Specificity and diversity of <i>Klebsiella pneumoniae</i> phage-encoded capsule depolymerases

Klebsiella pneumoniae is an opportunistic pathogen with significant clinical relevance. K. pneumoniae-targeting bacteriophages encode specific polysaccharide depolymerases with the ability to selectively degrade the highly varied protective capsules, allowing for access to the bacterial cell wall. Bacteriophage depolymerases have been proposed as novel antimicrobials to combat the rise of multidrug-resistant K. pneumoniae strains. These enzymes display extraordinary diversity, and are key determinants of phage host range, however with limited data available our current knowledge of their mechanisms and ability to predict their efficacy is limited. Insight into the resolved structures of Klebsiella-specific capsule depolymerases reveals varied catalytic mechanisms, with the intra-chain cleavage mechanism providing opportunities for recombinant protein engineering. A detailed comparison of the 58 characterised depolymerases hints at structural and mechanistic patterns, such as the conservation of key domains for substrate recognition and phage tethering, as well as diversity within groups of depolymerases that target the same substrate. Another way to understand depolymerase specificity is by analyzing the targeted capsule structures, as these may share similarities recognizable by bacteriophage depolymerases, leading to broader substrate specificities. Although we have only begun to explore the complexity of Klebsiella capsule depolymerases, further research is essential to thoroughly characterise these enzymes. This will be crucial for understanding their mechanisms, predicting their efficacy, and engineering optimized enzymes for therapeutic applications.