|Year : 2001 | Volume
| Issue : 1 | Page : 15-18
Bacterial adherence to polymethylmethacrylate posterior chamber IOLs
Shalini Tyagi, Jagat Ram, Pallab Ray, Gagandeep S Brar, Amod Gupta
MD. Department of Ophthalmology and Microbiology, Postgraduate Institute of Medical Education and Research, Chandigarh, India
MD. Department of Ophthalmology and Microbiology, Postgraduate Institute of Medical Education and Research, Chandigarh
Source of Support: None, Conflict of Interest: None
Purpose: Bacterial adherence to intraocular lenses (IOLs) has been incriminated in the pathogenesis of postoperative endophthalmitis. Staphylococcus epidermidis is the most common organism isolated. We studied the in-vitro adhesion of Staphylococcus epidermidis to Polymethylmethacrylate (PMMA) IOLs and the effect of duration of exposure to adherence.
Methods: Two groups of 10 IOLs each were incubated in Staphylococcus epidermidis suspension for 2 minutes and 20 minutes respectively. Adhesion of bacterial cells was determined by counting the number of viable bacteria attached to IOLs.
Results: The mean bacterial adherence with 2 minutes incubation was 12,889 ± 7,150 bacteria / IOL and with 20 minutes incubation was 84,226 ± 35,024 bacteria/IOL (P< 0.01).
Conclusion: Our results show that Staphylococcus epidermidis adheres to PMMA IOLs in vitro and the degree of adherence is less for shorter duration of exposure. We conclude that viable bacteria irreversibly adherent to IOLs may play a role in the pathogenesis of postoperative endophthalmitis. Shorter duration of operative manipulation and exposure to contaminating sources may decrease the chances of postoperative endophthalmitis.
Keywords: Intraocular lenses, bacterial adhesion, endophthalmitis, Staphylococcus epidermidis
|How to cite this article:|
Tyagi S, Ram J, Ray P, Brar GS, Gupta A. Bacterial adherence to polymethylmethacrylate posterior chamber IOLs. Indian J Ophthalmol 2001;49:15-8
|MEAN BACTERIAL ADHERENCE AFTER 2 MINUTES WAS 12889.00 ± 7151.79/IOL AND AFTER 20 MINUTES WAS 84226.00 ± 35024.32/IOL (P<0.01).|
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Postoperative pseudophakic endophthalmitis is a vision-threatening complication following cataract surgery and intraocular lens (IOL) implantation. Reported incidence varies from 0.06-0.1%. [1,2] Although it is a rare complication of intraocular surgery, its impact is significant if we consider the number of cataract surgeries done annually.
|MEAN BACTERIAL ADHERENCE AFTER 2 MINUTES WAS 12889.00 ± 7151.79/IOL AND AFTER 20 MINUTES WAS 84226.00 ± 35024.32/IOL (P<0.01).|
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Coagulase negative Staphylococci and in particular Staphylococcus epidermidis are currently the most commonly isolated organisms from cases of postoperative pseudophakic endophthalmitis, [1, 3, 4] Contaminated IOLs have been incriminated as one of the risk factors for development of postoperative endophthalmitis.[5-9] Coagulase-negative staphylococcal infections are also associated with implantation of other prosthetic surgical devices like cerebrospinal fluid (CSF) shunts, intravascular catheters, pacemaker leads and endotracheal tubes.[10-13] It seems likely that these microrganisms opportunistically infect the medical appliances because they preferentially adhere to the surface of the appliance.
Bacterial adhesion can be divided into two stages. The first stage involves reversible attraction due to electrostatic and van der Waals forces and hydrophobic bonds. The IOLs due to their surface electrostatic charge may become contaminated by bacteria on touching the ocular surface or even after exposure to the operating room air by this mechanism. Stage two involves irreversible adherence and persistence. In case of Staphylococcus epidermidis, this is achieved by production of a surface slime that enables bacteria to maintain colonization by adhering as macro-colonies on the foreign body surface. Surface slime also protects the organism from phagocytosis and action of antimicrobial agents.
This is significant since Staphylococcus epidermidis has been isolated from 69% of normal eyes, and is also the commonest organism isolated from the conjunctival sac at the time of cataract extraction. Molecular strain typing has also revealed eyelid isolates identical to intraocular isolates in 67.7% and 82% of cases of postoperative endophthalmitis in two series. [17,18] Hydrophobic bacteria may also be rejected from aqueous fluid and attached to non-aqueous phase implant surfaces. This is relevant since anterior chamber fluid is reported to be culture-positive in 1.7-43% cases following uncomplicated cataract extraction.[20-24]
In the present study, we demonstrated with an in-vitro model the adherence of Staphylococcus epidermidis to Polymethylmethacrylate(PMMA) IOLs and also studied the effect of duration of exposure on the degree of adhesion.
| Materials and Methods|| |
We used 20 sterile all PMMA posterior chamber IOLs (Aurolab, Madurai, India). A bacterial suspension of standard strain of Staphylococcus epidermidis (strain MTCC-155), at a concentration of 108 organisms/ml was used. The 20 IOLs were divided into two sets of 10 IOLs each. The sterile packing of each IOL was opened immediately before dipping in the bacterial suspension to ensure minimal exposure to room air. Bacterial adherence to IOLs was measured using the viable counting method. The IOLs were placed separately under sterile conditions in 1 ml of Staphylococcus epidermidissuspension and incubated at 37°C in 5% CO2. One set was incubated for 2 minutes and the other for 20 minutes.
After incubation, each IOL was removed from the suspension and placed in 5 ml of a standard buffer solution (RPMI-1640) and washed by vortexing for 60 seconds. This step was repeated with 5 ml of fresh RPMI-1640. Subsequently, the IOL was placed in 1 ml of RPMI-1640 in 10 ml plastic tubes and homogenised for 30 seconds. This regimen removes all adherent bacteria without affecting their viability. The number of organisms detached from each IOL was estimated by the method of Miles and Misra. The RPMI-1640 solution containing the dislodged organisms was serially diluted 6 times by a factor of 10 each time. 100 microlitre from each dilution was plated on a blood agar plate. The plates were incubated for 24 hours and the dilution yielding the highest number of discrete colonies was taken into consideration. On the assumption that each colony arises from one organism, the number of organisms present in the original RPMI-1640 solution containing the dislodged bacteria was calculated by multiplying the appropriate power of 10. This was equal to the number of bacteria adherent per IOL. Overall, 20 values were obtained, 10 each for incubation at 2 and 20 minutes. Mean + standard deviation was computed for each group and Student's 't' test was applied to determine statistical significance. P value of <0.05 was considered significant.
| Results|| |
The number of colonies of Staphylococcus epidermidis was found to be higher in IOLs incubated for 20 minutes [Figure:1b] compared to those incubated for 2 minutes [Figure:1a]. From serial dilution, the number of bacteria adherent to each IOL was calculated. Overall, there were 20 observations, 10 values for IOLs incubated for 2 minutes and 10 for those incubated for 20 minutes. The number of organisms (Staphylococcus epidermidis) adherent to each IOL varied from 3,300 to 43,300 with 2 minutes incubation and from 34,000 to 130,000 in IOLs incubated for 20 minutes Table.
The mean number of Staphylococcus epidermidis adherent to IOLs incubated for 2 minutes was 12,889.00 ± 7,151.79/IOL. The mean number of bacteria adherent/IOL for the 20 minutes group was 84,226.00 ± 35,024.32/IOL. The difference between the two groups was statistically significant (p <0.01).
| Discussion|| |
The first report of coagulase-negative Staphylococcus endophthalmitis confirmed by culture of aqueous removed by anterior chamber paracentesis was published in the early 1970s. Now these organisms are implicated in up to 60% of cases of postoperative endophthalmitis. The rise in incidence not only parallels an increase of the organism as a pathogen but also closely parallels the increased use of IOLs.
Coagulase-negative Staphylococci and in particular Staphylococcus epidermidis are the major cause of sepsis following prosthetic surgery of all types. Their role in the pathogenesis of prosthetic implant infection has been demonstrated in studies on different biomaterials.[10-13] In the present study, using viable counting, we have demonstrated that Staphylococcus epidermidis attaches to PMMA IOL surface. The calculated number of bacteria adherent per IOL represents irreversibly adherent bacteria as is evident by resistance of organisms to removal by vortexing (Stage II adherence). Various in-vitro studies on PMMA and other IOL materials have demonstrated that Staphylococcus epidermidis is capable of colonizing IOLs in vitro.[27-30]
In addition, we have also demonstrated that the extent of adherence to IOLs is related to time. Shorter duration of exposure to the organism significantly reduced bacterial adherence. This is in agreement with the findings of Griffith et al who reported increased number of adherent organisms with longer duration of suspension in Staphylococcus epidermidis culture broth. Their period of incubation ranged from 1 to 16 hours. Our choice of duration of exposure of 2 minutes and 20 minutes is more relevant in the clinical setting, as the time taken for the insertion of IOL and the period during which it is exposed to room air and periocular tissues may be closer to these values.
High degree of contamination on IOLs has been observed in various in-vivo studies. [5, 8, 9] Thus Staphylococcus epidermidis adhesion to IOLs may be an important factor in the pathogenesis of endophthalmitis, providing a surface to which bacteria can adhere. The bacteria may subsequently cross the posterior capsule to multiply in the vitreous cavity.
Given the high bacterial adherence observed, we would expect clinical endophthalmitis in a much higher percentage of pseudophakic eyes. Various reasons could explain the discrepancy between our in-vitro findings and clinical experience, namely, low virulence of bacteria involved and use of topical steroids and antibiotics in the postoperative period. Additionally the IOL surface may undergo changes after implantation such as protein absorption that could affect bacterial growth. Factors that combat a low inoculum might include ocular defence mechanisms, including immunoglobulins circulating in the aqueous that neutralize bacteria and potentiate phagocytosis, the filtration capacity of the trabecular meshwork and the phagocytic activity of the trabecular endothelium.
We do not known how low the bacterial counts must be to decrease the risk of infection. Whenever the number of intraocular bacteria exceed the critical limit, the host immunity is overwhelmed, and clinical endophthalmitis results. We have demonstrated that the extent of adherence is significantly greater with longer duration of exposure. Hence a longer peroperative manipulation and increased IOL insertion time may increase the inoculum of bacteria adherent to the IOL and predispose the patient to increased risk of endophthalmitis.
The usefulness of several currently recommended methods of reducing postoperative endophthalmitis [5, 9, 32, 33] which are can be substantiated by our study. These include minimising the number of viable microbes in the conjunctival sac and the lid margin by preoperative topical antibiotics and 5% povidone-iodine preparation of the conjunctiva prior to surgery. Adequate draping of lids and eyelashes away from the surgical site prevents contact with the IOL surface.The exposure time of the IOL prior to insertion can be minimised by removing the IOL from its case just before surgery and also reducing the operative time as far as possible.
In conclusion, our study demonstrates that Staphylococcus epidermidis adheres to PMMA IOLs in-vitro and the degree of adherence is directly related to the duration of exposure. In a clinical setting, this would imply that Staphylococcus epidermidis adherent to the IOL surface and carried into the eye may play a role in the pathogenesis of postoperative endophthalmitis. Further clinical studies on adherence characteristics of IOLs may provide more avenues for prevention and treatment of postoperative endophthalmitis.
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[Figure - 1], [Figure - 2]
[Table - 1]