Wednesday, December 26, 2012

What Are The Signs of Bioreactor Contamination?

For animal cell culture, look to dissolved oxygen (dO2) for signs of bioeactor contamination.

Animal (and certainly plant) cells grow more slowly than bacterial cells.  A population of animal cells will double once per day (specific growth rate ~ 0.70 day-1), where as bacterial cells can double as fast as once every 20 minutes (specific growth rate ~ 70 day-1).

Bacterial cells that make their way into a bioreactor (be it because you failed to kill them or because you failed to keep them out), have less cellular infrastructure to maintain, and when they find a nutrient-rich, oxygen-rich environment, they will multiply until the excess oxygen in their environment is exhausted.

Have a look at this batch:

dissolved oxygen contamination
What's plotted here is dissolved oxygen (blue) and volume (dark blue).  We inoculate the bioreactor and the dO2 falls to set point.  Once it hits setpoint, the controller comes on and maintains the setpoint by adding air or oxygen (blue line goes flat).  But a few days in, you can see that all of the sudden, the dissolved oxygen crashes for no reason.  This is usually the first sign of contamination... when dO2 in the culture gets consumed.

Let's have a look at the same trend, except with dO2 controller output also plotted.
dissolved oxygen contamination
You can see that the controller is gliding along on a slow upward path.  And then all of a sudden, an inflection point and then it shoots to 100%. (She's all I've got captain!)  And even at 100% controller output, the culture's dissolved oxygen still gets exhausted.

Watching the dissolved oxygen trend is a daily task for campaign monitoring.  If you're looking at these trends, you ought to be able to detect a contamination.

Of course, verify that the culture is contaminated by sampling the bioreactor and sending it off to QC Micro(biology).  They'll tell you for certain whether the culture is contaminated.

But you can use these trends to tell you when to suspect contamination.

Additional Reading:

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