Bacillus stearothermophilus as a model to evaluate membrane toxicity of a lipophilic environmental pollutant (DDT)

The thermophilic eubacterium Bacillus stearothermophilus has been used as a model system to identify DDT-promoted events in biological membranes putatively related with the insecticide toxicity. Two strategies have been approached: a) bacterial growth and viability were followed and the effects of DDT (2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane) determined; b) biophysical studies with fluorescent probes were performed to elucidate the effects of DDT on the organization of the membrane lipid bilayer. The effects of DDT on growth and physical properties of the membrane were also determined in the presence of Ca2+ to further identify the interference of the insecticide at the membrane level and its putative contribution to cell toxicity. Growth inhibition by DDT is concentration-dependent, being attenuated or removed by the addition of 2.5-mM Ca2+ to bacterial cultures. Consistently, fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) and its propionic acid derivative (DPH-PA) exhibited opposite effects of Ca2+ and DDT on the physical state of bacterial polar lipid dispersions. Growth and viability of bacterial cells are affected by DDT concentrations lower than those able to induce detectable bulk fluidity alterations, indicating high sensitivity of the intact bacterial system to alterations in limited membrane domains not directly probed by fluorescent probes that only report the average behavior of membrane lipid population.