Genetic basis of I-complex plasmid stability and conjugation

Plasmids are major drivers of increasing antibiotic resistance, necessitating an urgent need to understand their biology. Here we describe a detailed dissection of the molecular components controlling the genetics of I-complex plasmids, a group of antibiotic resistance plasmids found frequently in pathogenic Escherichia coli and other Enterobacteriaceae that cause significant human disease. We show these plasmids cluster into four distinct subgroups, with the prototype IncI1 plasmid R64 subgroup displaying low nucleotide sequence conservation to other I-complex plasmids. Using pMS7163B, an I-complex plasmid distantly related to R64, we performed a high-resolution transposon-based genetic screen and defined genes involved in replication, stability, and conjugative transfer. We identified the replicon and a partitioning system as essential for replication/stability. Genes required for conjugation included the type IV secretion system, relaxosome, and several uncharacterised genes located in the pMS7163B leading transfer region that exhibited an upstream strand-specific transposon insertion bias. The overexpression of these genes severely impacted host cell growth or reduced fitness during mixed competitive growth, demonstrating that their expression must be controlled to avoid deleterious impacts. These genes were present in >80% of all I-complex plasmids and broadly conserved across multiple plasmid incompatibility groups, implicating an important role in plasmid dissemination.

A Review of the Functional Annotations of Important Genes in the AHPND-Causing pVA1 Plasmid

Acute hepatopancreatic necrosis disease (AHPND) is a lethal shrimp disease. The pathogenic agent of this disease is a special Vibrio parahaemolyticus strain that contains a pVA1 plasmid. The protein products of two toxin genes in pVA1, pirAvp and pirBvp, targeted the shrimp's hepatopancreatic cells and were identified as the major virulence factors. However, in addition to pirAvp and pirBvp, pVA1 also contains about ~90 other open-reading frames (ORFs), which may encode functional proteins. NCBI BLASTp annotations of the functional roles of 40 pVA1 genes reveal transposases, conjugation factors, and antirestriction proteins that are involved in horizontal gene transfer, plasmid transmission, and maintenance, as well as components of type II and III secretion systems that may facilitate the toxic effects of pVA1-containing Vibrio spp. There is also evidence of a post-segregational killing (PSK) system that would ensure that only pVA1 plasmid-containing bacteria could survive after segregation. Here, in this review, we assess the functional importance of these pVA1 genes and consider those which might be worthy of further study.

Antirestriction Protein ArdB (R64) Interacts with DNA

The antirestriction ArdB protein inhibits the endonuclease activity of type I restriction/modification (RM) systems in vivo; however, the mechanism of inhibition remains unknown. In this study, we showed that recombinant ArdB from Escherichia coli cells co-purified with DNA. When overexpressed in E. coli cells, a portion of ArdB protein formed insoluble DNA-free aggregates. Only native ArdB, but not the ArdBΔD141 mutant lacking the antirestriction activity, co-purified with DNA upon anion-exchange and affinity chromatography or total DNA isolation from formaldehyde-treated cells. These observations confirm the hypothesis that ArdB blocks DNA translocation via the R subunits of the R₂M₂S complex of type I RM enzymes.

Characterization of a Novel Conjugative Plasmid in Edwardsiella piscicida Strain MS-18-199

Edwardsiella piscicida is a pathogenic bacterium responsible for significant losses in important wild and cultured fish species. E. piscicida strain MS-18-199 recovered from a diseased hybrid catfish from East Mississippi and showed resistance to florfenicol, chloramphenicol, oxytetracycline, doxycycline, erythromycin, tetracycline, azitromycin, spectinomycin, sulfonamide, and bacitracin. To explore the mechanisms of resistance in E. piscicida strain MS-18-199, genomic DNA was extracted and subjected to whole genome sequencing (WGS) using a combination of long (Oxford Nanopore) and short (Illumina) reads. The genome of strain MS-18-199 revealed a novel plasmid named pEPMS-18199. The 117,448 bp plasmid contains several antimicrobial resistance (AMR) elements/genes, including florfenicol efflux pump (floR), tetracycline efflux pump (tetA), tetracycline repressor protein (tetR), sulfonamide resistance (sul2), aminoglycoside O-phosphotransferase aph(6)-Id (strB), and aminoglycoside O-phosphotransferase aph(3)-Ib (strA). Two genes, arsA and arsD, that encode protein components related to transport/resistance to arsenic were also found in pEPMS-18199. In addition, pEPMS-18199 carried twelve conjugative transfer genes (tra), eight transposases and insertion elements, two plasmid stability proteins, two replication proteins, and three partitioning proteins (par system). Results from mobilization and stability experiments revealed that pEPMS-18199 is highly stable in the host cell and could be transferred to Escherichia coli and Edwardsiella ictaluri by conjugation. To our knowledge, this is the first detection of a multidrug resistance (MDR) conjugative plasmid in E. piscicida in the United States. Careful tracking of this plasmid in the aquaculture system is warranted. Knowledge regarding the molecular mechanisms of AMR in aquaculture is important for antimicrobial stewardship.