed modular structure dissection and sequence comparison. Then, the biological experiments were carried out to verify the biological characteristics of these isolates and MEGs.

Results

The novel MGEs identified in this study displayed diversification in modular structures, which showed complex mosaic natures. The seven types of 19 MGEs included in this study were divided into three groups: i) novel MGEs (firstly identified in this study): four IncpSE5381-aadB plasmids and three Tn7495-related integrative and mobilizable elements (IMEs); ii) newly defined MGEs (firstly designated in this study, but with previously determined sequences): four Tn7665-related IMEs; iii) novel transposons with reference prototypes identified in this study: two Tn6417-related integrative and conjugative elements (ICEs), two IS-based transposition units, two Tn501-related unit transposons, two Tn1403-related unit transposons. At least 36 ARGs involved in resistance to 11 different classes of antimicrobials and heavy metals were identified. Additionally, three novel blaOXA variants were identified. Antimicrobial susceptibility testing showed that these variants were resistant to some β-lactamase antibiotics and blaOXA-1204 was additionally resistant to cephalosporins.

Conclusion

The continuous evolution of ARG-carrying MGEs during transmission, leading to the emergence of novel MGEs or ARGs, which facilitates the spread of antibiotic resistance in P. aeruginosa and enhances the diversity of transmission modes of bacterial resistance.

Improved Method for Recovery of mRNA from Aquatic Samples and Its Application to Detection of mer Expression

Previously described methods for extraction of mRNA from environmental samples may preclude detecting transcripts from genes that were present in low abundance in aquatic bacterial communities. By combining a boiling sodium dodecyl sulfate-diethylpyrocarbonate lysis step with acid-guanidinium extraction, we improved recovery of target mRNA from both pure cultures and environmental samples. The most significant advantage of the new protocol is that it is easily adapted to yield high recovery of mRNA from 142-mm-diameter flat filters and high-capacity cartridge filters. The lysis and extraction procedures are more rapid than previously described methods, and many samples can be handled at once. RNA extracts have been shown to be free of contaminating DNA. The lysis procedure does not damage target mRNA sequences, and mRNA can be detected from fewer than 10 bacterial cells. We used the new method to examine transcripts of genes responsible for detoxification of mercurial compounds. Induction of merA (specifying mercuric reductase) transcripts in stationary-phase Pseudomonas aeruginosa containing Tn501 occurred within 60 s of HgCl(2) addition and was proportional to the amount of Hg(II) added. The new technique also allowed the detection of merA transcripts from the microbial community of a mercury-contaminated pond (Reality Lake, Oak Ridge, Tenn.). Significant differences in merA transcript abundance were observed between different locations associated with the lake. The results indicate that the new method is simple and rapid and can be applied to the study of mer gene expression of aquatic communities in their natural habitats.

Genes encoding mercuric reductases from selected gram-negative aquatic bacteria have a low degree of homology with merA of transposon Tn501

An investigation of the Hg2+ resistance mechanism of four freshwater and four coastal marine bacteria that did not hybridize with a mer operonic probe was conducted (T. Barkay, C. Liebert, and M. Gillman, Appl. Environ. Microbiol. 55:1196-1202, 1989). Hybridization with a merA probe, the gene encoding the mercuric reductase polypeptide, at a stringency of hybridization permitting hybrid formation between evolutionarily distant merA genes (as exists between gram-positive and -negative bacteria), detected merA sequences in the genomes of all tested strains. Inducible Hg2+ volatilization was demonstrated for all eight organisms, and NADPH-dependent mercuric reductase activities were detected in crude cell extracts of six of the strains. Because these strains represented random selections of bacteria from three aquatic environments, it is concluded that merA encodes a common molecular mechanism for Hg2+ resistance and volatilization in aerobic heterotrophic aquatic communities.

Genetic and molecular characterization of the Pseudomonas plasmid pVS1

A restriction map of the 30-kb nonconjugative Pseudomonas plasmid pVS1 was constructed. Derivatives of pVS1 obtained in vitro by successive deletions were used to localize on the physical map the determinant for resistance to mercuric ions (carried by transposon Tn501), the gene(s) encoding sulfonamide resistance, a 1.6-kb region affecting plasmid stability and establishment in P. fluorescens ATCC 13525, and a segment required for mobilization of pVS1 by plasmid RP1. The sulfonamide resistance determinant of pVS1 appeared to be closely related to that of transposon Tn21. A mini-pVS1 replicon, pME259, consisting of an essential 1.55-kb segment (designated rep and thought to carry the origin of replication) and a mercury resistance determinant was able to replicate P. aeruginosa PAO but selective pressure was needed for plasmid maintenance. The copy number of pVS1 derivatives was estimated to be 6-8 per chromosome equivalent. Plasmids possessing the essential rep segment plus the adjacent stability region could be established in strains of P. aeruginosa, P. putida, P. fluorescens, P. acidovorans, P. cepacia, P. mendocina, P. stutzeri, P. syringae, Agrobacterium tumefaciens, and Rhizobium leguminosarum.

Ordering of the flagellar genes in Pseudomonas aeruginosa by insertions of mercury transposon Tn501

The flagellar genes of Pseudomonas aeruginosa PAO cluster on the chromosome at two distinct regions, region I and region II. The order of the flagellar cistrons in this organism was established by using transducing phage G101 and plasmids FP5 and R68.45. A method to insert transposon Tn501 near the fla genes was devised. We obtained two strains in which Tn501 was inserted at sites close to the flagellar cistrons in region II. We isolated Fla mutants in which the chromosomal segment between the two Tn501 insertion sites was deleted. Using Tn501-encoded mercury resistance as an outside marker, we determined the order of 9 of the 11 flagellar cistrons in region II as follows: puuF-region I-flaG-flaC-flaI-flaH-flaD-flaB-flaA-flaF-flaE-pur-67. By using phage G101-mediated transduction, the mutation converting monoflagellated bacteria into the multiflagellated (mfl) form was closely linked to the five fla cistrons in region I. Using mfl as an outside marker, we determined the order of the five cistrons as follows: puuF-flaV-flaZ-flaW-flaX-flaY-region II. The mfl mutation was shown to be either located within the flaV cistron or linked very closely to this cistron. No linkage was observed in transductions between any of the fla cistrons in region I and any of the fla cistrons in region II.

Characterisation of Tn501, a transposon determining resistance to mercuric ions

The transposon encoding resistance to mercuric ions, Tn501, is 5.2 (+/-0.1) x 10(6) daltons and is bounded by small inverted repeats. The restriction sites for the restriction endonucleases EcoRI, HindIII and SalGI have been mapped on the element.

Characterization of a translocation unit encoding resistance to mercuric ions that occurs on a nonconjugative plasmid in Pseudomonas aeruginosa

The nonconjugative plasmid, pVS1, has a molecular weight of 18.5 X 10(6) and confers resistance to sulfonamides and to mercuric ions. In Pseudomonas aeruginosa PAO, the transfer can be mobilized by a variety of conjugative plasmids, and the process does not require a functional recombination system in the donor. Hybrid plasmids that arise by the relocation of the mer gene onto the mobilizing plasmid can be isolated readily, and, as far as can be determined, these hybrids retain the genome of the conjugative plasmid in toto. The relocation of mer occurs by a Rec-independent process and leads to a constant increase (about 6 X 10(6) daltons) in the size of the recipient plasmid. This suggests that the mer gene in pVS1 is located on a translocation unit, designated Tn501, of a molecular weight of about 6 X 10(6). The translocation of Tn501 into RP1 is not usually associated with the loss of any known plasmid-mediated function, but transfer-defective or tetracycline-sensitive derivatives do occur at frequencies of about 4%, whereas carbenicillin-sensitive or kanamycin-sensitive variants arise with a frequency of about 0.2% each. It seems therefore that the integration of Tn501 can occur at any one of a minimum of five sites in RP1.

Properties of derivatives of the Pseudomonas plasmid pVS1 that have inherited carbenicillin resistance from RP1

A procedure is described for the isolation, in Pseudomonas aeruginosa PAO, of bacteria carrying derivatives of pVS1 that inherited the carbenicillin-resistance determinant from RP1 either alone or together with that for aeruginocin resistance. Such bacteria occur among the transconjugant progeny from both recombination-proficient or -deficient pVS1+ RP1+ donors, suggesting that the formation of these plasmids is due to the translocation of TnA from RP1 into pVS1. It is possible, therefore, that the aeruginocin-resistance determinant is part of TnA or is closely linked to it. Unexpectedly, none of these plasmids showed the 3 x 10(6)- to 4 x 10(6)-dalton increase in size predicted for TnA+ derivatives of PVS1. It is suggested that an interaction between TnA and the Tn501 translocation unit in pVS1 could account for this result.

The properties of hybrids formed between the P-group plasmid RP1 and various plasmids from Pseudomonas aeruginosa

R38, R931-1, and R933 are conjugative plasmids derived from strains of Pseudomonas aeruginosa. They confer resistance to mercuric ions (Hg-r), and do not tranfer from P. aeruginosa to Escherichia coli at detectable frequencies. Hybrids between each of these plasmids and the P-group plasmid, RP1, have been detected among the rare Hg-r transconjugants arising from matings of P. aeruginosa PAO donors (RP1 + R+) and E. coli K12 recipients. Two independently isolated hybrid plasmids from each of the three mating combinations have been studied. All were found to confer the entire marker phenotype of RP1, but only the Hg-r phenotype of their second parent. Moreover, all were larger than RP1 but comprised only two groups of sizes; those increased by about 14 x 10(6) daltons (the RP1/R38 hybrids), and those increased by about 30 x 10(6) daltons (the RP1/R931-1 and RP1/R933 hybrids). The hybrid plasmids were all too large to be transduced intact by phage F116L, but tranduction of fragments was possible. Thus, the determinants for both carbenicillin-resistance (Cb-r) (from RP1) and mercuric-ion-resistance could be "rescued" by recipients that already carried an RP1-like plasmid and were recombination-proficient. A molecular analysis of the plasmids recovered from such transductants suggested that each of the parental hybrids was comprised of an entire RP1 genome into which a fragment of heterologous DNA had been inserted. In similar experiments in which the recipient carried a derivative of R931-1, the Hg-r but not the Cb-r determinant could be rescued. This suggested that R38, R931-1, and R933 shared sufficient homology in the region of the mer gene for recombination to occur between them. The reason for the inability to rescue the Cb-r determinant was also investigated.

Evolution and utility of a Pseudomonas aeruginosa drug resistance factor

We describe the addition to the Pseudomonas aeruginosa sex factor, FP2, of carbenicillin resistance encoded by the RP1 plasmid. This occurred in a step-wise manner as detected by variations in the characteristics of the FP2-RP1 plasmid aggregate. The addition of the carbenicillin resistance marker to FP2 facilitates estimates of FP2 transfer. Transfer frequencies for the presumed cointegrate plasmid, using carbenicillin selection, approached 10(-1) per donor bacterium. The chromosomal mobilization properties of the derived plasmid, designated pR0271, resembled those of the progenitor plasmid FP2. Plasmid pR0271 was also observed to mobilize a nontransmissible drug resistance plasmid sharing genetic homology at frequencies corresponding to those observed for chromosomal markers proximal to the origin of transfer.

A molecular analysis of transductional marker rescue involving P-group plasmids in Pseudomonas aeruginosa

The molecular properties of the P-group plasmids R26, R527 and R18-18- (a carbenicillin-sensitive derivative of R18) have been compared with those of RP1. R18-18 and RPI have a MW about 38 X 10(6) daltons, and R26 and R527 of 52 X 10(6) daltons (determined from contour lengths). All three plasmids have a bouyant density similar to that of RPI (1.719 g/cm3, 60% GN. From their molecular and phenotypic similarities, these plasmids probably represent two pairs of identical or closely similar elements. Resistant bacteria are not recovered following F116L-mediated transduction of R26 (or R527), and this correlates with the plasmids' larger size (phage genome=40 X 10(6) daltons). Fragments of R26 are, however, transduced and their resistance determinants may be "rescued" by recombination if the recipient harbours R1818. Such events are accompanied by an increase in the size of the recipient plasmid from 38 X 10(6) to 52 X 10(6) daltons following inheritance of the resistance determinants Sm Su Gm Hg, but not Cb. Thus, Sm Su Gm Hg are encoded in a DNA segment of MW about 14 X 10(6) daltons which apparently has no homologous region on R18-18. Since a piece of DNA of this MW also corresponds to the difference in size between R26 and R18-18, it is possible that the former is derived from an RPI-like element which has acquired these additional resistance determinants.

The use of bacteriophages for differentiating plasmids of Pseudomonas aeruginosa

Six bacteriophages have been used in the classification of 19 plasmids (antibiotic resistance-mediating R factors and FP sex factors which promote host chromosome transfer) ofP. aeruginosaisolated in different geographical regions. On the basis of phage-plating responses on isogenic strains of bacteria differing only in the plasmids carried, five groups of plasmids were distinguishable. In general the groups could be correlated with their geographical origin although differences between plasmids from the same region were found. The unique phage-plating responses were also useful in establishing the possible identity of plasmids isolated from the same original strain and given different designations by independent investigators. The classification of the plasmids derived here on the basis of phage-plating responses could be correlated with classifications based upon other phenotypic characteristics described elsewhere. The nature of inhibition of plating of phages B39 and G101 by R18–1 and R18–3 respectively was shown to be due to interference with some aspect of intra-cellular phage replication rather than to plasmid-mediated restriction.