Features to consider for suspension cell culture
There are two basic ways to grow cells in culture: as monolayers stuck to a surface or artificial substrate (adherent culture), or floating free in culture medium (suspension culture). When it comes to suspension cell culture, constant agitation ensures that the cells are aerated and nutrients are available to help the cells grow uniformly. The shaking “prevents the bacteria or other cell types from settling at the bottom of the flask, which would result in cell death from the lack of nutrient and oxygen availability,” says Jayne Bates, technical support manager at Cole-Parmer (Vernon Hills, IL). She adds that shaking also keeps clumps from forming.
To accomplish these vital functions in suspension cell culture, one requires a shaker that meets certain requirements.
The shaking motion needed when using an Erlenmeyer flask is a “nice swirl,” says Sara Livingston, global product manager for orbital shakers at Thermo Fisher Scientific (Waltham, MA), and there should be no sloshing. This circular (aka orbital) motion will increase the surface area of the top of the liquid to maximize gas exchange between the liquid and the air. Single-use bags, however, perform best on a two-dimensional rocking platform, says Brian Canna, vice president of marketing and business development at Boekel Scientific (Feasterville, PA). Bates notes that cultures in microtitre plates require rapid mixing, so more of a vibration mixing action is typically employed.
Variable speeds are needed to accommodate different culture vessel sizes, volumes, and numbers and types of cells. All these factors explain why determining the optimal speed for a particular application is such a common challenge encountered in suspension cell culture. “Some cells can be shaken faster than others,” says Livingston. For example, Bates explains that bacterial cells such as E. coli require vigorous shaking, whereas insect cell cultures should be shaken at much lower speeds. It is therefore important to use a shaker that has adjustable speeds, says Canna.
Shakers must also accommodate vessels of a variety of sizes, “from large culture flasks down to small culture bottles and even microtitre plates,” says Bates. She notes that it is unlikely that a single type of shaker would effectively agitate cells in both large culture flasks and the very small wells of microtitre plates, so different designs must be employed. Tube-holder accessories can also be useful.
The cell environment should be considered when selecting a shaker. When culturing bacterial cells, which are generally grown at 37°C, the shaker must be inside or built into an incubator. Temperature control is an important feature, says Bates. Canna adds that when working with a CO2 incubator, users must select a shaker that is compatible with a CO2-rich environment, i.e., one specifically designed for corrosive environments.
As with any type of cell culture, contamination is a constant concern. Livingston recommends shaker models that provide HEPA filtration to continuously protect against circulating contaminants that could jeopardize fragile cultures. She notes that another feature to look for is “easy-to-remove platforms and clamps, which facilitate cleaning so that unaddressed spills can be removed, preventing microbial colonization in otherwise-hard-to-access areas.”
In terms of convenience, Bates suggests choosing an incubator shaker with clear windows, which enables the user to easily check whether the mixing is efficient and to spot problems. Additionally, an easy-to-access platform allows users to remove individual samples for periodic testing.
The shaker may be just one component of a successful suspension cell culture, but it is one that requires careful consideration.
For additional resources on shakers, including useful articles and a list of manufacturers, visit www.labmanager.com/shakers