| Impedance basics | High impedance --> delay/stop flow of electric current
Low impedance --> allows flow of electric current
Bacterial metabolic activity can lower electrical impedance of a medium (charged products like amino acids, lactate) |
| Impedance detection time (IDT) | - time for initial population of microbes in food to reach a specified population (10^4 - 10^5 CFU/g) to give detectable electrical signal
- IDT is inversely proportional to the initial number of microorganisms |
| Standard Curve for Impedance | Initial Aerobic Plate Count (log CFU per g/mL) vs. Impedance Detection Time (Hours)
- aerobic plate count: standard conditions required, use selective or non-selective detection |
| Impedance Sample Instrumentation | - Bactometer
- Malthus System V
- BacTrac 4300
automated, can process large number of samples |
| Flow Cytometry basics | - science of measuring components (cells) and their properties in a liquid suspension
- properties: fluorescence, absorbance, light scatter
- factors: laminar flow, velocity, angle at which cell passes the detector, emitted or absorbed light |
| Fluorescein isothiocyanate | dye used in flow cytometry to stain protein |
| Propidium iodide | dye used in flow cytometry to stain DNA |
| Flow cytometry use | adjustments to measure and characterize thousands of cells/s
- one or more excitation sources (argon, krypton, or helium-neon lasers)
- one or two fluorescent dyes (fluorescein isothiocyanate, acridine orange)
- excitation spectrum of the dye must match the wavelength of the excitation source (laser beam) for efficient detection |
| Example flow cytometry method: DNA and protein in yeast cells | 1. grow yeast and fix cells
2. incubate in RNAase solution for 1 hour
3. Stain with dyes. Fluorescein isothiocyanate for protein, propidium iodide for DNA
4. Wash cells and suspend in buffer
5. Test in flow cytometer |
