Influence of low (radio)metal concentrations on bacterial growth using calorimetric metabolic monitoring

Introduction: The transition of industrially caused metal contaminations into the food chain constitutes a serious risk for the environment and human health. It is particularly a major challenge to develop ecotoxicological biomonitors that provide a physical readout based on measurable metabolic effects rather than extrapolating risks from restricted physical and chemical environmental parameters that account for neither bioavailability nor metabolic responses to toxicity.

Objectives: We have studied bacterial growth by measuring metabolic heat using state ofthe art microcalorimetry to observe the effects of low doses of heavy (radio)metals on three different bacterial strains. Escherichia coli and Lactococcus lactis were used as genetically well defined test organisms and Peanibacillus sp. JG-TB8 as a natural isolate recovered from a soil sample of the uranium mining waste pile “Haberland” (Johanngeorgenstadt, Germany).

Material and Methods: Liquid cultures of Escherichia coli, Lactococcus lactis and Peanibacillus sp. JG-TB8 were exposed to micromolar concentrations of europium(III), copper(II) and uranium(VI) salts and the metabolic heat release was measured as a function of time and temperature using a thermal activity monitor (TAM-III, TA-instruments).

Results: Reproducible effects of europium and copper on the time dependent heat release are observed already at concentration of 10 µM. In contrast to europium and copper, for which the inhibitory action scales with concentration, uranium influences bacterial growth in a more complicated manner which strongly depends on temperature and pH, probably as a consequence of its different speciations. In contrast to conventional optical monitoring of cell growth, much more subtle effects, such as consecutive exponential growth phases can be distinguished.

Conclusions: The results demonstrate that microcalorimetric monitoring is an extremely sensitive tool to investigate the influence of low heavy metal and radionuclide concentrations on the metabolic activity of microorganisms. The bacterial growth rates were determined with high accuracy continuously in real time. The proven long-term stability will also allow the monitoring of higher living organisms (e.g. C. elegans).