TREK Diagnostic Systems supplies Sensititre automated AST/ID instrumentation to clinical diagnostic laboratories throughout the world. Sensititre susceptibility plates can be read automatically using the ARIS® 2X or AutoReader instruments. Recently, the Sensititre automated system accurately detected four Vancomycin-resistant Staphylococcus aureus (VRSA) strains at the Centers for Disease Control (CDC).* A brief summary is presented here, full details can be obtained from Trek Diagnostics.

In regards to strains referenced by the CDC, the Sensititre automated method detected the New York (NY5734), Michigan and Pennsylvania VRSA isolates after 24 hours incubation generating MIC's of 16 - >32 µg/ml. Sensititre manual MIC methods also exhibited MICs of >32 µg/ml for these strains after 24 hours incubation. Reference microbroth dilution (MBD) plates exhibited MICs for 32-1024 µg/ml for all three strains. Other automated tests systems failed to detect resistance or generated MICs well below the reference microbroth dilution method.

The New York strain (NY595) was detected automatically with Sensititre MIC plates after isolation on BHI-V6 screen plates (Brain heart infusion agar with 6 ug/mL Vancomycin agar), thereby generating an MIC of > 32 µg/ml. Although the Sensititre automated method did not detect resistance in the original New York strain (NY595) on blood agar (MIC < 1µg/ml), no automated method was able to detect Vancomycin-resistance with this challenging isolate. This strain was characterized as having a Vancomycin-resistant phenotype that was unstable while others were stable.

Laboratories typically choose Sensititre susceptibility products based on superior performance for many bug/drug combinations, whether the test is automated or manual. Sensititre Gram-positive MIC plates have the flexibility of allowing automated and manual MIC read methods after overnight incubation; therefore, technologists can examine MIC results for unusual MIC patterns, like VRSA, in conjunction with "SWIN™ expert susceptibility rules". The Sensititre automated system has an increased chance of detecting resistance** on these isolates as it yields a true microbroth dilution result only after 24 hours incubation; other tests systems may rely on faster detection algorithms and extrapolated MIC results, thereby missing important resistance mechanisms. However, along with our system, the CDC recommends the set-up of BHI-V6 screen plates to enhance detection of these challenging VISA/VRSA strains.

TABLE: Results from Poster D-66. Failure of Automated Systems to Detect Vancomycin-Resistant Staphylococcus aureus, R.B. Carey, et. al. CDC, Atlanta, GA.

Isolates were tested on MIC plate (Part No. GPN2F); this plate contains the following antimicrobial agents: Ampicillin, Cefazolin, Ceftriaxone, Ciprofloxacin, Clarithromycin, Erythromycin, Gatifloxacin, Gentamicin, Levofloxacin, Linezolid, Oxacillin, Penicillin, Quinupristin/Dalfopristin, Rifampin, Streptomycin, Tetracycline, Trimethoprim/Sulfamethoxazole, Vancomycin.

TREK Diagnostic Systems recently added a new Gram-positive plate (Part No. GPN3F) containing the following antimicrobics: Ampicillin, Ceftriaxone, Ciprofloxacin, Clindamycin, Daptomycin, Erythromycin, Gatifloxacin, Gentamicin, Levofloxacin, Linezolid, Oxacillin, Penicillin, Quinupristin/Dalfopristin, Rifampin, Streptomycin, Tetracycline, Trimethoprim/Sulfamethoxazole, Vancomycin.

If you would like additional details regarding system performance, please contact Liz Lloyd at TREK Diagnostic Systems, contact details above

* NY 595 strain detected when isolated on 6 µg/mL Vancomycin agar prior to automated read methods; original strain isolated on BAP yielded an automated MIC of <= 1 µg/ml
** Because there are not an adequate number of strains available for testing, the ability of the Sensititre system to detect resistance to Vancomycin is unknown. Additional testing methods should be used. An alternate method as recommended by CDC should be followed.