Numerous studies have compared the use of solid and liquid media for bacterial identification by MALDI-TOF MS. While these studies showed that visible differences in mass spectra were observed, the identification and accuracy were not affected. Since culture purity is essential, isolates should be grown on solid media for routine applications. Using solid media, purity can be visually assessed, and individual colonies can be selected for identification. An approximate biomass of 105 cfu/mL is required for identification.
For MALDI-TOF MS evaluation, microorganisms grown in liquid media typically require manipulation prior to analysis. This includes concentration by centrifugation or filtration to enhance the biomass generally with a series of washing steps to remove medium from the sample. Individual manufacturer’s give recommendations to determine when it is appropriate to use liquid media. Some applications include identification of mycobacteria, molds, and blood culture isolates. Prior to testing, liquid media should be sub-cultured to solid media to ensure culture purity. Laboratories may elect to consider microorganism identifications from liquid media as preliminary until culture purity is confirmed.