Targeting Plasmids to Limit Acquisition and Transmission of Antimicrobial Resistance

Abstract

Antimicrobial resistance (AMR) is a significant global threat to both public health and the environment. The emergence and expansion of AMR is sustained by the enormous diversity and mobility of antimicrobial resistance genes (ARGs). Different mechanisms of horizontal gene transfer (HGT), including conjugation, transduction, and transformation, have facilitated the accumulation and dissemination of ARGs in Gram-negative and Gram-positive bacteria. This has resulted in the development of multidrug resistance in some bacteria. The most clinically significant ARGs are usually located on different mobile genetic elements (MGEs) that can move intracellularly (between the bacterial chromosome and plasmids) or intercellularly (within the same species or between different species or genera). Resistance plasmids play a central role both in HGT and as support elements for other MGEs, in which ARGs are assembled by transposition and recombination mechanisms. Considering the crucial role of MGEs in the acquisition and transmission of ARGs, a potential strategy to control AMR is to eliminate MGEs. This review discusses current progress on the development of chemical and biological approaches for the elimination of ARG carriers.

Keywords: CRISPR; antibiotics; infection; plasmid curing; resistance.

FIGURE 1

Schematic representation of the predominant HTG mechanisms involved in the acquisition and dissemination of genetic material such as ARG. From top to bottom: Conjugation, DNA transfer between a donor cell (left) and a recipient cell (right) mediated by plasmids; Transduction, transfer of bacterial DNA between a donor cell (left) and a recipient cell (right) mediated by phages; Transformation, release of DNA by a donor cell (left) and uptake by a recipient cell (right).

FIGURE 2

Schematic representation of the predominant MGEs involved in acquisition and dissemination of ARGs. (A), IS element (IR: inverted repeats; tnp: transposase gene). (B), Tn3 complex transposon (tnpB: resolvase gene; ARG-antibiotic resistance gene). (C), composite transposon. (D), class I integron and the acquisition of a gene cassette (Int1: integrase gene; att1: recombination site of the integron; qacEδ: truncated segment belonging to a gene that encodes resistance to quaternary ammonium compounds; sul1: sulfonamide resistance gene; orf5/orf6: open reading frames, attC: recombination site of the gene cassette). (E), the mechanism of acquiring adjacent DNA by ISCR elements (oriIS: origin of replication; terIS: end of replication; a second stop sign is located after the ARG, allowing transposition of the entire segment by recombination). (F), complex class 1 integrons (Int1: integrase gene, followed by the attI site; VR1/VR2: variable regions e.g., ARGs, followed by the attC site).

FIGURE 3

Schematic representation of CRISPR-based plasmid system capable of removing MGE-like resistance plasmids. This system contains two sgRNA transcripts, the cas9 nuclease, and other structural elements. Firstly, sgRNA forms a complex with cas nuclease. The sgRNA transcripts guide cas9 nuclease to introduce double-stranded breaks at the ends of the target DNA, leading to cleavage. Direct target recognition is achieved through recognition of protospacer adjacent motifs (PAM), short DNA sequences that are not found in CRISPR loci, so there is no risk of self-degradation (So et al., 2017). Subsequently, the gap is filled through homologous recombination by an editing template. This system can be used to edit the genome of several antibiotic-resistant bacterial strains, leading to the removal of resistance determinants.