Strategies to combat antimicrobial resistance: anti-plasmid and plasmid curing

Abstract

Antimicrobial resistance (AMR) is a global problem hindering treatment of bacterial infections, rendering many aspects of modern medicine less effective. AMR genes (ARGs) are frequently located on plasmids, which are self-replicating elements of DNA. They are often transmissible between bacteria, and some have spread globally. Novel strategies to combat AMR are needed, and plasmid curing and anti-plasmid approaches could reduce ARG prevalence, and sensitise bacteria to antibiotics. We discuss the use of curing agents as laboratory tools including chemicals (e.g. detergents and intercalating agents), drugs used in medicine including ascorbic acid, psychotropic drugs (e.g. chlorpromazine), antibiotics (e.g. aminocoumarins, quinolones and rifampicin) and plant-derived compounds. Novel strategies are examined; these include conjugation inhibitors (e.g. TraE inhibitors, linoleic, oleic, 2-hexadecynoic and tanzawaic acids), systems designed around plasmid incompatibility, phages and CRISPR/Cas-based approaches. Currently, there is a general lack of in vivo curing options. This review highlights this important shortfall, which if filled could provide a promising mechanism to reduce ARG prevalence in humans and animals. Plasmid curing mechanisms which are not suitable for in vivo use could still prove important for reducing the global burden of AMR, as high levels of ARGs exist in the environment.

Figure 1.

Organisation of two antibiotic resistance plasmids. (A) pCT_CTX-M (IncK). Brown, pseudogenes; orange, hypothetic proteins; light pink, insertion sequences; light blue, tra locus; green, pil locus; dark pink, antimicrobial drug resistance gene; yellow, putative sigma factor; red, replication-associated genes. Arrows show the direction of transcription. Reproduced with permission from Cottell et al. (2011). (B) pHNSHP45_mcr-1. Light blue, type IV pilus; dark blue, transfer region; yellow, plasmid stability; dark green, plasmid replication; red, insertion sequence; light green, antimicrobial resistance; purple, other proteins; grey, hypothetical proteins. Reproduced with permission from Liu et al. (2016).

Figure 2.

CRISPR/Cas as an anti-plasmid strategy. (A) CRISPR/Cas systems (purple) which target plasmid encoded genes cause double-stranded breaks in the AMR plasmid, leading to plasmid degradation. In plasmids with toxin (Tox, blue) antitoxin (AT, green) systems, loss of plasmid leads to active toxin. The toxin then mediates cell death, resulting in removal of AMR plasmid carrying bacteria from a population. (B) CRISPR/Cas system prevents uptake of plasmid DNA. Bacteria encoding CRISPR/Cas system that targets plasmid genes degrade incoming DNA, including conjugative (Tra, orange) AMR plasmids, thus preventing spread of AMR palsmids. (C) CRISPR/Cas system combined with lysogenic and lytic phages selects for an antimicrobial sensitive population. Lysogenic phages encoding CRISPR/Cas systems which target both AMR plasmid and lytic phage are administered to bacteria. Production of the CRISPR/Cas system results in degradation of the AMR plasmid, and protection from lytic phages. Administration of lytic phages kills all non-sensitised bacteria, which do not encode the lytic phage resistance, thus producing evolutionary pressure for an antimicrobial sensitive population.