Microbiology and Silicone Hydrogels

Prof. Mark Willcox - Associate Professor Mark Willcox, BSc(Hons), PhD Associate Professor Mark Willcox is the Director of Science at the Vision Cooperative Research Centre (Vision CRC) He was awarded his BSc (Hons) from the University of the West of England, Bristol, in 1983, and in 1987 he gained his PhD in medical microbiology at the University of Manchester. In 1993 he took up a position at CRCERT where he is responsible for directing the various scientific studies, including research into the mechanisms of contact lens inflammation, biocompatibility of contact lenses and surfaces and infection models.

Successful continuous wear of silicone hydrogel s depends on many factors, but perhaps the most important of these is the prevention and proper management of adverse events. This can only be achieved by a proper understanding of what causes these events, and how they can best be treated. Bacterial colonisation of contact lens surfaces causes many adverse responses encountered during contact lens wear (Poggio et al., 1989; Holden et al., 1996; Sankaridurg et al., 1996; Sankaridurg et al., 1999; Sankaridurg et al., 2000; Corrigan et al., 2001; Jalbert et al., 1999; Keay et al. 2000). The most devastating of these is Microbial Keratitis (MK) which, if not treated promptly or with the proper antibiotics can lead to scarring and blindness. Microbial colonisation of lenses is also associated with other adverse responses, including Contact Lens Induced Acute Red Eye (CLARE), Contact Lens Induced Peripheral Ulceration (CLPU), Infiltrative Keratitis (IK) and Asymptomatic Infiltrates (AI). For a detailed clinical description of these conditions read the article by Holden et al. (2000).

Adverse events can be caused by a range of different bacteria. MK, for example, is most commonly caused by colonisation of contact lenses with the bacterium Pseudomonas aeruginosa (Schein et al. 1989), a gram-negative bacterium that is commonly found in many environments, including water. This is illustrated by the fact that historically, Pseudomonas infections of daily wearers is associated with poor hygiene practices, for example, the use of tap water. CLARE is associated with colonisation of contact lenses with gram-negative bacteria (in particular Haemophilus influenzae; Sankaridurg et al., 1996) or the gram-positive bacterium Streptococcus pneumoniae; Sankaridurg et al., 1999). H. influenzae and Strep. pneumoniae are commonly isolated from the throat, especially after acute flu-like symptoms and probably originate in that area and subsequently colonise the eye causing the inflammation that is characteristic of CLARE. As such, it is important to be careful with recommendations for patients with upper respiratory tract conditions, or at risk of such infections. CLPU, on the other hand, is produced by gram-positive colonisation of contact lenses, with the bacterium Staphylococcus aureus being the most commonly associated bacterium (Jalbert et al., 1999). . This highlights the importance of vigilantly treating patients with blepharitis, a condition caused by such gram-positive bacteria..Causes of IK and AI are many, but include colonisation by large numbers of gram-negative bacteria for IK and large numbers of gram-positive bacteria for AI.

We have examined the colonisation of the new silicone hydrogel contact lenses, worn on a 30 night continuous wear basis, by bacteria during wear and during adverse events. We have also studied the microbial numbers and types on the lids and conjunctiva during silicone hydrogel wear. We have compared this data to data collected on similar subject groups but wearing HEMA-based lenses on a 6 nights extended wear basis.

There is no difference in the frequency of isolation (Table 1) or numbers of colonies of the bacterium Staph. epidermidis colonising silicone or HEMA-based contact lenses. However, table 1 does demonstrate that there was a significant increase in percentage of lenses colonized by Propionibacterium sp. after 2 years of wear. The significance of this remains uncertain as the bacterium has not been reported to cause adverse responses during wear. The colonisation of lenses by the other normal ocular microbiota, Corynebacterium sp. did not alter during wear. We have previously reported that extended wear of HEMA-based lenses does not alter the frequency of isolation or the spectrum of microorganisms that colonise these lenses during 18 months extended wear (Gopinathan et al., 1997) and the data presented in the current study agree with this previous data. Table 2 demonstrates that there were no differences in the frequency of colonisation between HEMA-based or silicone hydrogel lenses. In addition, this table demonstrates that no differences in bacterial colonisation were evident when comparing lens contamination for 6 night extended wear HEMA-based vs silicone hydrogel lenses. Comparison of 6 night extended wear vs 30 night continuous wear silicone hydrogel lenses demonstrated a small increase in number of Propionibacterium sp. isolated.

As mentioned, we also examined whether the colonisation of the lids or conjunctiva were changed during silicone hydrogel wear vs HEMA-based lens wear. Table 3 demonstrates that the only differences noted were with Corynebacterium sp. which gave an increase in colonisation of lids during silicone hydrogel EW wear. The significance of this is again uncertain as this bacterium is not associated with adverse responses (it is another member of the normal microbiota). Previously we have reported that HEMA-based extended contact lens wear does increase the numbers of microorganisms that colonise those lenses (Stapleton et al., 1995).

Finally, we have cultured contact lenses during adverse responses (Figure 1). We have found no difference in the types of bacteria that are associated with adverse responses when subjects wear either silicone or HEMA-based hydrogel lenses. CLARE is associated with colonisation of lenses by gram-negative bacteria or Strep. pneumoniae and CLPU is associated with colonisation of lenses with gram-positive bacteria, in particular Staph. aureus or Strep. pneumoniae.

In conclusion, our studies have show that there are very few differences in the lens or ocular microbiota associated with wear silicone or HEMA-based hydrogel lenses. Similar types of the normal ocular microbiota are found at similar frequencies during uncomplicated lens wear. During adverse responses, the same kind of bacteria are isolated from lenses. Therefore, it will be of interest to determine whether the adverse response rate, and in particular the MK rate, is altered by wearing silicone hydrogel lenses. While there may be the same level and types of bacteria, it is possible that the added corneal health given by the high DK/t of the silicone lenses “protects” the cornea and makes, at least MK, less likely.

Figure 1. Changes in microbiota of silicone hydrogel lenses over 2 years of wear.

*, p<0.05 – greater colonisation at 2 years compared to either 1 month or 6 months extended wear
Data from Willcox et al., 2002

Figure 2. Comparison of lens colonisation on different soft contact lenses after 1 years wear

*, p<0.05 – greater frequency of isolation compared to 6N EW silicone hydrogel wear

Figure 3. Comparison of colonisation of eyes during wear of silicone hydrogel or HEMA-based hydrogel lenses

p<0.05 – greater frequency of isolation of Corynebacterium sp. From the lids of subjects wearing silicone hydrogel lenses than from those subjects wearing HEMA-based lenses
Data from Willcox et al., 2000

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Figure 4. Contact lenses cultured from subjects experiencing an adverse event. A. A typical contact lens cultured at the time of a CLARE. B. A contact lens cultured at the time of a CLPU. Note the agar plate surrounding the CLARE lens has many bacterial colonies that were initially attached to the lens and released during culture. The CLPU lens is typical in that only a few (in this case 2) bacterial colonies were isolated at the time of the event.

References

1. Corrigan KM, Harmis NY, Willcox MDP. 2001. Association of Acinetobacter sp. with contact lens induced adverse responses. Cornea 20, 463-466.

2. Gopinathan U, Stapleton F, Sharma S, Willcox MDP, Sweeney DF, Rao N, Holden BA 1997 Microbial contamination of hydrogel contact lenses during extended wear. J Appl Bacteriol. 82, 653-658.

3. Holden BA, Sankaridurg PR, Jalbert I. 2000. Adverse events and infections: which ones and how many? In: Sweeney DF (ed). Silicone Hydrogels, the rebirth of continuous wear contact lenses, Butterworth Heinemann, Oxford, UK.

4. Holden BA, La Hood D, Grant T, Newton-Howes J, Baleriola-Lucas C, Willcox MDP, Sweeney DF. 1996. Gram-negative bacteria can induce Contact Lens Related Acute Red Eye (CLARE) responses. CLAO J. 22, 47-51.

5. Jalbert I, Willcox MDP, Sweeney DF. 1999. Isolation of Staphylococcus aureus from a contact lens at the time of a contact lens induced peripheral ulcer: a case report. Cornea 19, 116-120.

6. Keay L, Harmis N, Corrigan K, Sweeney D, Willcox M. 2000. Infiltrative keratitis associated with extended wear of hydrogel lenses and Abiotrophia defectiva. Cornea. 19, 864-869.

7. Poggio EC, Glynn RJ, Schein OD, Seddon JM, Shannon MJ, Scardino VA, Kenyon KR. 1989. The incidence of ulcerative keratitis among users of daily wear and extended-wear soft contact lenses. New Eng. J. Med. 321, 779-783.

8. Sankaridurg PR, Willcox MDP, Sharma S, Gopinathan V, Janakiraman D, Hickson S, Vuppala N, Sweeney DF, Rao GN, Holden BA. 1996. Haemophilus influenzae adherent to contact lenses associated with production of acute ocular inflammation. J. Clin. Microbiol. 34, 2426-2431.

9. Sankaridurg PR, Sharma S, Willcox M, Sweeney DF, Naduvilath TJ, Holden BA, Rao GN. 1999. Colonization of hydrogel lenses with Streptococcus pneumoniae: risk of development of corneal infiltrates. Cornea. 18, 289-295.

10. Sankaridurg P, Sharma S, Willcox MDP, Sweeney DF, Holden BA, Rao GN. 2000. Increased bacterial colonization of disposable soft contact lenses during corneal infiltrative events versus asymptomatic extended lens wear. J Clin Microbiol. 38, 4420-4424.

11. Schein OD, Ormerod LD, Barraquer E, Alfonso E, Egan KM, Paton BG, Kenyon KR. 1989. Microbiology of contact lens-related keratitis. Cornea 8, 281-285.

12. Stapleton F, Willcox MDP, Fleming CM, Hickson S, Sweeney DF, Holden BA. 1995. Changes in the ocular biota with time in extended and daily wear disposable contact lens use. Infect Immun. 63, 4501-4505.

13. Willcox MDP, Harmis NY, Holden BA. 2002. Bacterial populations on high-Dk silicone hydrogel contact lenses: effect of length of wear in asymptomatic patients. Clin Exp. Optom. 85, 172-175.

14. Willcox M, Sankaridurg P, Lan J, Pearce D, Thakur A, Zhu H, Keay L, Stapleton F . 2000. Inflammation and infection and effects of the closed eye. In: Sweeney DF (ed). Silicone Hydrogels: the rebirth of continuous wear contact lenses. Butterworth-Heinemann, Oxford, UK.