The aim of this study was to evaluate the effects of the photosensitizers Rose Bengal and erythrosin combined with a light-emitting diode (LED) on Enterobacteriaceae. Twelve Enterobacteriaceae strains isolated from the oral cavities of patients undergoing prolonged antibiotic therapy, including three Escherichia coli, three Enterobacter cloacae, three Klebsiella oxytoca and three Klebsiella pneumoniae, were studied. An Enterobacteriaceae suspension (10(6) cells/ml) was prepared from each clinical strain isolated from the human oral cavity and subjected to the following treatments: LED and Rose Bengal, LED and erythrosin, LED and physiological solution, and physiological solution only as control. A blue LED unit (460 nm), and Rose Bengal and erythrosin at a concentration of 50 micromol/l were used. After incubation at 37 degrees C for 48 h, the number of colony-forming units (CFU) was calculated and subjected to analysis of variance (ANOVA). The Enterobacterial strains were sensitive to photodynamic therapy with Rose Bengal. There was a reduction of approximately 7.14 log10 for Enterobacter cloacae, 7.73 log10 for Escherichia coli, 6.76 log10 for Klebsiella pneumoniae and 7.21 log10 for Klebsiella oxytoca. However, photodynamic therapy using erythrosin did not reduce the numbers of CFUs per milliliter compared to the control group. The use of the LED alone had no toxic effect on the strain tested. The Enterobacteriaceae strains studied were sensitive to photodynamic therapy with Rose Bengal.