Heat shock gene expression in continuous cultures of Escherichia coli

Temperature inducible systems for the controlled expression of recombinant genes are finding increasing industrial applications. These involve either short or long term exposure of the process culture to superoptimum temperatures. It is well known that bacteria respond to a sudden increase in their environmental temperature with an immediate transient increase in the synthesis rates of specific heat shock proteins. The use of continuous flow processes for the production of recombinant proteins would allow higher productivity and smaller scale bioreactors. However, the induction patterns of heat shock proteins in continuous culture after defined heat shocks are not well defined despite a large amount of information which is now available concerning heat shock protein induction in batch cultures. An overview of this information is presented to enable a better understanding of the response in continuous cultures. The latter was investigated by monitoring the transient expression of a representative heat shock gene, htpG, in E. coli in continuous culture. The relative magnitude of the response was found to be both temperature and exposure time dependent, but growth rate independent. Changing medium composition resulted in both different steady and transient state expression levels.