A Novel Microfluidic Assay for Rapid Phenotypic Antibiotic Susceptibility Testing of Bacteria Detected in Clinical Blood Cultures

Background
Appropriate antibiotic therapy is critical in the management of severe sepsis and septic shock to reduce mortality, morbidity and health costs. New methods for rapid antibiotic susceptibility testing are needed because of increasing resistance rates to standard treatment.
Aims
The purpose of this study was to evaluate the performance of a novel microfluidic method and the potential to directly apply this method on positive blood cultures.
Methods
Minimum inhibitory concentrations (MICs) of ciprofloxacin, ceftazidime, tigecycline and/or vancomycin for Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae and Staphylococcus aureus were determined using a linear antibiotic concentration gradient in a microfluidic assay. Bacterial growth along the antibiotic gradient was monitored using automated time-lapse photomicrography and growth inhibition was quantified by measuring greyscale intensity changes in the images. In addition to pure culture MICs, vancomycin MICs were determined for S. aureus from spiked and clinical blood cultures following a short centrifugation step. The MICs were compared with those obtained with the Etest and for S. aureus and vancomycin also with macrodilution.
Results
The MICs obtained with the microfluidic assay showed good agreement internally as well as with the Etest and macrodilution assays, although some minor differences were noted between the methods. The time to possible readout was within the range of 2 to 5 h.
Conclusions
The examined microfluidic assay has the potential to provide rapid and accurate MICs using samples from positive clinical blood cultures and will now be tested using other bacterial species and antibiotics.