By Dr. Kirk Doing, Eastern Maine Healthcare, Affiliated Laboratory, Inc., Bangor, Maine

Susceptibility studies remain among the most important functions performed in the clinical microbiology laboratory; however, mounting resistance, coupled with sometimes novel mechanisms, continue to challenge the ability of commercial Antimicrobial Susceptibility Testing (AST) systems to accurately detect some resistant phenotypes. Indeed, for some organisms, microdilution methods with full 18-24 hours of incubation yield the most accurate phenotypic results. This has forced many laboratories to maintain multiple AST methodologies to generate reliable results for different clinical isolates, which adds cost to overall laboratory operations.

The Sensititre ARIS 2X System has been designed to incubate and then auto-read Sensititre microdilution susceptibility and identification plates. Sixty-four plates can be incubated in each ARIS 2X instrument, with up to four modules (256 plate capacity) being linked to a single computer. Inventory is tracked using barcode information present on each plate. A temperature controlled, and timed incubation is maintained within the instrument, after which, robotics are used to transport plates to the reading unit. Hydrolysis of a fluorogenic substrate by the bacterial isolate is used to measure growth in each well and determine MIC endpoints. Appealing features of Sensititre plates include a traditional doubling dilution format, a large selection of antimicrobics, and the ability to test both fastidious and non-fastidious bacteria using a single AST system.

The Sensititre ARIS 2X System was developed in the 1980s, however, its widespread use in the clinical arena has been limited. Improved plate design, instrumentation, and data management software support the need for direct comparisons of the ARIS 2X to other available instrumentsand AST methods.

This study focused on verifying the accuracy of Sensititre MIC trays incubated and read automatically by the ARIS 2X for a variety of clinical isolates. Susceptibility results were compared to those obtained with either the Vitek Legacy (bioMerieux, Marcy L’Etoile, France), Pasco frozen microdilution panels (Becton Dickinson, Sparks, MD), the E-test gradient diffusion method (AB Biodisk, Solna, Sweden), or combinations of these methods for 401 clinical isolates comprising multiple genera and antimicrobial resistant patterns. Gram-negative identifications were evaluated using Sensititre GNID panels. The ability to consolidate multiple AST methods currently used in our laboratory onto the ARIS 2X platform was also explored.

A total of 401 clinical isolates were tested against clinically appropriate antibiotics. Bacterial isolates included a “defined” collection of 318 strains (200 gram-negative and 118 gram-positive) collected up to 90 days prior to the start of the study. These isolates were maintained frozen at -70°C until testing, and were selected for species diversity and phenotypic resistant patterns. An additional 83 (69 gram-negative and 14 gram-positive) organisms were tested concurrently with the Vitek Legacy. No duplicate isolates were tested.

Identifications using Sensititre GNID plates were compared with those obtained using Vitek GNI Plus cards (V1311) for 205 gram-negative isolates. Discrepant identifications were settled using an additional commercial identification kit, Crystal ID (Becton Dickinson), API 20 NE strips (bioMerieux), and/or conventional tubed biochemical media (Remel, Inc). Isolates included both patient and laboratory stock cultures.

An essential agreement of 98%, after discrepant analysis, was obtained between the ARIS 2X and the AST methods evaluated. Categorical error rates obtained with the ARIS 2X were within accepted limits, and a very major error rate of <1.5%, was similar to recent studies evaluating the Sensititre ARIS 2X and MicroScan System.

The Sensititre® ARIS 2X® System provides an excellent platform to consolidate microbiology test methods:

Consolidation
Currently many laboratories are forced to maintain multiple AST methods in order to accurately detect antibiotic resistance expressed by the multiple bacterial genera encountered in the clinical setting. An attractive feature of the Sensititre ARIS 2X system is the potential to consolidate susceptibility testing onto a single platform. In the current study, all susceptibility studies, including those for fastidious organisms, were completed on the ARIS 2X using just three different susceptibility plates.

Streamline Set-Up
Inoculation and incubation procedures are basically the same for all Sensititre plates further streamlining workflow. In contrast, three separate Vitek cards, three different Pasco panel configurations, and multiple Etest strips were necessary to complete the same work. Adapting to an 18-24 hour incubation time for susceptibilities may also seem problematic, given data that supports improved patient outcome when rapid susceptibility results are made available to clinicians. However, rapid phenotypic susceptibility testing of gram-positive organisms presents problems, and may not accurately detect beta-lactam or glycopeptide resistance. While combining genotypic and phenotypic susceptibility methods help in addressing this issue, at present, phenotypic susceptibility studies for gram-positive cocci require a full 24 hours of incubation.

Flexible Test Formats
The 96-well microtiter format, number of approved antimicrobials and testing dilutions, coupled with automated plate inoculation procedures lend flexibility to the system and create the potential for unique enhancements to Sensititre plate design.

Reliable System
Unlike Vitek cards, Sensititre plates can be incubated off-line and read visually with results entered directly into the data management software for report generation, computer interface transmission, and statistical archiving; this virtually eliminates instrument down-time.

This study was performed to verify the performance of the ARIS 2X Automated
Susceptibility and Identification System for routine use in the clinical microbiology laboratory. The Sensititre ARIS 2X System performed well, and offers a reliable and accurate means to perform routine susceptibility testing of rapidly growing bacteria frequently encountered in the clinical laboratory. The ARIS 2X also addresses the difficulty of maintaining multiple AST methods in the laboratory with the ability to consolidate most susceptibility testing onto a single platform. Instrument design, capacity, and capability to link multiple units to a single computer further support the ARIS 2X as a suitable automated susceptibility and identification system worthy of consideration.

Taken from poster A-045 presented at the ASM 2006 in Orlando, FL, Evaluation of Sensititre Plates Read on the Automated Incubation and Reading System (ARIS) with Comparison to Vitek Legacy, Pasco Frozen Microdilution Panels, and E-test Strips for Determining Susceptibility Profiles of Commonly Encountered Bacteria, K. M. Doing and E.C. Rioux, Eastern Maine Healthcare, Affiliated Laboratory, Inc., Bangor, Maine 04401.