660 - Evaluation of Qvella’s FAST-Prep™ Liquid Colony™ for Early Antimicrobial Sensitivity Testing of Positive Blood Culture by Disk Diffusion Method

Background: Conventional antimicrobial susceptibility testing (AST) of microorganisms from positive blood cultures (PBC) can take ≥ 2 days.  In order to improve the turnaround time for AST on a PBC, CLSI and EUCAST have made efforts to standardize procedures for disk diffusion (DD) direct from a PBC.  Qvella Corporation (Richmond Hill, ON, Canada) has recently developed FAST-Prep, an automated centrifugal sample preparation system that rapidly delivers a Liquid Colony consisting of a purified, concentrated, viable cell suspension directly from a PBC. This study was performed to investigate the feasibility of DD AST off of a PBC using a FAST-Prep Liquid Colony.

Methods: Contrived PBC samples were prepared by spiking 6 species of Gram-positive and 4 species of Gram-negative bacteria (3-5 strains per species) into FA^® Plus bottles and incubating in the BACT/ALERT^® VIRTUO^® System (bioMerieux, Durham, NC).  After positivity, 3 mL of PBC was added to the FAST-Prep cartridge.  After 20 minutes of processing in the FAST-Prep instrument, the Liquid Colony was removed from the cartridge and a 0.5 McFarland sample was prepared for DD AST.  In parallel, the DD AST from a PBC was performed using 4 drops of PBC (CLSI direct method).  Both methods were compared to conventional colony-based DD AST.  After 16-18 hours of incubation zone diameters and S/I/R interpretations were determined.  Categorical agreement (CA) and errors for both DD AST methods were calculated. In addition, colony plate counting was performed on 0.5 McFarland suspensions of Liquid Colony and the plate colony to determine biomass recovery and sample purity.

Results: CA for a FAST-Prep DD AST for Gram-positive and Gram-negative bacteria was 95.6% and 98.6%, respectively, compared to CA for CLSI DD AST of 77.2% and 81.9%, respectively. Biomass in the Liquid Colony was 7.2x10⁸ and 1.2x10⁹ CFU for Gram-positive and Gram-negative bacteria, respectively.  Cell concentration in the 0.5 McFarland suspension of the Liquid Colony was 3.7x10⁷ and 5.9x10⁷ CFU/mL for Gram-positive and Gram-negative bacteria, respectively, which was similar to the concentration for the reference colony suspension.

Conclusion: The results support the potential role of FAST-Prep in providing a Liquid Colony for use in rapid AST.