Heavy metals have toxic effects for almost all the living things, but because of different species variety, heavy metal toxicity affects in different ways for different organisms. For example, bacteria as a group are more sensitive to the heavy metal toxicity than fungi, (Rajapaksha et al, 2004). Heavy metal pollution can come from natural source (e.g. silver metal mine), but most of the times, they are caused by human activities. Industrial wastes are the biggest source of heavy metal pollution, (Bae et al, 2000). People need to use heavy metal salts during plating procedure, dyeing, chemical synthesis and other areas. The common heavy metal wastes are silver salts, copper salts, and mercury compounds. This heavy metal can cause a lot of negative effects in living things. First of all, heavy metal can destroy protein structures, and that will make the protein lost its function. Secondly, heavy metal can accumulate in organism's body, and that might cause long-term effects such as mutation. Because it is too complex and inhumane to do experiment on human body, we need to find another organism, which allow us to see the effects of toxicity by those heavy metals in a controllable constant environment. In fact, bacteria are the good choice. They are unicellular, and have a short generation period. These advantages let us can easily monitor them for even a long term study. I am going to discuss the effects of three different heavy metal salts on the growth of four species of bacteria. My hypothesis is different heavy metal have different levels toxicity to the organisms, and different species have different responses to the same heavy metal environment. This test will let us understand which one in these three metals are the most toxic one, and it also can show us the degree of toxicity for different metals applied.
             We followed the procedure from Biology 108 Laboratory Manual 2004-2005.
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