Abstract
In his Comment article in the May issue of Microbiology (Ferenci, 2006), Dr Ferenci provides a very timely pull on the reigns and reminds us that what we all have been taught classically about continuous cultures is a gross oversimplification. Pragmatically, or by necessity if you like, the theory of the continuous culture is based upon a simple mathematical model. If we look at the constants in that model, µmax and Ks, we really should stop to think. These are not invariable within a population of cells and, as such, mutations will sweep through a population due to selective pressures (as indicated by Dr Ferenci). These mutant take-overs have been described in several studies, and have been shown to be strain-dependent. Indeed, our laboratories work mainly with Streptomyces coelicolor, where visually obvious mutants (asporulent and pigmented antibiotic variants) occur frequently upon prolonged continuous culture. These strain variabilities and mutational effects are approached in our laboratories by restricting prolonged studies (reduction in generation times) when the mutational aspect is not the reason for study, thus material required for analysis is harvested on or at the establishment of (quasi-) steady-state, especially for global studies. With respect to our previous article (Hoskisson & Hobbs, 2005), we failed to indicate that the establishment of (quasi-) steady-state under empirically established conditions may be problematic in the initiation of studies, and a careful experimental design strategy is required.