2007 OMIG, Abstract 3

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Use of Cumulative MIC Distributions to Document Shifting (declining) Vancomycin Susceptibility Trends  among Ocular Gram Positive Pathogen Populations
Darlene Miller DHSC, MPH, Eduardo C. Alfonso, MD, Edith Perez BS, Martha Diaz BS
Bascom Palmer Eye Institute, Department of Ophthalmology, University of Miami School of Medicine,  Miami, FL

Purpose: To document incremental decline in in vitro susceptibility to vancomycin among ocular gram positive pathogens populations.
Methods:  Cumulative  vancomycin MIC distribution data for Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pneumoniae and Streptococcus viridans group were collected and compared for susceptibility by concentration range for time periods 1990-1994, 1995-1999, 2000-2004, and 2005-June 30, 2007. MICs were determined using the Vitek Auto Microbial System and supplemented with the E tests. MIC concentrations ranged from 0.25 µg-32 µg/ml (vitek) and 0.016-256 µg/ml for the E tests. Susceptible MICs were <1 µg/ml for S. pneumoniae and Streptococcus viridans grp.; <2 µg/ml for S. aureus and <4 µg/ml for S. epidermidis.
Results: Modal MIC (MIC₅₀)-<1 µg/ml and MIC₉₀ (2 µg/ml) for S. aureus remain stable from baseline (1990-1994, N=634) to the current period (N=452).  However, at baseline 98% of isolates were susceptible to a vancomycin concentration of <1 µg/ml, but the percentage of isolates susceptible to this concentration declined to 62% for the most recent evaluation period. At baseline 94% of S. epidermidis isolates (N=125) were inhibited at <1 µg/ml, but only 20% of the 2005-June 2007 (N=63) isolates were susceptible to this concentration. Nonsusceptible vancomycin isolates were detected among S. pneumoniae (1, 3%, N=33) and Streptococcus virdians (2000-2004, 3, 7%, N=45, 2005-2007, 1,2.6%, N=39) populations during the most current evaluation periods. Greater than 95% of the Streptococcus isolates had MIC₉₀s of <1 µg/ml.
Conclusions.  Increasingly higher concentrations of vancomycin are needed to kill common ocular gram positive pathogens. Vancomycin should be reserved for severe, nonresponsive, gram positive infections.

Disclosure code: N

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