Vol.13  No. 1,  October 2003

A STUDY OF FACTORS AFFECTING BACTERIAL ADHERENCE TO VARIOUS IOL MATERIALS (POLYMETHYL METHACRYLATE AND FOLDABLE INTRA-OCULAR LENSES)

Dr. Jagat Ram,¹ Dr.  Jaspreet Sukhija,¹ Dr.  Gangandeep S. Brar,¹ 
Dr.  Pallab Ray, ² Dr.  Amod Gupta¹

INTRODUCTION

Postoperative pseudophakic endophthalmitis is a rare but vision threatening complication following cataract extraction and intra-ocular lens implantation¹.  Various studies have reported an incidence of postoperative pseudophakic endophthalmitis ranging from 0.06% - 0.1%. ²

Contaminated intraocular lenses have been incriminated as one of the various risk factors for postoperative endophthalmitis.³  There is evidence to suggest that chronic low-grade endophthalmitis developing after intraocular lens implantation is caused by intraoperative contamination with a variety of low-virulent microorganisms and subsequent colonization of organisms on the intraocular lens and capsular bag.⁴

AIMS AND OBJECTIVES

A.    To evaluate the surface properties of intraocular lenses affecting bacterial adherence using Scan Electron Microscopy.

B.    To evaluate the factors affecting in vitro adherence of Staphylococcus epidermidis to the PMMA and foldable IOLs materials.

MATERIALS AND METHODS

INTRAOCULAR LENSES

For the purpose of this study forty eight sterile posterior chamber IOLs with biconvex optic were used.  Each IOL had an optic diameter of 6.0 mm.  Three different types of lenses were used in each experiment.  The sterile packing of each IOL was opened immediately prior to suspension to ensure minimum  exposure to room air.  The lenses used were:

1. One piece PMMA  posterior chamber IOLs ( Aurolab S-3603 n= 16)

2. Heparin Surface Modified PMMA IOLs ( Pharmacia & Upjohn 811C n= 16)

3. Acrylic optic PMMA haptic foldable IOLs ( Allergan, USA, Sensar AR40 n= 16)

TEST ORGANISMS

In this study, we used Staphylococcus epidermidis strain, ATCC 155.  It is a well characterized laboratory staphylococcus strain whose adherence characteristics are known.  The strain was obtained from Microbial Type Culture Collection of the Institute of Microbial Technology, Chandigarh.

PROCEDURE

PREPARATION OF BACTERIAL SUSPENSION

The organism was harvested by centrifugation and resuspended in balanced salt solution (BSS) to 0.5 Mc Farland turbidity standard ( approximately 10 organisms/ ml).

ASSAY PROCEDURE

16 IOLs of each material were divided into two sets of 8 IOLs each.  Each IOL was in a sterile packing, which was opened immediately prior to suspension.  Two IOLs of each material were placed under aseptic condition into 1ml of Staphylococcus epidermidis suspension (at two different concentrations of 10 ⁵ CFU / ml and 10 ⁹ CFU / ml) contained in a 1.5 ml microcentrifuge tube and incubated at 37 ⁰ C in 2-3% CO₂ for 2 and 15 minutes.

After incubation each IOL was removed from the suspension and placed in 5ml of sterile BSS in a 30ml plastic container, and vortexed for 60 seconds to wash the IOL.  This step was repeated with 5ml of fresh BSS.

The number of organisms detached from each IOL was estimated by the method of Miles and Misra as described below.

Therefore, the dilutions that we had were:

1.	N	Neat preparation undiluted
2.	N /10 ( N x 10 -1 )	First dilution
3.	N /100 ( N x 10 -2 )	Second dilution
4.	N /1000 ( N x 10 -3 )	Third dilution
5.	N /10000 ( N x 10 -4 )	Fourth dilution
6.	N /100000 ( N x 10 -5 )	Fifth dilution
7.	N /1000000 ( N x 10 -6 )	Sixth dilution

INOCULATION OF FLUID

Inoculation was done on blood agar culture plates.  These were divided into six sectors, i.e. one sector for each dilution and marked appropriately.  Using a 10 microlitre auto pipette, three drops of fluid ( 10 microlitre volume each ) from each dilution was placed on the corresponding sector of the blood agar plates.  After the drops dried the culture plates were inverted and incubated at 37 ⁰ C in 2-3% CO₂ for 24 hours  and the number of colony forming units were counted.

On the assumption that each colony arises from one organism it is possible to calculate the number of organisms present in the original undiluted sample and therefore, on the surface of the IOL.

SCAN ELECTRON MICROSCOPY

One set of each material was evaluated for Scan Electron Microscopy (SEM) for surface evaluation without sonication.  All the IOLs were sputter-coated with gold.  Scanning Electron Microscopy was done using Jeol JM 6300 at RSIC, Punjab University, Chandigarh.  Special attention was paid to the optic surfaces, edges , haptic optic junction and haptic itself.  The area of the IOL with maximum bacterial adherence was observed for any irregularity, tear line, furrows, fracture, cracks and surface finish or any defects.

RESULTS

Twenty four values of adherent organisms / IOL were obtained, eight each for PMMA, Acrylic and HSM IOLs.  The number of organisms adherent / IOL to HSM IOLs ( 239875±224863.4109) were significantly less than the number of organisms adherent to PMMA IOLs ( 431000 ± 498508.633) p < 0.05 and also significantly less as compared to the organisms adherent to acrylic IOLs ( 531500 ± 7761448.687). Table - 1

TABLE 1.    BACTERIAL ADHERENCE IN DIFFERENT IOL MATERIALS

Intraocular Lens No.	No. of organisms adherent / PMMA IOL	No. of organisms adherent / HSM IOL	No. of organisms adherent /  ACRYLIC IOL
1 ( 2 mts.)	1.2 X 10 5	1.2 X 10 5	1.4 X 10 5
2 ( 2 mts.)	8.0 X 10 3	1.6 X 10 6	0.3 X 10 4
3 ( 2 mts.)	1.8 X 10 5	2.0 X 10 6	1.6 X 10 4
4 ( 2 mts.)	1.2 X 10 4	10.0 X 10 6	2.0 X 10 4
5 ( 15 mts.)	1.3 X 10 6	2.1 X 10 6	6.0 X 10 5
6 ( 15 mts.)	1.1 X 10 6	2.5 X 10 6	4.0 X 10 5
7 ( 15 mts.)	5.0 X 10 5	2.3 X 10 7	3.6 X 10 5
8 ( 15 mts.)	1.2 X 10 5	1.2 X 10 6	3.8 X 10 5
Total	417500 ± 509475.6	5315000 ± 7761448.687	239875 ± 224863.4109

Thus we observed that the adherence of Staphylococcus epidermidis to intraocular lenses was found to vary with the material of intraocular lenses and the duration of exposure to the organisms, being significantly less with shorter duration exposure ( two minutes ) as compared to longer duration of exposure ( fifteen minutes ) and also significantly less for Heparin Surface modified IOL's compared to PMMA and acrylic IOLs.

DISCUSSION

In the present in-vitro study, we have demonstrated using viable counting that Staphylococcus epidermidis attaches to IOL surfaces.  The calculated number of bacteria adherent / IOL represented irreversibly adherent bacteria as evidenced by resistance of organisms to removal by vortexing. This adherence involves slime production which is very characteristic of pathogenic strains of Staphylococcus epidermidis.  This is in contrast to bacterial adherence which involves increased attraction due to electrostatic and Van der Walls forces and hydrophobic bonds and is reversible. ⁵

In summary, Staphylococcus epidermidis adhesion to IOLs may play role in the pathogenesis of postoperative endophthalmitis, and longer duration of preoperative intraoperative exposure of IOL to the offending organism may increase the chances of infection by increasing the inoculum of bacteria introduced into the eye.  Selection of the IOL biomaterial may also influence the rates of postoperative endophthalmitis.  Acrylic IOLs have higher adherence compared to PMMA or HSM IOLs.  Although no direct correlation has been observed till date between the use of foldable IOL materials like Acrylic or Silicone and the occurrence of postoperative endophthalmitis, it would be prudent to keep the adherence characteristics of these materials in mind before IOL selection, especially in high risk cases.

REFERENCES

1. Lawin-brussel  CA, Rafojo MF, Kenyon KR. In vitro adhesion of Pseudomonas aeruginosa and Staphylococcus aureus to surface passivated PMMA intraocular lenses. J Cataract Refract Surg 1992; 18: 598-601.

2. Kattan HM, Flynn HW, Pfugfelder SC, Forster RK, Robertston C. Nosocomial endophthalmitis survey- Current incidence of infection after intraocular surgery.  Ophthalmology 1991; 98: 227-238.

3. Driebe WT, Jr  Mandelbaum S, Forster RK et al. Pseudophakic endophthalmitis- diagnosis and management.  Ophthalmology 1986; 93: 442-448.

4. Nobe JR, Finegold SM, Smith RE. Chronic anaerobic endophthalmitis in pseudophakic rabbit eye. Invest Ophthalmol Vis Sci 1986;27 ( ARVO abstract. Suppl): 38.

5. Griffiths PJ, Elliot TS, Mc Taggart L.  Adherence of Staphylococcus epidermidis to intraocular lenses.  Br J Ophthalmol 1989; 73: 402-406.

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Departments of Ophthalmology¹and  Microbiology² , 
Post Graduate Institute of Medical Education and  Research, Chandigarh

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