Ralstonia eutropha JMP 134 was continuously grown on phenol and 2,4-dichlorophenoxyacetate under nutristatic conditions at elevated stationary concentrations of 90–650 mg phenol/l and 25–100 mg 2,4-D/l, respectively, in order to study the response of the bacterial population to long-term exposure to these potentially toxic substrates. The course of the cells' response over time was observed by determining distinctive growth parameters and by the on-line measurement of fluorescence spectra of intracellular and extracellular fluorophores. The latter were monitored using a modified fluorescence spectrophotometer. The results of the nutristat experiments indicate that the adaptation of the culture to long-term exposure to phenol and 2,4-D exhibited dynamic characteristics of the growth pattern determined by the individual substrates and their concentration, including enforced and reduced levels of substrate conversion. This growth pattern is interpreted as an expression of superimposing cellular events in order to withstand unfavorable environmental conditions. Finally, the growth rate attained retarded levels under stationary conditions, slowing down to almost zero for example in the case of about 100 mg 2,4-D/l.

The growth rate profile within the various phases of adaptation was well reflected by the fluorescence signals. The NAD(P)H fluorescence was almost exclusively emitted by the cellular pool of NADPH and behaved inversely to the growth rate. A similar relationship was obtained for the cellular fluorescence of a flavin-containing compound. Sharply reduced growth was additionally accompanied by a rapid rise of the background fluorescence. These data indicate that fluorescence-derived signals provide a useful reflection of cellular events in inhibited growth situations.