Revision as of 18:48, 21 October 2009

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Overall project

Abstract

In this project our aim is to engineer a biosensor that will be able to detect simultaneously the presence of the nitrite, nitrate and ortophosphate ions in water. One of the tools that our biosensor uses is de nirK/nsrR system from N.europaea. We can use it too as a new transcriptional regulation system, as a protein coding sequence joined to Pnir (nirK promoter) without the presence of nitrite won't be expressed due to the coupling of NsrR (nitrite sensitive repressor)to Pnir. If we add nitrite to the media, NsrR will be released and the transcription will begin. For our sensitive bacteria, we will join Pnir to a reporter gene , so we will be able to detect when NO2- concentration reaches a threshold. To detect the phosphate we will make use of the phoA promoter from E. coli. It's upregulated in the absence of phosphate, so it can be used to detect its presence in waters after coupling it with an inverting system like tetR and then adding a reporter gene. The nitrate will be detected using narG promoter from E.coli. This promoter takes part in the nitrate reductase system and it has its expression enhanced with the presence of NO3-. We will be able to detect the presence of nitrate coupling the promoter with a reporter protein. We notice it when the concentration of nitrate reaches a threshold.

Nitrite, nitrate and phosphate

Nitrates

Nitrogen compounds occur naturally into the soil from decaying plants and animals. Main sources of nitrogen compounds include human sewage and livestock manure; inorganic sources examples are plant fertilizers such potassium nitrate and ammonium nitrate. Many nitrificant bacteria are able to convert the rest of nitrogen compounds to nitrate ions; nitrate is also the form absorbable by plants. However, nitrate is highly leachable so that it can reach and pollute water easily.

The main risk for nitrate is its ability to oxidizes to nitrite, which has the responsibility for the toxic effects (See “nitrites” below). Also, they contribute to increase the cultural eutrophication effect (see “phosphates” below). The Environmental Protection Agency (EPA) has adopted the 10 mg/L standard as the maximum contaminant level (MCL) for nitrate-nitrogen

Nitrite

Nitrite ions have the same primary origin than nitrate ones, owing to the fact that the majority of nitrite ions are produced by the oxidation of nitrate-nitrogen.

It's very important to measure these ions in drinking water, especially for infants, pregnant and nursing women, and elderly people. Nitrite is able to oxidize Fe2+ to Fe3+ and thus transform hemoglobin to methemoglobin. When reaching the first year of life, most people can revert this transformation easily, but there are some risk groups, including babies under 1 year, pregnant and nursing women and elderly people. It is possible for them to get their enzymatic systems saturated and cannot convert methemoglobin to oxyhemoglobin, acquiring methemoglobinemia. This is a pathology in which there isn't hemoglobin enough to satisfy the tissues demands, so that symptoms should be: difficulty to breath and blueness of skin, and in long term diuresis, increased starchy deposits and hemorrhaging of the spleen.

To sum up, it is very important to regulate nitrite concentrations in drinking waters. EPA establish the maximum contaminant level (MCL) for nitrite-nitrogen in about 1 mg/L

Phosphate

Phosphorus allows plants and animals to grow and function, as it is an essential macronutrient. Inside living being, phosphorus is found in its oxidized form, phosphate. It is part of lots and lots of biochemical molecules, such as DNA, proteins, ATP, phospholipids, etc. Thus, phosphates are found in every wastewater and sewage, which constitute the main source of these ions. Also, they can increase their concentration due to the presence of fertilizers and so other farming products which can increase water phosphate levels.

When drinking water with very high levels of phosphates, digestive problems could occur, but they don't have any important toxic effect. The main risk of getting a high concentration of phosphates is produced on the environment; they allow the phenomenon of eutrophication. It is a term for what happens when algae becomes unusually productive on water systems, and is due to the presence in water of a large amount of nutrients. Eutrophication consists of the formation of masses of algae's blooms which choke rivers and lakes.

The consequence of generating such those algae blooms is the massive consumption of resources as oxygen, as well as the cut of light supply. Eutrophicated areas are dead areas, where is impossible for life to develop. The equilibrium of ecosystems is, thus, deeply altered.

Therefore, it is very important to regulate and limit the phosphate concentration in water, even more if this water will end in a reservoir like a lake. EPA establishes the MCL for phosphate in no more than 0,1 mg/L for streams that do not empty into reservoirs, 0.05 mg/L for streams which empty into reservoirs, and 0,025 for reservoirs.

Project Details

The beginning of the laboratory work

Results and discussion

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-=='''MilliQ® or distilled water?'''==
-Distilled water has a small amount of chloroform, and we thought it could be able to activate gfp expression.
-We made parallel experiments with both milliQ® and distilled water in order to discover if that amount could affect our research. So we made mediums and PBS solutions with those kind of water.
-Finally, results showed us that the difference in chloroform between the two types of water produced no observable variation in gfp expression
-=='''PBS'''==
-	Previous studies proved that sample incubation in PBS solution may amplify the fluorescence signal when working with GFP protein.
-	However, when we were making our fluorescence measurement, we noticed that it was lower in samples which were incubated less. Then, we decided to do a kinetic experiment about how PBS affected our samples.
-	The results showed that there was a top fluorescence at 30 minutes, but after this maximum, fluorescence declined with time. Thus the optimum time for incubation were 30 minutes.
-	It is due to the small amount of GFP in our samples. If we had used GFP-full samples, the optimum time would be higher.
-=='''Volatilization'''==
-	When we performed the assays, one of the problems we had to think about was related to chloroform volatility. It was reasonable to suppose that chloroform concentrations could be modified due to that factor. Consequently, we tried to avoid chloroform losses by completely filling up test tubes, but it was useless.
-	After that, a gas chromatography was done to check that fact; nevertheless, there was no appreciable variation in chloroform levels with time.
-	This is a logic result because all chloroform concentrations we tested were under its maximun water solubility. All of these conclusions can be contrasted in the following graphic.
-=='''Induction experiment'''==
-	For the induction experiment we followed a series of steps:
-•	The day before the tests we inoculated a LB medium with our bacteria.
-•	The day of the tests:
-o	We pelleted the culture and resuspended in new LB medium.
-o	We added the chloroform in several concentrations, from 0 to 3000μL (0, 7, 33.5, 84, 100, 168, 300, 1000, 1500 and 3000 μL). The standard test used 0, 33.5, 84 and 100  μL.
-o	We incubated the aliquots in different times, from 30 minutes to 120 minutes. The standar test used 30 and 90 minutes.
-o	We pelleted again and resuspended in PBS.
-o	We incubated 30 minutes in PBS.
-o	We measured fluorescence.
-The highest concentrations turned out toxic to our cultures. This is reasonable considering they were higher than those which there are in drinking water.
-The tests were run with three strains, a DH5alpha ''E.coli'', a mbla promoter-mutant ''E.coli'' and a clpb promoter-mutant ''E.coli''. But results showed an increase in fluorescence with time in all the strains, thus there were no conclusive conclusions.
-=='''AMO'''==
-	Our project was based on the ability of gfp-recombinant ''Nitrosomonas europaea'' strain to detect and metabolize chloroform, and thus express GFP. It’s supposed that ''N.europaea'' is able to do that due to the presence AMO, which is believed to recognize unspecifically this pollutant and oxidizes it to phosgene. Then, gfp expression would be activated.
-	So that, primer sequences were dessigned in order to clone amo gene by PCR. Nevertheless, after performing the PCR, purification and electrophoresis, we could observe the absence of the expected DNA.
-	It was obvious that something was wrong, and it was thought that it could be due to the small amount of “mould” DNA. Therefore a ''N.europaea'' strain was grown up, but it took us about two weeks. When whe had a suitable optical density, we proceed to repeat the PCR. Unfortunately, we reached the same results. This conducted us to conclude that there was a mistake designing primer sequences.
-	So, we could not obtain an amo-recombinant ''E.coli'' strain, and thus we could not reach our goal.
-=='''Inclusion bodies'''==
-	As it was mentioned above, we tried to add chloramphenicol in order to inhibit protein synthesis and avoid producing those inclusion bodies. However, we didn't obtain any evidence about this; even more, it seemed that chloramphenicol contributed to slightly decrease fluorescence signal.
-	After that, we could check, by carrying out confocal microscopy, that there is not any inclusion bodies production. Thus, this fact could not affect neither gfp concentrations nor signal strength in our assays.

Team:UAB-Barcelona/Project2

From 2009.igem.org