Bacteriophage lysins as effective antibacterials

Abstract

Lysins are highly evolved enzymes produced by bacteriophage (phage for short) to digest the bacterial cell wall for phage progeny release. In Gram-positive bacteria, small quantities of purified recombinant lysin added externally results in immediate lysis causing log-fold death of the target bacterium. Lysins have been used successfully in a variety of animal models to control pathogenic antibiotic resistant bacteria found on mucosal surfaces and infected tissues. The advantages over antibiotics are their specificity for the pathogen without disturbing the normal flora, the low chance of bacterial resistance to lysins, and their ability to kill colonizing pathogens on mucosal surfaces, a capacity previously unavailable. Thus, lysins may be a much needed anti-infective in an age of mounting antibiotic resistance.

Figure 1. Basic structure of phage lytic enzymes

With one exception [16], the majority of the enzymes that have been characterized so far are built with two domains separated by a short linker (L): an N-terminal catalytic domain and a C-terminal cell binding domain ranging from 25–40 kDa in size. The catalytic domain retains the activity to cleave one of the four major bonds in the bacterial peptidoglycan. Thus, they fall into one of five classes, either a glucosaminidase an N-acetylmuramidase, an endopeptidase, an N-acetylmuramoyl-L-alanine amidase or a γ-D-glutaminyl-L-lysine endopeptidase. In rare cases, lysins may have two or three different catalytic domains. The C-terminal half of the molecule binds to a substrate in the target bacterial cell wall (usually carbohydrate). Sequence comparisons of enzymes in the same enzyme class indicate that the catalytic region is highly conserved while the C-terminal region is variable.

Figure 2. Electron microscopy of lysin treated bacilli

Thin section electron micrograph of B. cereus (RSVF) [9] after treatment with phage lytic enzyme PlyPH) [34]. A. High magnification of a bacillus exhibiting externalization of the cytoplasmic membrane after treatment with enzyme for 1 minute. B. Low magnification field of bacilli showing bacterial ghosts after 10 minutes of treatment with lysin.

Figure 3. Survival of pneumococcal bacteremia after i.v. treatment with Cpl-1

A typical protection experiment in which mice were given 10⁸ pneumococci intravenously One hour later they were treated with Cpl-1 lysin by the same route and followed for 48 hrs. All mice treated with lysin survived while most of the control animals that received buffer died.