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A. A. Ghanekar, C. Leong, E. Li, J. J. Mun, N. R. Acharya, S. M. J. Fleiszig; P. aeruginosa Isolates from the SCUT Study (Non-Contact Lens Wearing Population) Reveals Multiple Strain Types, With a Majority Invasive Strains. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5114.
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The Steroid for Corneal Ulcers Trial (SCUT) is an NEI-funded, randomized, placebo-controlled trial enrolling patients at the F.I. Proctor Foundation at the University of California San Francisco, Aravind Eye Hospital in South India and Dartmouth-Hitchcock Medical Center. The objective is to study the effect of topical corticosteroids on bacterial corneal ulcers, as measured by clinical outcomes. Our previous studies demonstrated that P. aeruginosa corneal isolate type (invasive vs. cytotoxic) influences treatment outcome in an animal model. Here, we examined a subset of P. aeruginosa isolates collected from SCUT study patients to determine the feasibility of exploring if strain type also influences treatment outcome in people.
Genotyping was done to test for presence or absence of the genes encoding for the Type III secretion effectors ExoS, ExoU and ExoY. Typically, invasive strains are exoS+/exoU-, while cytotoxic strains are exoS-/exoU+. Phenotyping was performed in vitro by Trypan blue staining (to examine cytotoxic capacity) and gentamicin survival assays (to quantify invasion capacity).
Genotyping and phenotyping have been completed for 24 isolates with 15 displaying a typical invasive genotype (exoS+/exoU-) and 5 a typical cytotoxic genotype (exoS-/exoU+). Phenotypes are all as predicted, i.e. cytotoxic strains kill corneal cells and invasive strains enter cells efficiently. Of the remaining 4 isolates, 2 possess both genes (exoS+/exoU+) and are phenotypically unusually invasive, while two possess neither gene (exoS-/exoU-), are phenotypically invasive and lack exoY. Two other strains lack exoY, one invasive and one cytotoxic. An additional 29 isolates have been received. All 53 have been examined for the presence of exoY (44/53, 84%) and 40 for exoU (10/40, 25%).
In this population (non-contact lens wearers) a majority of invasive strains and a small number of non-typical genotypes were found. This differs from previous studies that have included contact lens wearers in which cytotoxic strains were noted in larger numbers, but is similar to canine corneal infections, another non-lens wearing situation. It has been previously suggested that cytotoxic strains are more common in contact lens-related infections based on trends noted in two separate studies. Nevertheless, the existence of multiple strain types in the SCUT study population should enable the impact on different clinical signs at enrollment and response to treatment to be examined.
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