Costs and benefits of natural transformation in Acinetobacter baylyi.

BACKGROUND: Natural transformation enables acquisition of adaptive traits and drives genome evolution in prokaryotes. Yet, the selective forces responsible for the evolution and maintenance of natural transformation remain elusive since taken-up DNA has also been hypothesized to provide benefits such as nutrients or templates for DNA repair to individual cells. RESULTS: We investigated the immediate effects of DNA uptake and recombination on the naturally competent bacterium Acinetobacter baylyi in both benign and genotoxic conditions. In head-to-head competition experiments between DNA uptake-proficient and -deficient strains, we observed a fitness benefit of DNA uptake independent of UV stress. This benefit was found with both homologous and heterologous DNA and was independent of recombination. Recombination with taken-up DNA reduced survival of transformed cells with increasing levels of UV-stress through interference with nucleotide excision repair, suggesting that DNA strand breaks occur during recombination attempts with taken-up DNA. Consistent with this, we show that absence of RecBCD and RecFOR recombinational DNA repair pathways strongly decrease natural transformation. CONCLUSIONS: Our data show a physiological benefit of DNA uptake unrelated to recombination. In contrast, recombination during transformation is a strand break inducing process that represents a previously unrecognized cost of natural transformation.

Low biological cost of carbapenemase-encoding plasmids following transfer from Klebsiella pneumoniae to Escherichia coli.

OBJECTIVES: The objective of this study was to determine the biological cost, stability and sequence of two carbapenemase-encoding plasmids containing blaKPC-2 (pG12-KPC-2) and blaVIM-1 (pG06-VIM-1) isolated from Klebsiella pneumoniae when newly acquired by uropathogenic Escherichia coli clinical isolates of different genetic backgrounds. METHODS: The two plasmids were transferred into plasmid-free E. coli clinical isolates by transformation. The fitness effect of newly acquired plasmids on the host cell was assessed in head-to-head competitions with the corresponding isogenic strain. Plasmid stability was estimated by propagating monocultures for ∼312 generations. Plasmid nucleotide sequences were determined using next-generation sequencing technology. Assembly, gap closure, annotation and comparative analyses were performed. RESULTS: Both plasmids were stably maintained in three of four E. coli backgrounds and resulted in low to moderate reductions in host fitness ranging from 1.1% to 3.6%. A difference in fitness cost was observed for pG12-KPC-2 between two different genetic backgrounds, while no difference was detected for pG06-VIM-1 between three different genetic backgrounds. In addition, a difference was observed between pG12-KPC-2 and pG06-VIM-1 in the same genetic background. In general, the magnitude of biological cost of plasmid carriage was both host and plasmid dependent. The sequences of the two plasmids showed high backbone similarity to previously circulating plasmids in K. pneumoniae. CONCLUSIONS: The low to modest fitness cost of newly acquired and stably maintained carbapenemase-encoding plasmids in E. coli indicates a potential for establishment and further dissemination into other Enterobacteriaceae species. We also show that the fitness cost is both plasmid and host specific.

Fitness costs of various mobile genetic elements in Enterococcus faecium and Enterococcus faecalis.

OBJECTIVES: To determine the fitness effects of various mobile genetic elements (MGEs) in Enterococcus faecium and Enterococcus faecalis when newly acquired. We also tested the hypothesis that the biological cost of vancomycin resistance plasmids could be mitigated during continuous growth in the laboratory. METHODS: Different MGEs, including two conjugative transposons (CTns) of the Tn916 family (18 and 33 kb), a pathogenicity island (PAI) of 200 kb and vancomycin-resistance (vanA) plasmids (80-200 kb) of various origins and classes, were transferred into common ancestral E. faecium and E. faecalis strains by conjugation assays and experimentally evolved (vanA plasmids only). Transconjugants were characterized by PFGE, S1 nuclease assays and Southern blotting hybridization analyses. Single specific primer PCR was performed to determine the target sites for the insertion of the CTns. The fitness costs of various MGEs in E. faecium and E. faecalis were estimated in head-to-head competition experiments, and evolved populations were generated in serial transfer assays. RESULTS: The biological cost of a newly acquired PAI and two CTns were both host- and insertion-locus-dependent. Newly acquired vanA plasmids may severely reduce host fitness (25%-27%), but these costs were rapidly mitigated after only 400 generations of continuous growth in the absence of antibiotic selection. CONCLUSIONS: Newly acquired MGEs may impose an immediate biological cost in E. faecium. However, as demonstrated for vanA plasmids, the initial costs of MGE carriage may be mitigated during growth and beneficial plasmid-host association can rapidly emerge.