Cell Wall Hydrolytic Enzymes Enhance Antimicrobial Drug Activity Against Mycobacterium

Cell wall hydrolases are enzymes that cleave bacterial cell walls by hydrolyzing specific bonds within peptidoglycan and other portions of the envelope. Two major sources of hydrolases in nature are from hosts and microbes. This study specifically investigated whether cell wall hydrolytic enzymes could be employed as exogenous reagents to augment the efficacy of antimicrobial agents against mycobacteria. Mycobacterium smegmatis cultures were treated with ten conventional antibiotics and six anti-tuberculosis drugs-alone or in combination with cell wall hydrolases. Culture turbidity, colony-forming units (CFUs), vital staining, and oxygen consumption were all monitored. The majority of antimicrobial agents tested alone only had minimal inhibitory effects on bacterial growth. However, the combination of cell wall hydrolases and most of the antimicrobial agents tested, revealed a synergistic effect that resulted in significant enhancement of bactericidal activity. Vital staining showed increased cellular damage when M. smegmatis and Mycobacterium bovis bacillus Calmette-Guérin (M. bovis BCG) were treated with both drug and lysozyme. Respiration analysis revealed stress responses when cells were treated with lysozyme and drugs individually, and an acute increase in oxygen consumption when treated with both drug and lysozyme. Similar trends were also observed for the other three enzymes (hydrolase-30, RipA-His₆ and RpfE-His₆) evaluated. These findings demonstrated that cell wall hydrolytic enzymes, as a group of biological agents, have the capability to improve the potency of many current antimicrobial drugs and render ineffective antibiotics effective in killing mycobacteria. This combinatorial approach may represent an important strategy to eliminate drug-resistant bacteria.