Role of lysozyme as an antibacterial in tears

Purpose: Tears contain a number of antimicrobial proteins that help in protecting the ocular surface from microbial pathogens. Among these lysozyme is the major protein. Given its abundance in tears, it is expected to play a major role in tear defence. To investigate it further, this study aimed at studying the efficacy of lysozyme against ocular pathogenic bacteria to evaluate its role in antimicrobial protection of the ocular surface. Methods: The inhibition of growth of the ocular pathogenic bacteria (Staphylococcus aureus 31 and Pseudomonas aeruginosa 19) by a range of concentrations of lysozyme was tested in Mueller-Hinton broth using the broth dilution method. Time-kill assay was performed to observe total viable counts of bacteria surviving the lysozyme treatment. Scanning electron microscopy was used to observe the morphology of cells treated with lysozyme. Results: Lysozyme inhibited bacterial growth only to a small extent. A low percentage of growth inhibition (9 to 18% for 2.5 to 10mg/mL of lysozyme) was observed for Staphylococcus aureus 31, and a very little growth inhibition (2 to 6% for 2.5 to 10mg/mL of lysozyme) was observed for Pseudomonas aeruginosa 19. Time kill assay also showed that the bacterial cell death was not much affected by lysozyme. Scanning electron microscopy showed similar cell morphology for control and treated cells except that there was loss of aggregation in the treated cells in case of Staphylococcus aureus 31. Conclusions: In spite of its abundance in tears, lysozyme in not an efficient antimicrobial of the ocular surface by itself. This suggests that other proteins either individually or in combination with lysozyme might be more effective in protecting the ocular surface from pathogens. This will be tested in further investigations to understand the role of different proteins in tear defence.