Discussion
Under carbon-limiting conditions in retentostats, microbial physiology is a reflection of both carbon limitation and slow growth. The response to carbon limitation is expected to be char-cterized by a relief from carbon catabolite repression (CCR), increase in the uptake capacity of limiting substrates, and expression of enzymes degrading alternative carbon substrates (not currently present in the medium) [11] , while the response to slow growth would be independent from the source of limitation (e.g. decrease in protein and DNA synthesis and increase in signal transduction).