A common and rapid technique to measure cell growth in microplates is light scattering using the absorbance mode of microplate readers, typically using a wavelength of 600 nm. This choice is due to the relative transparency of culture constituents at this wavelength and the fact that scattering of light is proportional to cell density. Since this is a measure of optical density and not molecular absorbance defined by extinction coefficients, results do not follow Beer’s law and are not necessarily linear. The extent of light scattering is a function of the reader’s optics, including slit geometry and size, measuring distance, and light intensity as well as cell morphology and, as a result, normalized comparisons from reader-to-reader and organism-to-organism are impossible.
Additionally, light scattering does not differentiate between living and dead cells, and thus alternative methods may be required in conjunction with absorbance methods. Nonetheless, light scattering is a rapid and nondestructive analytical method commonly used for cell-growth analyses (Figure 1).
A benefit to performing light-scattering measurements in microplate format is obviously higher throughput, allowing more samples or growth conditions to be tested in parallel. Furthermore, small well volumes mean that less reagent and sample are necessary, resulting in cost savings.
Additional savings are seen by eliminating the time spent and potential for operator error by alleviating the need for sample transfer from culture vessel to sample cuvette. Finally, one is assured that environmental conditions remain constant throughout testing, as the incubation chamber is integrated in the microplate reader.
Microplate-based cell-growth assays using light scattering fall into two general categories: end-point and kinetic. End-point assays use a single absorbance or fluorescent measurement at the end of a growth period to quantify cell density, thus aiding in the quantitative determination of product formed or expressed, and substrate consumption rates. Typically, end-point measurements are more automation friendly, less affected by minor external influences, and require a blank measurement.
Kinetic assays use continuous monitoring throughout the cell-growth cycle to detect subtle changes that may be affected by metabolite production, protein expression, promoters/inhibitors, and cell-signaling events. These changes may provide valuable information during the growth cycle, including insights into signaling pathways that would otherwise be missed in an end-point assay.
Kinetic measurement using light scattering, especially in a microplate format, is not without its challenges. Accumulation of expressed proteins or by-products of metabolism can quickly cause toxicity resulting in cell death. Additionally, a notoriously common problem for yeast strains and some bacterial or mammalian cell lines is cell aggregation. Both can be problematic for microplate-based assays with little culture area and volume per well.
In larger culture vessels, clumping is minimized through continuous shaking on an incubated orbital platform. In microplate format, a kinetic-capable reader must be equipped with temperature-controlled shaking.
A robust microplate reader, such as BioTek Instruments’ Synergy™ H1 Hybrid Multi-Mode Microplate Reader, provides precise temperature control and optical density measurements for bacterial, yeast, and mammalian cell growth assays, and also includes orbital shaking for cells prone to aggregation.
In addition to kinetic assays, Synergy H1 offers end-point, spectral scanning and well-area scanning in absorbance mode, and is also capable of running fluorescence, fluorescence polarization, time-resolved fluorescence, and luminescence assays. A four-zone heating system monitors incubation temperatures up to 45°C for uniform temperature across the plate, while continuous shake/read cycles are possible up to 48 hours or more without interruption.