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experimental methods

Cu²⁺ Growth Assay Design

The Copper growth assay tests the correlation between the growth of the bacteria and copper disappearance in solution. In our experimental system, the formation of covellite should be directly related to the disappearance of Cu²⁺ with time because covellite is a complex of copper and sulfur. To test this hypothesis, three growing conditions were tested. In the first condition, the E. coli grew for 6hrs in Luria Broth medium with no IPTG. In the second condition, the E. coli were grown for the same amount of time but with IPTG added at time 0. In the last condition, IPTG was added after three hours of growth. The E. coli need IPTG because it triggers the production of H₂S which reacts with the copper in the solution to form covellite. As a result, over time, as covellite is formed, the concentration of copper should decrease in the media.

We hypothesize that in the first condition, the copper concentration should remain constant because H₂S produce was never activated. In the third condition, copper levels should level until IPTG is added spurring H₂S production and copper disappearance. Lastly, we hypothesize that in the second condition copper would decrease slowly at first because of the low density of bacteria producing H₂S. However, as the density of bacteria increases, the rate of decrease of copper should increase.

We measure the concentration of copper using bathocuproinedisulfonic acid (BCS) which turns orange when it reacts with Cu+. First, a standard curve of copper concentration of Cu+ from the 10mM to 1uM range. Afterwards, the concentration of copper was determined in the stored supernatants of the three growth conditions.

Assay Results