INVITED REVIEW Disturbing the balance: effect of contact lens use on the ocular proteome and microbiome Clin Exp Optom 2017; 100: 459–472 DOI:10.1111/cxo.12582 Maureen Boost* † DPhil FIBMS Pauline Cho* † PhD FAAO FBCLS Zhaoran Wang* BSc (Hons) *School of Optometry, The Hong Kong Polytechnic University, Hong Kong † Squina International Centre for Infection Control, Hong Kong E-mail: sopaulin@polyu.edu.hk Submitted: 18 March 2017 Revised: 29 May 2017 Accepted for publication: 5 June 2017 Contact lens wear is a popular, convenient and effective method for vision correction. In recent years, contact lens practice has expanded to include new paradigms, including ortho- keratology; however, their use is not entirely without risk, as the incidence of infection has consistently been reported to be higher in contact lens wearers. The explanations for this increased susceptibility have largely focused on physical damage, especially to the cornea, due to a combination of hypoxia, mechanical trauma, deposits and solution cytotoxicity, as well as poor compliance with care routines leading to introduction of pathogens into the ocular envi- ronment. However, in recent years, with the increasing availability and reduced cost of molec- ular techniques, the ocular environment has received greater attention with in-depth studies of proteins and other components. Numerous proteins were found to be present in the tears and their functions and interactions indicate that the tears are far more complex than for- merly presumed. In addition, the concept of a sterile or limited microbial population on the ocular surface has been challenged by analysis of the microbiome. Ocular microbiome was not considered as one of the key sites for the Human Microbiome Project, as it was thought to be limited compared to other body sites. This was proven to be fallacious, as a wide variety of micro-organisms were identified in the analyses of human tears. Thus, the ocular environ- ment is now recognised to be more complicated and interference with this ecological balance may lead to adverse effects. The use of contact lenses clearly changes the situation at the ocu- lar surface, which may result in consequences which disturb the balance in the healthy eye. Key words: contact lens, microbiome, ocular infection, proteome The eye is the primary sensory organ and its importance is reflected in terminology such as ‘windows to the soul’. For light to enter the eye, the epithelial layer has to be thin and as such, is more vulnerable to damage or infec- tion. Therefore, defence of the eye is of para- mount importance, as loss of integrity of the cornea can lead to sight-threatening condi- tions. Defence of the eye comprises physical structures, production of a wide range of anti- microbial substances and the maintenance of a stable microbiome. Disturbance of any of these elements can lead to further conse- quences and possibly diseases of the eye. Use of contact lenses has become increasingly popular, but while this form of vision correc- tion has many advantages, it does involve a disturbance to the environment of the eye. DEFENCE IN THE NORMAL EYE Tears Tears are produced by the lacrimal glands of which there are two types, the main and the accessory lacrimal glands. 1 The main lacrimal glands are located in the upper outer part of the eye sockets and release extra tears into the conjunctiva if the eyes are irritated or during crying. The acces- sory lacrimal glands, known as glands of Wolfring and glands of Krause, are located within the palpebral conjunctiva and main- tain the tear film on the surface of the cor- nea and conjunctiva. The tears drain to form the marginal tear strips or tear prisms at the upper and lower lid margins, and eventually, via the lacrimal canals to the naso-lacrimal duct. This process is effected by lid closure which facilitates tear move- ment. Lacrimal glands are larger in women, reaching their maximum volume in the teenage years. Differences have been observed between ethnic groups, with lar- gest volumes observed in Asian women. 2 The tear film is generally considered to consist of three layers. 1 Although more recent researchers have suggested that there may be as many as six layers, 3 a three-layer model better explains the characteristics and functions of tears. 4 The major components of the outer layer are lipids, produced by the meibomian glands in the eyelids. 5 The pres- ence of this layer prevents evaporation of the tears, as well as providing a smooth optical surface. This helps prevent contamination of the eye by dust and bacteria, as well as by skin lipids, which could otherwise destabilise the tear film by altering surface tension, leading to spilling of tears onto the skin. 6 The lipids in this layer vary, with non- polar lipids, including wax and sterol esters and tryglycerides, being outermost, cover- ing more polar lipids, such as phospholi- pids and sphingolipids. 7 Beneath this is the aqueous layer which contains ions, soluble mucins, enzymes and other proteins, which are predominantly produced by the lacri- mal glands, although other proteins may come from the corneal and conjunctival epithelial cells, as well as serum exudates and neutrophils, especially in the morn- ing. 8 The major proteins present are lacto- ferrin, immunoglobulin A (IgA), lysozyme, lipocalin and complement factors, which © 2017 Optometry Australia Clinical and Experimental Optometry 100.5 September 2017 459 CLINICAL AND EXPERIMENTAL