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Category: Bacterial Pathogenesis
The Dissemination of Antibiotic Resistance by Bacterial Conjugation, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555815615/9781555813031_Chap18-1.gif /docserver/preview/fulltext/10.1128/9781555815615/9781555813031_Chap18-2.gifAbstract:
In areas where trachoma is highly endemic, that is, where about 70% of the population is infected, many individuals typically carry Streptococcus pneumoniae. Of these, 2% were originally macrolide resistant. After 2 weeks of trachoma treatment, the carrier rate for macrolide-resistant pneumococci rose to 50%, with an increase in the carrier rate of the more highly resistant strains. This study dramatically illustrates the potential for rapid in vivo selection of resistance. In this study it was also found that one chronically ill child was carrying a strain with very high level resistance. This chapter defines gene dissemination mechanisms and shows how and why conjugative mechanisms are the most proficient in multiple drug resistance (MDR) transfer. The plasmid is an interesting variation on the theme of MDR dissemination. The chapter reviews aspects of bacterial conjugation in terms of basic science and clinical relevance. A unifying model can be developed because the general mechanism for classical bacterial conjugation appears to be conserved in conjugative transposons. It is reasonable to suggest that bacterial conjugation is the greatest mover of genes in the microbial world and, in the clinical world, that these genes are often antibiotic resistance genes. The cycle of antibiotic resistance and pathogen genome sequencing are showcasing the prominent role of bacterial conjugation in gene dissemination.
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The cycle of antibiotic development and resistance. The cycle illustrates the inevitability of bacterial resistance. Rate 1 indicates the time required for the development of a new antibiotic, and Rate 2 is the rate at which clinical bacteria develop resistance to the new antibiotic.
Genetic map of the RK2 plasmid. Conjugative transfer genes are clustered in two regions, Tra1 and Tra2. The transfer origin, oriT, is a 250-bp noncoding region within Tra1. The genes for DNA processing and coupling are in Tra1, and the genes for the type IV secretion system are in Tra2. Also shown are the genes encoding resistance to tetracycline (TcR), kanamycin (KmR), and ampicillin (ApR), the origin of vegetative replication (oriV), and selected restriction sites.
Conjugative transfer in cartoon form to illustrate cell-to-cell DNA transfer in concert with rolling-circle replication. Single-stranded DNA cleavage at nic precedes rolling-circle replication. Relaxation complex proteins TraI and TraJ are shown interacting with the inverted repeat and nick region, and the arrow indicates the nick site. Single-stranded binding (SSB) proteins are shown bound to the incoming single-stranded DNA.
DNA transfer and DNA relaxation of RK2 mutations in the nick region and surrounding DNA. G-C–to–A-T transitions were generated by hydroxylamine mutagenesis. The wild-type strain is shown with high transfer proficiency and 100% relaxosome formation, and mutant donor base pair change and values are shown below for comparison. E. coli—to—E. coli conjugative transfer in a 1-h experiment was measured as the relative number of recipient cells that received DNA ( 100 ).
DNA nick regions, flanking sequences, and nick sites (…G ▾,…) of IncP oriT and related sequences. Shown are sequences of gram-negative bacterial plasmids, including plant tumor-inducing plasmids, gram-positive plasmids, and phage ϕX174 DNA. All of the molecules represented by these sequences have rolling-circle replication originating at the nick site. Consensus nucleotides are in upper case, and four invariant nucleotides are in boldface.
Electron micrograph of conjugating cells as mediated by RK2 on a solid surface. E. coli–to–E. coli filter mating is visualized by cryofixation. Arrows indicate junctions between donor and recipient cells, showing the points of contact and fusion of outer membranes. Fusion events require the presence of the RK2 plasmid in the donor cells ( 73 ).
Theta (left) and rolling-circle (right) DNA replication in cartoon form. In theta bidirectional replication, growing forks enlarge the loop, resulting in a structure that resembles a Greek letter θ. Theta replication is a common form for plasmid vegetative replication. In rolling-circle replication, the 3′ (–) end is indicated by the arrow. The 3′ end uses the (+) strand DNA as the template as it leads continuous DNA synthesis around the template.
DNA transfer by plasmid conjugation and conjugative transposons. The basic steps are shown for plasmid conjugation (A), starting with a double-stranded plasmid undergoing vegetative replication in the donor cell, followed by relaxosome formation, cell-cell transfer of single-stranded DNA molecule, single-to-double-stranded DNA synthesis, and vegetative replication in the recipient. Conjugative transposition (B) has these steps plus excision and insertion. The conjugative transposon excises in the host cell as a double-stranded molecule incapable of vegetative replication. This is shown by a lack of oriV sequence. DNA transferred to a recipient cell integrates into host cell DNA. Not shown is an alternative event in which the CTn transposes into another donor cell DNA site without first transferring to another cell.
Model for the membrane bridge in RK2-mediated E. coli–to–E. coli conjugation. The donor cell is shown with the basal part of the membrane bridge and a portion of the attached relaxosome at the inner membrane (IM). TraJ interacts with oriT DNA to align the TraI relaxase enzyme with the nick region. TraG and TraF interact with the relaxosome and inner membrane. TraF is further out, shown at the merged outer membranes (OM) of the donor and recipient cells. Residual pilin subunits are shown at the top of the bridge, at the inner membranes of the recipient cell. Nicking by TraI at the nick site, within the underwound portion of DNA, is thought to initiate the DNA transfer event.
Mr. and Mrs. E. coli bacterial cells in cartoon form. Both cells have chromosomes as indicated by the large hearts. The male cell, Mr. E. coli DH5, carries the RK2 plasmid, ready for transfer to Mrs. E. coli HB101, the female cell.
Genetic mechanisms in bacteria a