PASSAGING CELLS IN SUSPENSION CULTURE

A suspension culture is grown in culture flasks in a humidified 37°C, 5% CO2 incubator.
Passaging of suspension cultures is somewhat less complicated than passaging of monolayer
cultures. Because the cells are suspended in medium rather than attached to a surface,
it is not necessary to disperse them enzymatically before passaging. However, before
passaging, cells must be maintained in culture by feeding every 2 to 3 days until they
reach confluency (i.e., until the cells clump together in the suspension and the medium
appears turbid when the flask is swirled).
NOTE: All culture incubations should be performed in a humidified 37°C, 5% CO2
incubator unless otherwise specified. Some media (e.g., DMEM) may require altered
levels of CO2 to maintain pH 7.4.
1. Feed cells as follows every 2 to 3 days until the cultures are confluent:
a. Remove flask of suspension cells from incubator, taking care not to disturb those
that have settled to the flask bottom.
b. Aseptically remove and discard about one-third of the medium from flask and
replace with an equal volume of prewarmed (37°C) medium. If the cells are
growing rapidly, add an additional 10% medium by volume in order to maintain
optimum concentration of 1 × 106 cells/ml. Gently swirl flask to resuspend cells.
c. Return flask to incubator. If there is <15 ml of medium in the flask, incubate flask
in horizontal position to enhance cell/medium contact.
At higher volumes of medium the flask can be incubated in the vertical position.
If using a 25-cm2 flask, there should be 20 to 30 ml of medium in the flask at confluency.
2. On the days cultures are not being fed, check them by swirling flask to resuspend
cells and observing color changes in the medium that indicate good metabolic growth.
3. When cultures are confluent (∼2.5 × 106 cells/ml), passage culture as follows:
a. Remove flask from incubator and swirl flask so that cells are evenly distributed in
the medium.
b. Aseptically remove half of the volume of cell suspension and place into a fresh
flask.
c. Feed each flask with 7 to 10 ml prewarmed medium and return flask to incubator.
Some labs prefer to split the cells 1:3 or 1:4, although increasing the split ratio will
result in a longer interval before subcultures reach confluency.