[Cadmium toxicity and spectrum of resistance/ tolerance to cadmium by bacterial species isolated from water and sediments of the Kor River, Fars province]

Agricultural and industrial activities, as well as population growth, have resulted in increases in the amounts of pollutants in the environment, the human body, and other organisms. Cadmium is one of the common heavy metals entering the biological cycles through different paths causing disease in humans, animals and plants. This study aimed at determining cadmium levels and physicochemical and biological factors in water and sediment samples, as well as determining relationships between culturable bacterial counts and cadmium toxicity and assessing bacterial resistance/tolerance spectrum through MIC and MBC. Surface water and sediment samples were taken from five stations in areas believed to be polluted along the Kor River. The stations included 1. Dorudzan Dam outflow, 2. Petrochemistry Bridge, 3. Khan Bridge, 4. Doshakh Bridge, and 5. the mouth of the Kor River to the Bakhtegan Lake. Two sets of samples were taken. Set 1was used for measuring cadmium level, physicochemical and biological factors, while Set 2 was used for the isolation and identification of cadmium resistant/tolerant bacteria and determination of their resistance spectrum. Pure cultures of each bacterial species were prepared and the more resistant bacteria were identified by exposure to media with different cadmium concentrations. Stations 2 and 3 were found to be significantly more polluted with cadmium than the other 3 stations (p<0.001). The cadmium level in sediments of all the stations was higher than in water (p<0.001) and quite high as compared with the existing standards. The data also showed a lower BOD/COD ratio in the last 2 stations and higher phosphate levels in the last 4 stations. As compared to control, the bacterial count decreased in the presence of 1 mM cadmium chloride solution (p<0.001). Cadmium-resistant bacterial counts were higher in stations 3 and 5 than in the others (p<0.001). The most cadmium-resistant/tolerant bacteria were P. aeruginosa ETs and Bacillus ABs with MIC 6 and 4 and MBC 7 and 5 mM, respectively. The most resistant bacteria were gram-negative, isolated from the sediments. Cadmium pollution, especially in stations 2 and 3, are due to man's activities. Although many bacterial types were found to be sensitive to cadmium in this study, some of them showed resistance/tolerance to it to different extents. Our findings also show that unlike water, which is an unstable environment, sediments are stable and provide a suitable environment for bacteria to form biofilms as a way to reduce cadmium accumulation. The most resistant bacteria were found in the stations with the highest cadmium concentrations. This may be a result of cadmium-resistant gene expression in the presence of this heavy metal. The use of these resistant strains in biofilters and wastewater bioremediation can potentially help to reduce cadmium pollution, a problem in some rivers.