Table 2.

Comparison of methods for determining viability of microorganisms

MethodSpeedNo. of cells analyzedEase of useTypical costs (excluding labor)
Plate countingPreparation of dilutions and plating take minutes. Hundreds of plates can be prepared per day. Incubation of plates for 1 to 7 days typical before results are obtained.Viable counts are typically based on plates with 30 to 300 cells.Minimal training required in aseptic technique and safe handling of microbes.Plastic consumables and media components. Incubation at growth temp.
MicroscopyDilution (if necessary) and staining take minutes. Some stains may require incubation of, e.g., 10 to 30 min. Manual microscopic analysis may take several minutes per sample. A hundred samples could conceivably be processed in a day. Results are obtained immediately.Typically 100 to 500 cells per sample are scored as viable or dead. Image analysis can be used to automate the process of identifying and scoring viable/dead cells.Minimal training in safe handling of microbes and stains (some of which are carcinogenic).Microscope slides and coverslips. Stains. Cost of purchasing and maintaining microscope or fluorescence microscope.
Flow cytometryDilution (if necessary) and staining take minutes. Some stains may require incubation of, e.g., 10 to 30 min. Manual sample presentation may take several minutes per sample. Automated samplers can be loaded with, e.g., a 96-well plate of samples. Hundreds of samples can be processed in a day. Results are obtained immediately, although postacquisition analysis of data is common.Typically 10,000 to 100,000 cells per sample are analyzed. As stain uptake is quantified, intermediate results between live and dead are possible.In addition to the above, training is needed in operation and quality control of flow cytometer. Experience required for protocol development and data analysis.Sample tubes and stains. Costs of purchasing and maintaining flow cytometer.