The multiple variants of Salmonella Genomic Island 1 (SGI1) and IncC conjugative plasmids are two functionally interacting families of mobile genetic elements commonly associated with multidrug resistance in the Gammaproteobacteria. SGI1 and its siblings are specifically mobilised in trans by IncC conjugative plasmids. Conjugative transfer of IncC plasmids is activated by the plasmid-encoded master activator AcaCD. SGI1 carries five AcaCD-responsive promoters that drive the expression of genes involved in its excision, replication, and mobilisation. SGI1 encodes an AcaCD homologue, the transcriptional activator complex SgaCD (also known as FlhDCSGI1) that seems to recognise and activate the same SGI1 promoters. In a new article published today in the NAR Breakthrough section of Nucleic Acids Research, we demonstrated the importance of SgaCD in SGI1′s lifecycle. Mating assays revealed the requirement for SgaCD and its IncC-encoded counterpart AcaCD in the mobilisation of SGI1. An integrative approach combining ChIP-exo, Cappable-seq, and RNA-seq confirmed that SgaCD activates each of the 18 AcaCD-responsive promoters driving the expression of the plasmid transfer functions. A comprehensive analysis of the activity of the complete set of AcaCD-responsive promoters of SGI1 and the helper IncC plasmid was performed through reporter assays. qPCR and flow cytometry assays revealed that SgaCD is essential to elicit the excision and replication of SGI1 and destabilise the helper IncC plasmid. The system biology approach used in this article provides a better and deeper understanding of the complex interactions that drive the formidable spread of SGI1-like elements.
The increasing association of the etiological agent of cholera, Vibrio cholerae serogroup O1 and O139, with multiple antibiotic resistance threatens to deprive health practitioners of this effective tool. Drug resistance in cholera results mainly from acquisition of mobile genetic elements. Genomic islands conferring multidrug resistance and mobilizable by IncC conjugative plasmids were reported to circulate in non-O1/non-O139 V. cholerae clinical strains isolated from the 2010 Haitian cholera outbreak. As these genomic islands can be transmitted to pandemic V. cholerae serogroups, their mechanism of transmission needed to be investigated. Today, we published a new article in mSphere in which we reveal plasmid- and genomic island-encoded factors required for the resistance island’s excision, mobilization and integration, as well as regulation of these functions. We also present the discovery of related genomic islands carrying diverse phage resistance genes but lacking antibiotic resistance-conferring genes in a wide range of marine dwelling bacteria. This discovery suggest these elements are ancient and recently acquired drug resistance genes.
The Salmonella genomic island 1 (SGI1) and its variants propagate multidrug resistance in several species of human and animal pathogens with the help of IncA and IncC conjugative plasmids that are absolutely required for SGI1 dissemination. These helper plasmids are known to trigger the excision of SGI1 from the chromosome. In a new paper published today in PLOS Genetics, we showed that IncC plasmids also trigger the replication of the excised, circular form of SGI1 by enabling the expression of an SGI1-borne replication initiator gene. In return, high-copy replication of SGI1 interferes with the persistence of the IncC plasmid and prevents its cotransfer into a recipient cell, thereby allowing integration and stabilization of SGI1 into the chromosome of the new host. Transient SGI1 replication seems to be a key feature of the life cycle of this family of genomic islands. Sequence database analysis revealed that SGI1 variants encode either a replication initiator protein with a RepA_C domain, or an alternative replication protein with N-terminal replicase and primase C terminal 1 domains. This finding is important to better understand the complex interactions between SGI1-like elements and their helper plasmids that lead to widespread and highly efficient propagation of multidrug resistance genes to a broad range of human and animal pathogens.
Bacteria have evolved multiple defence mechanisms against bacteriophages. For instance, restriction-modification systems provide innate immunity by degrading invading DNAs that lack proper methylation. CRISPR–Cas systems provide adaptive immunity by sampling the genome of past invaders and cutting the DNA of closely related DNA molecules. These barriers also restrict horizontal gene transfer mediated by conjugative plasmids. We recently found that several families of conjugative plasmids are able to fight back. In a paper featured as a NAR Breakthrough article, we show that IncC conjugative plasmids are highly resilient to host defence systems during entry into a new host by conjugation. Using a TnSeq strategy, we uncovered a conserved operon of five genes that confer a novel host defence evasion (hde) phenotype. hde promotes both resistance against type I restriction-modification and CRISPR–Cas evasion by repairing double-strand DNA breaks via recombination between short sequence repeats. All or parts of hde are also found in lambdoid bacteriophages including Lambda, in IncA and untyped conjugative plasmids, and in the integrative and conjugative element R391, which is also resilient against CRISPR–Cas. Hence, the conserved hde operon considerably broadens the host range of large families of mobile elements that spread multidrug resistance. Congratulations to David, Kevin and Frédéric for this interesting contribution to our understanding of plasmid biology.
We are currently looking for MSc or PhD candidates to work on various aspects of the interactions between IncC conjugative plasmids and the resistance island SGI1.
Due to Covid-19 restrictions, start date is expected to be January 2021. Applicants must have a strong background in microbiology, genetics and molecular biology, have had a past experience in a research laboratory and be perfectly fluent in written and spoken English and/or French. Only serious applications from excellent candidates with the specified background will be considered for these positions.
Before applying, applicants are strongly advised to refer to the Université de Sherbrooke website (here) regarding the specific costs related to tuition fees for international students in Québec.
Applicants must apply by e-mail and provide a complete CV, their complete transcripts and a copy of their last diploma, a letter of motivation, and three letters of recommendation from three different academic references. Documents must be provided in PDF format only. Incomplete application will not be considered.
Nicolas Rivard received the Terry Beveridge Poster Award for his poster entitled “Antimicrobial resistance dissemination in Vibrio cholerae: mechanistic insights into the insidious role of IncC plasmids” that he presented in the Molecular Genetic & Cellular Microbiology section at CSM 2019.
IncA and IncC conjugative plasmids drive the spread of antibiotic resistance among several pathogenic species of Gammaproteobacteria. While historically grouped as “IncA/C”, IncA and IncC replicons were recently confirmed to be compatible, and to abolish each other’s entry into the cell they reside in during conjugative transfer by an unknown mechanism. In a new research article published in Journal of Bacteriology, we identified an entry exclusion system (Eex) that is shared by IncA and IncC plasmids. It impedes DNA transfer to recipient cells bearing a plasmid of either incompatibility group. The entry exclusion protein of this system is unrelated to any other known entry exclusion proteins.
Inter-ICE recombination plays an important role in ICE evolution
This month we published a new paper in Applied and Environmental Microbiology that reports on a comparative genomics study of a large set of unique representatives of the SXT/R391 family of integrative and conjugative elements (ICEs). These mobile elements are key players in the spread of antibiotic resistance in Vibrio cholerae and other pathogens. The SXT/R391 family of ICEs was initially defined based on the conservation of a core set of 52 genes and site-specific integration into the 5′ end of the chromosomal gene prfC. Hence, the integrase gene int has been intensively used as a marker to detect SXT/R391 ICEs in clinical and environmental isolates. With the recent reports of closely related elements that carry an alternative integrase gene, it became urgent to investigate whether ICEs that have been left out of the family are a liability for the accuracy of such screenings. Thus, we explored the prevalence and diversity of atypical ICEs in GenBank databases and their relationship with typical SXT/R391 ICEs. Our results led us to broaden the SXT/R391 family of ICEs to include atypical ICEs that are often associated with heavy metal resistance.
Alterations of the IncC mating pore by SGI1 – analysis of the working combinations
Acquisition and dissemination of multidrug resistance among Enterobacteriaceae is in part driven by IncA and IncC (A/C) conjugative plasmids, and Salmonella genomic island 1 (SGI1). Although unrelated, SGI1 relies on the self-transmissible A/C plasmids to spread within bacterial populations. How SGI1 hijacks the mating apparatus synthesized by A/C plasmids had yet to be established.
Our team published a new report in PLOS Genetics that describes yet another twist in the parasitic behavior of SGI1 towards IncC conjugative plasmids. In this report, we show that IncC plasmids trigger the expression of three SGI1-borne genes that code for functional mating pore subunits distantly related to those encoded by the IncC helper plasmids. These subunits alter the mating pore encoded by IncC plasmids to ensure optimal transfer of SGI1 and promote SGI1 dissemination in cell populations harboring IncC plasmids. Apart from SGI1 and relatives, documented mobilizable genomic islands are not known to code for mating pore components, possibly because of redundancy with those encoded by helper conjugative elements. Instead they usually code for mobilization proteins such as a relaxase and auxiliary factors involved in DNA recognition, processing and docking to the mating pore encoded by their helper conjugative element.
From an ecological and epidemiological perspective, the strategy used by SGI1 likely confers a strong competitive advantage to SGI1 over IncC plasmids in clinical settings and could account for the high prevalence of SGI1 and relatives in multidrug-resistant Salmonella enterica and Proteus mirabilis.