Intestinal cells are the first line of defense against toxic substances that may enter the body orally. Some of these substances may emanate from wastewater treatment plants and thus may eventually enter the food chain. In this study, human intestinal Caco-2 cells were incubated with biologically treated activated sludge and lipopolysaccharide (LPS) to determine changes in protein expression and cell monolayer integrity. Caco-2 cells incubated for 3 h with 24-h-aerated activated sludge or 220 microg/ml LPS showed enhanced expression of the heat shock protein 90b (Hsp90b). The results further show that 220 microg/ml LPS is sufficient to irreversibly disrupt tight-junction permeability in 30 min. The Hsp90b expression by the Caco-2 cells incubated with biologically treated activated sludge may be a cellular protective mechanism against LPS-induced stress.