Natural Transformation Facilitates Transfer of Transposons, Integrons and Gene Cassettes between Bacterial Species

Abstract

We have investigated to what extent natural transformation acting on free DNA substrates can facilitate transfer of mobile elements including transposons, integrons and/or gene cassettes between bacterial species. Naturally transformable cells of Acinetobacter baylyi were exposed to DNA from integron-carrying strains of the genera Acinetobacter, Citrobacter, Enterobacter, Escherichia, Pseudomonas, and Salmonella to determine the nature and frequency of transfer. Exposure to the various DNA sources resulted in acquisition of antibiotic resistance traits as well as entire integrons and transposons, over a 24 h exposure period. DNA incorporation was not solely dependent on integrase functions or the genetic relatedness between species. DNA sequence analyses revealed that several mechanisms facilitated stable integration in the recipient genome depending on the nature of the donor DNA; homologous or heterologous recombination and various types of transposition (Tn21-like and IS26-like). Both donor strains and transformed isolates were extensively characterized by antimicrobial susceptibility testing, integron- and cassette-specific PCRs, DNA sequencing, pulsed field gel electrophoreses (PFGE), Southern blot hybridizations, and by re-transformation assays. Two transformant strains were also genome-sequenced. Our data demonstrate that natural transformation facilitates interspecies transfer of genetic elements, suggesting that the transient presence of DNA in the cytoplasm may be sufficient for genomic integration to occur. Our study provides a plausible explanation for why sequence-conserved transposons, IS elements and integrons can be found disseminated among bacterial species. Moreover, natural transformation of integron harboring populations of competent bacteria revealed that interspecies exchange of gene cassettes can be highly efficient, and independent on genetic relatedness between donor and recipient. In conclusion, natural transformation provides a much broader capacity for horizontal acquisitions of genetic elements and hence, resistance traits from divergent species than previously assumed.

Bacterial natural transformation by highly fragmented and damaged DNA

“…Further studies will reveal the broader impact of short fragment transformation in different species and environments. The outcomes of natural transformation with short DNA fragments will differ substantially from those of transformation by longer DNA segments, such as those containing entire genes, operons, and mobile genetic elements (34). Natural transformation with very short DNA will, due to size constraints, lead to base pair substitutions resulting in modification or loss of resident gene functions, rather than acquisition and integration of entire genes.…”

Integrons: Past, Present, and Future

“…They have readily spread by conjugation and natural transformation (145,146), such that they are now found in some 40 to 70% of Gram-negative pathogens from clinical contexts (147,148). They are common in the pathogens and commensal flora of livestock and companion animals (144,149,150).…”

“…These xenogenetic DNA elements are released back into the environment, simultaneously with the antibiotics, disinfectants, and heavy metals that originally drove their selection (175). Wastewaters and effluent then become a giant reaction vessel for recombination and rearrangement of resistance determinants (17,229,250) and for extensive lateral gene transfer between clinical, commensal, and environmental bacteria (145,236). There is also considerable scope for coselection.…”

The Evolution of Natural Competence: Disentangling Costs and Benefits of Sex in Bacteria

“…More than 60 species of bacteria have been reported to undergo transformation (Johnsborg et al 2007), including important human pathogens such as Streptococcus pneumoniae and Haemophilus influenzae. Despite its importance for bacterial evolution (Lorenz and Wackernagel 1994;Domingues et al 2012), the evolutionary forces responsible for the evolution and the maintenance of natural competence in bacteria are still under debate (Redfield 2001;Michod et al 2008;Vos 2009). …”