Cellular stress induced in cultured human cells by exposure to sludge extracts from water treatment plants.

Sludge extracts from three water treatment plants, corresponding either to the readily water-soluble fraction or to the heavy metal content found in the solid fraction were tested separately or in combination on human cultured cells for their ability to affect the growth rate and/or to trigger a synthesis of the stress-related hsp72 and metallothionein proteins. When given separately, the soluble extract or the metal mixture corresponding to the solid fraction of sludge failed to exert significant effects on cell growth rate and expression level of the stress proteins. However, when given in combination, they were found to exert a strong synergistic effect, as they impaired cell growth and induced a significant overexpression of both hsp72 and metallothionein. This result points out the complex molecular interactions in actual environmental samples when acting on biological structures. It underscores the need for biological tests to complement chemical analyses in environment monitoring.