Simplified aquatic toxicity test based on fluorescein diacetate method using esterase activities of the bacteria Bacillus Cereus

Abstract

In the present study, the possibility of using Fluorescein Diacetate (FDA) method for assessing toxicity of chemicals in an aquatic environment was evaluated. Inhibition of enzyme esterase of bacteria Bacillus cereus was used as experimental end points. The optimum conditions for a toxicity test using a Fluorescein Diacetate (FDA) for assessing toxicity of chemicals, as well as the toxicity of four chemical groups were examined, including : 1) heavy metals cadmium and copper, 2) organic dye, 2,4 dinitrophenol, 3) the aromatic hydrocarbon benzene, and 4) solvents ethanol and DMSO. Comparisons of esterase inhibition observed from different cell cultures of Bacillus cereus, i.e. fresh cell and lyophilized cell in response to toxics of the four chemical groups, were also performed. The results showed that the suitable incubation time and viable cell count for toxicity testing with the FDA method were 2 hours 30 minutes and 3.5-4.5x107 CFUs/ ml, respectively. Results from toxicity testing of both fresh cell and lyophilized cell cultures with the 4 chemical groups indicated that the heavy metal group was the most toxic. The highest inhibition effect was found with copper. The toxic effect observed from fresh cell culture was IC50= 1.12±0.33 mg Cu2+/l, while from lyophilized cell culture was IC50 = 1.46±0.40 mg Cu2+/l. The toxicity ranking of chemical groups on both fresh cell and lyophilized cell cultures, were heavy metal> organic dye> aromatic hydrocarbon > solvent, accordingly. Sensitivity comparison of fresh and lyophilized bacteria on the four reference chemical groups showed that fresh cell cultures of Bacillus cereus seemed to be more sensitive to toxicity than lyophilized cell cultures. The high sensitivity to heavy metal of the FDA toxicity test using Bacillus cereus indicated a good possibility exists for applying to screening for heavy metal contamination in the environment. This could greatly reduce the cost of using commercially available bacteria test kits for such a purpose