Efficacy of Phage- and Bacteriocin-Based Therapies in Combatting Nosocomial MRSA Infections

Abstract

Staphylococcus aureus is a pathogen commonly found in nosocomial environments where infections can easily spread - especially given the reduced immune response of patients and large overlap between personnel in charge of their care. Although antibiotics are available to treat nosocomial infections, the increased occurrence of antibiotic resistance has rendered many treatments ineffective. Such is the case for methicillin resistant S. aureus (MRSA), which has continued to be a threat to public health since its emergence. For this reason, alternative treatment technologies utilizing antimicrobials such as bacteriocins, bacteriophages (phages) and phage endolysins are being developed. These antimicrobials provide an advantage over antibiotics in that many have narrow inhibition spectra, enabling treatments to be selected based on the target (pathogenic) bacterium while allowing for survival of commensal bacteria and thus avoiding collateral damage to the microbiome. Bacterial resistance to these treatments occurs less frequently than with antibiotics, particularly in circumstances where combinatory antimicrobial therapies are used. Phage therapy has been well established in Eastern Europe as an effective treatment against bacterial infections. While there are no Randomized Clinical Trials (RCTs) to our knowledge examining phage treatment of S. aureus infections that have completed all trial phases, numerous clinical trials are underway, and several commercial phage preparations are currently available to treat S. aureus infections. Bacteriocins have primarily been used in the food industry for bio-preservation applications. However, the idea of repurposing bacteriocins for human health is an attractive one considering their efficacy against many bacterial pathogens. There are concerns about the ability of bacteriocins to survive the gastrointestinal tract given their proteinaceous nature, however, this obstacle may be overcome by altering the administration route of the therapy through encapsulation, or by bioengineering protease-resistant variants. Obstacles such as enzymatic digestion are less of an issue for topical/local administration, for example, application to the surface of the skin. Bacteriocins have also shown impressive synergistic effects when used in conjunction with other antimicrobials, including antibiotics, which may allow antibiotic-based therapies to be used more sparingly with less resistance development. This review provides an updated account of known bacteriocins, phages and phage endolysins which have demonstrated an impressive ability to kill S. aureus strains. In particular, examples of antimicrobials with the ability to target MRSA strains and their subsequent use in a clinical setting are outlined.

Keywords: MRSA; bacteriocins; bacteriophage; endolysins; nosocomial environment.

FIGURE 1

Staphylococcus aureus mechanisms of virulence. (1) The agr quorum sensing system uses AIP levels to regulate cell wall proteins. (2) Phages incorporate their DNA into the bacterial cell genome. (3) The core genome of S. aureus contains a variety of virulence genes, including mecA which encodes for methicillin resistance. (4) Adhesins are surface proteins used to bind to host tissue. (5) Toxins secreted by the bacterial cell damage host tissue and can be encoded by the core genome or MGEs. (6) Autolysins break down peptidoglycan in the cell wall and so can attack other cells. (7) Plasmids carry virulence and antibiotic resistance genes that can be incorporated into the genome of bacterial cells. Created using Biorender.com.

FIGURE 2

Mechanisms for intracellular preservation of S. aureus and induction of cell death. (1) The ADAM10 receptor is a target for formation of α-toxin pores on the cell membrane. (2) S. aureus may survive and grow once engulfed in endosomes. (3) or in the cytoplasm of the cell. (4) S. aureus present in the cytoplasm are recognized by NOD2 and subsequent activation of NFκB occurs resulting in the production of cytokines. Mitochondrial permeabilization may result in apoptosis. (5) This is brought about by a potassium efflux caused by α-toxin build up and subsequent caspase 2 activity. (6) The presence of PVL also cause disruptions to the mitochondrial membrane. This leads to chain of activation involving cytochrome C, caspase 9 and executioner caspases resulting in apoptosis of the cell. (7) Damaged phagosomes release cathepsin, activating the inflammasome and caspase 1. This activation results in cellular secretion of IL1β, followed by cell death. A process known as pyronecrosis. (8) Bacteria can replicate within autophagosomes, which have collected cellular contaminants, until they can escape at which point they can induce cell death. Macroautophagy is also said to be activated by Ca^{2 +} (Fraunholz and Sinha, 2012). Created using Biorender.com.

FIGURE 3

Bacteriocins can function as colonizing peptides (by opening up a niche in the environment for their producing bacteria to occupy), as signaling peptides [to communicate with the immune system or other surrounding bacteria, or as killing peptides (to eradicate bacteria threatening the survival of their producing bacteria)] (Dobson et al., 2012). Created using Biorender.com.

FIGURE 4

Class I and class II bacteriocins produced from Gram positive bacteria (Newstead et al., 2020). Created using Biorender.com.

FIGURE 5

Morphology of podoviridae, myoviridae, and siphoviridae phages (Newstead et al., 2020).

FIGURE 6

(a) Initial biofilm exposure to proteins produced by the phage such as polysaccharide depolymerase. (b) Breakdown of the extracellular polymer substance allows for phage to infect the bacterial cells. (c) Phage infection of the bacterial cells in the exterior layer of the biofilm leads to infection of bacterial cells deeper within the biofilm. (d) A combination of the breakdown of the extracellular polymer substance due to polysaccharide depolymerase exposure and phage replication leads to the eradication of the biofilm. (Gutiérrez et al., 2016).

FIGURE 7

Staphylococcus aureus phage recognition of wall teichoic acids (WTA) in Staphyococcus. (A) Staphylococcal Myoviridae (Myo) and S. aureus Siphoviridae (Rbo-Sipho) phages can recognize α-GlcNAc and β-GlcNAc residue containing ribitol phosphate (RboP)-type WTA. (B) Podoviridae, Siphoviridae and Myoviridae phages are capable of infecting S. aureus when α-GlcNAc residues are absent. (C) Glycerol phosphate (GroP)-type WTA is only recognized by Myoviridae phages and Gro-Siphoviridae phages (including CoNS and S. pseudointermedius phages) (Azam and Tanji, 2019). Created using Biorender.com.