Factors influencing the vitek gram-positive susceptibility system's detection of vanB-encoded vancomycin resistance among enterococci

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Abstract
Studies were conducted to identify factors contributing to the inability of the Vitek Gram-Positive Susceptibility system (GPS; bioMerieux, Vitek, Inc., Hazelwood, Mo.) to reliably detect vanB-mediated vancomycin resistance among enterococci. To some extent the accuracy of the GPS depended on a particular strain's level of resistance, as all isolates for which vancomycin MICs were > or = 128 mu g/ml were readily detected but detection of resistance expressed by several strains for which MICs were < or = 64 mu g/ml was sporadic. Factors besides the level of resistance were studied in two vanB strains. For one strain (Enterococcus faecium U8304), the ability of GPS to detect resistance was accurate and consistent, while for the other (Enterococcus faecalis V583), GPS results were inconsistent and unreliable. Using these isolates, we established that growth medium had the most notable effect on the detection of resistance. In the absence of vancomycin, Vitek GPS broth supported growth comparable to that obtained with brain heart infusion broth for both E. faecium U8304 and E. faecalis V583. However, in the presence of vancomycin the growth patterns changed dramatically so that neither VanB strain grew well in Vitek broth, and growth of V583 was barely detectable after 8 h of incubation. In contrast, good growth of both strains was observed in brain heart infusion broth supplemented with vancomycin. Additionally, the same medium effect was observed with other inducibly resistant VanB strains. In conclusion, although Vitek broth can support good enterococcal growth, this medium does not sufficiently support expression of vancomycin resistance by certain strains to allow them to be detected by the Vitek automated system. Furthermore, this observation establishes that the type of growth medium used can substantially influence the expression of vancomycin resistance and indicates that medium-based strategies should be explored for the enhancement of resistance detection among commercial systems.

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