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K.S. Ambrus, N. Azizi, W. Chang, T. Denbo; Effect of Growth Media on the Kill of Staph. aureus by Contact Lens Multipurpose Solutions . Invest. Ophthalmol. Vis. Sci. 2005;46(13):919.
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Contact lens multipurpose solutions sold in the US and in Europe are tested according to the FDA (510(k)) guidelines, or ISO guidelines. This testing includes disinfection efficacy testing of a panel of five test organisms. Staph aureus, ATCC 6538 is one of the five organisms. Both guidelines require growth of this organism on Tryptic Soy Agar (TSA) prior to testing.
Various journal publications have reported growing the organism in the liquid version of the above media instead – Tryptic Soy Broth (TSB). The scope of our study was to establish the effect of the growth media on the outcome of disinfection efficacy studies vs. S. aureus.
We prepared the test culture in two ways:
1) We grew the test organism on TSA slant for 18–24 hours, harvested it with sterile saline, and adjusted on a spectrophotometer to result in a final inoculum in the product of 1–10 x 105 CFU/mL.
2) We grew the test organisms in a test tube containing 10 mL TSB for 18–24 hours, then centrifuged the cells at 4000 rpm for 10 minutes. We washed the cells once more with Dulbecco's PBS, using centrifugation as above. We suspended the final pellet in Dulbecco's PBS, and adjusted the culture spectrophotometrically as above.
We then inoculated three marketed contact lens multipurpose solutions with the two culture preparations to contain the final inoculum level of 1–10 x 105 CFU/mL:
Solution A – 1 ppm polyhexamethylene biguanide, taurine.
Solution B – 0.001% POLYQUAD®, 0.0005% ALDOX®.
Solution C – MoistureLocTM, 0.00045% alexidine.
We evaluated the levels of survivors at 4 and/or 6 hour contact time points, using Letheen Broth.
The results are presented in terms of log–drops at the 4 and/or 6 hour contact time points.
* no recovery of test organism (<10 CFU/mL)
These data show that the growth medium affects the test results to the degree where it could affect the outcome of the performance criteria set by the FDA or ISO. To claim "stand–alone" disinfection, the log–drop for S. aureus must be at least 3.0 logs at the soak time. The guidelines also prescribe the growth medium prior to testing, and specify that this must be Tryptic Soy Agar (TSA), and not the broth (TSB).
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