Bacteriophage endolysins against gram-positive bacteria, an overview on the clinical development and recent advances on the delivery and formulation strategies.

Facing the increasing threat of multi-drug antimicrobial resistance (AMR), humans strive to search for antibiotic drug candidates and antibacterial alternatives from all possible places, from soils in remote areas to deep in the sea. In this "gold rush for antibacterials," researchers turn to the natural enemy of bacterial cells, bacteriophage (phages), and find them a rich source of weapons for AMR bacteria. Endolysins (lysins), the enzymes phages use to break the bacterial cells from within, have been shown to be highly selective and efficient in killing their target bacteria from outside while maintaining a low occurrence of bacterial resistance. In this review, we start with the structures and mechanisms of action of lysins against Gram-positive (GM+) bacteria. The developmental history of lysins is also outlined. Then, we detail the latest preclinical and clinical research on their safety and efficacy against GM+ bacteria, focusing on the formulation strategies of these enzymes. Finally, the challenges and potential hurdles are discussed. Notwithstanding these limitations, the trends in development indicate that the first, approved lysin drugs will be available soon in the near future. Overall, this review presents a timely summary of the current progress on lysins as antibacterial enzymes for AMR GM+ bacteria, and provides a guidebook for biomaterial researchers who are dedicating themselves to the battle against bacterial infections.

Bacteriophage-derived endolysins to target gram-negative bacteria.

Bacteriophage-encoded endolysins (lysins) have emerged as a novel class of antibacterial agents to combat the surging antibiotic resistance. Lysins have specific structures and mechanisms to exert antibacterial effect against both Gram-positive (G+ve) and Gram-negative (G-ve) bacteria. However, its use against G-ve bacteria is limited because the outer membrane (OM) of G-ve bacteria hinders the permeation of exogenously applied lysins. Besides identifying lysins with intrinsic OM permeability, several other approaches including combining lysins with outer membrane permeabilizers (OMPs), protein engineering and formulating with nanocarriers have been proposed to enhance the permeability and activity of lysins. In the present review, we summarize strategies that have been developed to enable lysins to target G-ve bacteria in the past decade. While lysins demonstrates clear potential in managing bacterial infections caused by the drug-resistant G-ve bacteria, there are still challenges hindering their translation into clinical settings, including safety issues with OMP use, low efficiency against stationary phase bacteria and problems in stability. The applicability of protein engineering and formulation sciences to improve enzyme stability, and combination therapy with other classes of antibacterial agents to maximize the therapeutic potential have also been reviewed.