A Threshold Substrate Concentration Is Required to Initiate the Degradation of 3-Phenylpropionic Acid in Escherichia coli

A central question concerning the fate of chemicals (or pollutants) in the environment is that of their degradation at low concentrations. This problem was tackled in well-defined experiments using the bacterium Escherichia coli growing with two compounds of quite different chemical structure that are degraded via different catabolic pathways, namely glucose and 3-phenylpropionic acid (3-PPA). The study was performed in carbon-limited continuous cultures supplied with mixtures of glucose (always 100 mg/l) and 3-PPA (varied from 0.25 to 25 mg/l) in the inflowing medium. Cells grown only with glucose exhibited no 3-PPA-degrading activity. When such a glucose culture was exposed to low 3-PPA concentrations (by adding < 3 mg/l 3-PPA to the feed), the catabolic pathway was not induced and 3-PPA accumulated unutilised in the culture. When higher concentrations of 3-PPA were supplied in the feed (5.1, 7.7, 25.2 and 50 mg/l) 3-PPA was always utilised. Hence, an apparent threshold concentration existed (∼ 3 mg/l) below which the degradation of 3-PPA was not initiated. The experimental data suggest that neither a certain defined proportion of 3-PPA to glucose in the feed mixture, nor a particular initial specific flux of 3-PPA, but a certain extracellular concentration of 3-PPA is needed to initiate 3-PPA utilisation. Once this threshold concentration is exceeded, the specific flux of 3-PPA affects both the time required until 3-PPA degradation becomes detectable and the steady-state residual 3-PPA concentration. When induced, the cells are able to utilise 3-PPA down to concentrations (< 2 mg/l) lower than the required induction threshold. Also the utilisation of 3-PPA is initiated faster at lower specific growth rates.