Effect of hyperosmolarity on the antimicrobial properties of tear proteins

Purpose: Tear proteins play a protective role against microbial infections at the ocular surface. Lysozyme and lactoferrin are the most abundant antimicrobial proteins in tears. Our previous research showed that lysozyme and lactoferrin exhibit synergistic antimicrobial effects in normal tears. The purpose of this study was to determine if antimicrobial properties of tear proteins are affected by hyperosmolarity of tears, such as those occur in dry eye, to evaluate if individuals with dry-eye would be more prone to ocular infections due to the decreased antimicrobial defence at the ocular surface. Methods: Artificial tear (AT) solutions containing normal and 1.5 times salt concentration were prepared, and their osmolarity was measured using a vapour pressure osmometer. Overnight culture of an ocular pathogenic bacteria Pseudomonas aeruginosa, grown in M9 medium, was inoculated into the two types of AT solutions containing lysozyme and lactoferrin in concentrations found in tears. These were incubated at 37ºC for 24h and the bacterial growth was observed by taking viable counts on LB plates. Controls did not contain proteins. Results: In case of AT with osmolarity value near normal tears (305 mOsm/Kg), the bacterial growth decreased in the presence of lysozyme and lactoferrin as compared with the control where no proteins were present. In case of 1.5xAT with very high osmolarity value (478 mOsm/Kg), the bacterial growth was found to be more in the presence of proteins as compared with control. Conclusions: Antimicrobial effects of tear proteins decrease in hyperosmolar conditions leading to enhanced bacterial proliferation. This indicates that people with hyperosmolar tears or dry-eye will have decreased antibacterial defence at the ocular surface. The possible mechanism might be the high salt concentration affecting the structure and function of tear proteins rendering them less active as antimicrobials.