Advancements in bacteriophage therapies and delivery for bacterial infection

Abstract

Having co-evolved with bacteria over hundreds of millions of years, bacteriophage are effective killers of specific bacterial hosts. Therefore, phage therapies for infection are a promising treatment avenue, can provide a solution for antibiotic resistant bacterial infections, and have specified targeting of infectious bacteria while allowing the natural microbiome to survive which systemic antibiotics often wipe out. Many phages have well studied genomes that can be modified to change target, widen target range, or change mode of action of killing bacterial hosts. Phage delivery can also be designed to increase efficacy of treatment, including encapsulation and delivery via biopolymers. Increased research into phage potential for therapies can allow new avenues to develop to treat a larger range of infections.

Fig. 1. Lytic bacteriophage replication schematic. Modified from Düzgünes et al. (2021).

Fig. 2. (A) Schematic of homologous recombination of phage receptor binding proteins to alter phage host target range. Modified from Lenneman et al. (2021). (B) Schematic of phage triggered CRISPR-Cas9 pathway for activation of endogenous bacterial nuclease via CRISPR RNA (crRNA). (1) Phage adsorbs to target bacterium and injects its genome into the cell. (2) Phage enters replication cycle, where spacers contained in the phage genomes CRISPR array are expressed and mature into crRNAs and phage structural proteins are produced. (3) The phage cRNAs guide endogenous Cas9 proteins to cut targeted loci in the bacterial genome and/or plasmids. (4) Cuts lead to cell death if the bacterium is unable to repair genome damage or re-sensitization to antibiotics if antibiotic resistance gene on plasmid is damaged. Phage completes its replication cycle and assembles new progeny virions, which lyse the cell and are released into surroundings. Modified from Fage et al. (2021).

Fig. 3. Benefits of encapsulation or hydrogel delivery systems compared to free phage delivery. Stability of phage improved by encapsulation in liposomes or hydrogel to limit exposure to extreme pH and other degradation factors. Protection from enzymes increases with liposome encapsulation and hydrogel suspension compared to free phage easily targeted and degraded by enzymes. Hydrogel application to wound sites allows for localization of phage to a wound bed compared to free phage application. This leads to increased concentration of phage at wound sites, and phage titer is a known to enhance phage therapy efficacy. Adhesion and active site delivery can be facilitated by liposome encapsulation to improve direct interaction with target tissues. Modified from Loh et al. Appl. Environ. Microbiol. (2021, 87, e01979–20, DOI: https://doi.org/10.1128/AEM.01979-20, amended with permission from American Society for Microbiology).