Printing microarrays of bacteria for identification by infrared microspectroscopy

Fourier transform infrared (FTIR) spectroscopy has been established as a rapid bacteria identification technique. To increase the throughput of this methodology, the feasibility of applying contact deposition microarray technology to print intact bacterial cells as arrayed deposits (150 μm diameter) on optical substrates and on agar slides was demonstrated for the first time. This contact deposition technology delivers nanoliter (nL) droplets of bacterial suspensions from a microtiter plate onto a slide surface. Protocols for printing microarrays of whole-cells on agar and on infrared (IR)-transparent slides were evaluated and optimized for subsequent measurement by IR microspectroscopy and IR imaging. Parameters that were investigated included pin capacity, deposition mode, and spatial distribution of microarrays. Bacteria representing eight genera (Yersinia, Staphylococcus, Salmonella, Listeria, Enterobacter, Citrobacter, Klebsiella, and Escherichia) were used in this proof-of-concept study. The resulting dendrograms generated by hierarchical cluster analysis (HCA) demonstrated clustering of the descendants of the foodborne bacteria investigated into their respective branches. The suitability of microarray printing coupled to focal-plane-array (FPA) FTIR detection for the rapid identification of bacteria was demonstrated.