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Updated: May 9, 2025
Site-specific Bacterial Chromosome Engineering: ΦC31 Integrase Mediated Cassette Exchange (IMCE)
Published on: March 16, 2012
Large DNA fragment ISEc9-mediated transposition during natural transformation allows interspecies dissemination of
Sara Domingues1,2, Tiago Lima3,4,5, Corentin Escobar6
1Faculty of Pharmacy, University of Coimbra, Coimbra, Portugal. saradomingues@ff.uc.pt.
Purpose:
Antimicrobial resistance poses a significant global health challenge, contributing to a lack of effective therapeutic agents, especially against Gram-negative bacteria. Resistance dissemination is accelerated by horizontal gene transfer (HGT) mechanisms. The extended-spectrum beta lactamases CTX-M confer resistance to several beta-lactams, are usually embedded into plasmids and thought to be mainly disseminated by conjugation. However, an increasing number of isolates carry these enzyme-encoding genes in the chromosome, suggesting that they can spread by other means of HGT. In this study, we aimed to test the involvement of natural transformation in the chromosomal acquisition of a blaCTX-M gene.
Methods:
Natural transformation assays were performed during motility on wet surfaces. Acquisition of foreign DNA by transformants was screened by antimicrobial susceptibility testing, polymerase-chain reaction (PCR) and whole genome sequencing (WGS).
Results:
Acinetobacter baumannii A118, a naturally competent clinical strain, was transformed with naked DNA from Salmonella enterica serovar Typhimurium Sal25, which was isolated from swine meat. The transformation occurred at low frequency (2.7 × 10- 8 ± 2.04 × 10- 8 transformants per recipient) and blaCTX-M was acquired in one transformant, which was named ACI. WGS of the transformant revealed the acquisition of the blaCTX-M-32 as part of a ca. 36 Kb DNA fragment through an ISEc9-mediated transposition event; various mobile genetic elements and other resistance genes were co-transferred. The blaCTX-M-32 gene was subsequently transferred within A. baumannii at a higher frequency (1.8 × 10- 6 ± 2.49 × 10- 6 transformants per recipient).
Conclusion:
Our results highlight the importance of natural transformation events in the dissemination of antimicrobial resistance genes and mobile genetic elements between and within species.
Insights
Natural transformation facilitates the chromosomal acquisition of antimicrobial resistance genes, like blaCTX-M, between bacterial species. This horizontal gene transfer mechanism contributes to the spread of antibiotic resistance, posing a global health threat.
Area of Science:
- Microbiology
- Genetics
- Molecular Biology
Background:
- Antimicrobial resistance (AMR) is a critical global health issue, particularly against Gram-negative bacteria.
- Horizontal gene transfer (HGT) accelerates the spread of AMR genes, with extended-spectrum beta-lactamases (ESBLs) like CTX-M being significant contributors.
- While plasmid-mediated spread (conjugation) is common for CTX-M genes, chromosomal integration suggests alternative HGT mechanisms.
Purpose of the Study:
- To investigate the role of natural transformation in the chromosomal acquisition of blaCTX-M genes.
- To determine if natural transformation can mediate the transfer of blaCTX-M genes into the bacterial chromosome.
Main Methods:
- Natural transformation assays were conducted using motile bacteria on moist surfaces.
- Antimicrobial susceptibility testing, PCR, and whole-genome sequencing (WGS) were employed to screen for and characterize DNA acquisition.
- Acinetobacter baumannii A118 (competent strain) was transformed with DNA from Salmonella enterica Typhimurium Sal25.
Main Results:
- Natural transformation of Acinetobacter baumannii with Salmonella DNA resulted in low-frequency acquisition of the blaCTX-M-32 gene.
- WGS revealed the blaCTX-M-32 gene was integrated into the chromosome via an ISEc9-mediated transposition event within a 36 Kb DNA fragment.
- Co-transfer of mobile genetic elements and other resistance genes was observed.
- Subsequent transfer of blaCTX-M-32 within Acinetobacter baumannii occurred at a higher frequency.
Conclusions:
- Natural transformation is a significant mechanism for the dissemination of antimicrobial resistance genes and mobile genetic elements.
- This study demonstrates the potential for chromosomal acquisition of resistance genes via natural transformation.
- Findings underscore the importance of considering natural transformation in understanding AMR spread between and within bacterial species.
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