inoculating an aliquot of cells into a sterile flask containing a finite amount of
medium. Such systems are referred to as closed, since nutrient supply is limited to
that provided at the start of culture. Under these conditions, growth will continue until
the medium is depleted of nutrients or there is an excessive build-up of toxic waste
products generated by the microbes. Thus, in this system, the cellular composition
and physiological status of the cells will vary throughout the growth cycle.
In continuous cultures (also referred to asopen systems) the medium is refreshed
regularly to replace that spent by the cells. The objective of this system is to maintain
the cells in the exponential growth phase by enabling nutrients, biomass and waste
products to be controlled through varying the dilution rate of the cultures. Continuous
cultures, although more complex to set up, offer certain advantages over batch
cultures in that they facilitate growth under steady-state conditions in which there
is tight coupling between cell division and biosynthesis. As a result, the physiological
status of the cultures is more clearly defined, with very little variation in the cellular
composition of the cells during the growth cycle. The main concern with the open
system is the high risk of contamination associated with the dilution of the cultures.
However, applying strict aseptic techniques during feeding or harvesting cells may
help to reduce the risk of such contaminations. In addition, the whole system can be
automated by connecting the culture vessels to their reservoirs through solenoid
valves that can be triggered to open when required. This minimises direct contact
with the operator or outside environment and thus reduces the risk of contamination.Pressure release valveNutrientsSterile airCooling
waterTapProductsStirrerWater
jacketCooling
waterSample
Motor tubeTemperature and
pH monitorsFig. 2.13Schematic representation of a fermenter.70 Cell culture techniques