Team:Dundee/Forensic Toolkit/Chromate Fiona
Introduction
Background
Stainless steel is an alloy of different metals and apart from the base material, iron, contains a large proportion >11% of chromium. There is a wide range of stainless steels for a variety of different applications, ranging from surgical steel to alloys used in the aeronautic industry. For our application we assumed stainless steels that are used in bulk production of standard kitchen knives with a chromium content between 12% and 14%. We also utilise the fact that chromate is a salt containing chromium. Bone incisions and kerfmarks, distinctive marks left behind, are current indicators for different types of weapons such as saws or knifes.
Initial Ideas
At the beginning of the project we met with Dr. Lucina Hackman, an expert in dismemberment cases at the University of Dundee. In dismemberment cases it is obvious enough that a body has been dismembered. However, if the body is not found for a long time and the only remains are bones, it is hard to tell if a person has been stabbed or if an animal was chewing on the bone. This is where a chromate detector would be useful. By detecting minute amounts of chromate on a bone, the use of a weapon may be inferred which will justify searching for trace evidence.
We found that a chromate detector had already been submitted by the Beijing Institute of Technology 2013 iGEM team, where they had created a biobrick for this (BBa_K1058008). The biobrick is a modified version of a chromate resistance operon of ochrobactrum tritici 5bvl1, a bacterial strain that was discovered in 2004 in a chromate contaminated wastewater treatment plant. (image of plasmid map) We aimed to modify and improve this existing biobrick by taking the brick apart and use its constituent parts in a new arrangement (image). This should have the advantage that each part can be modified individually and the level of GFP and/or the repressor can be modulated more easily by simply cloning it into a higher or lower copy number plasmid.
What did we do?
To ensure that the modified system was valid we used mathematical modelling to compare the original biobrick and our modified system. We also looked at GFP production with varying levels of chromate added to the system. The models of both the original and modified pathways revealed two key points; the modified pathway is just as reliable as the original pathway and the level of GFP produced will not be changed. However the response will be faster for increasing levels of chromate added to the system.
The models allowed us to ensure that it will be valid to use our modified system. In the lab to test the system we used the two compounds potassium chromate and potassium dichromate, as well as the two controls, Chromium(III)Chloride and the isostructural potassium sulphate and observed if it had any influence on the level of fluorescence. Our cloning strategy focused on taking this brick apart and use its constituent parts in a new arrangemet.
To complement our chromate sensor originally aimed to find out how much chromate would be left on an incision in a bone from a stab of a certain force. However this was quickly turned around to measure the wear of bone instead of stainless steel. By calculating the force required to make the incision, this could potentially indicate the size or strength of a possible suspect, while the sliding distance would correspond to the length of the blade. The bone incision experiments allowed us to create a relationship between the applied force, sliding distance and wear volume when a stainless steel knife collides with bone. However some irregularities were noticed and could be due to experimental error.
What next?
Since most of the parameters were assumed in the mathematical models then there is room for error. Practical experiments could be done to produce values for the kinetic parameters or sensitivity analysis done to compare varying kinetic parameter values. This would ensure that the models are more realistic and therefore give a more valid description of the systems.
The next step for this project is to find an ideal concentration of ChrB in relation to pChrGFP to work in a balance way of expression/repression. This involves finding plasmids with an appropriate copy number and promoters of appropriate strength. The modularity of this system makes this process easier. Probing the concentration range for detecting different chromate salts will be essential to feed back into the modelling which is currently largely based on assumptions. Once the concentration range has been established, we can test the system by using actual shavings of stainless steel. A problem that might occur is that the chromium contained in the alloy is in the wrong oxidation state. Oxidising chromium to yield Cr(VI) can be done by subjecting it to a strong acid. The solution will have to be neutralised again to be used as an additive for an experiment. The question is then if the concentration is still high enough to induce expression of GFP in our system. So basically at first, we need to make the system work and test it under conditions that are as real as possible. Good insights into the amount of steel retained after sharp-force trauma and blunt-force trauma on bones can be found in a series of papers (reference). Of great importance will also be potential contaminants. Hence testing the system with similar compounds will be one test. Checking for the prevalence of inducing compounds in or around bones is essential to see how prone the test is to leading to false negatives.
To remove doubts in the data from the bone incision experiments, due to possible experimental error we should repeat the experiments. This would ensure that the experiments are reproducible and reliable.
