Does salt really kill microbes ?
By Daniel Loeb, CRI
For centuries, before we had refrigerators and other ways of conserving of our
food for long periods of time, people used salt. This chemical compound helped
people from falling sick because bacteria had grown all over their food and
contaminated it. Even today, this method is used for meats of various kinds and it is therefore important to make sure that salt really is as effective as popular belief suggests. We decided to look at how different amounts of salt in cultures of
Escherichia coli would affect the number of cells inside. While we do this, we can
also study how salt acts on the cells and why it has it's effects. We thought that as salt concentration increased in the cells' environment; eventually it would be higher outside of the cells than inside, in what is called hypertonic conditions. In order to change this, the cells would release water and shrivel up as their size decreased. We chose to work on E. coli because it is an organism that is easy to cultivate and study, and it is present in humans' everyday lives, in their intestines, their drinking water and their food, so our results could put to good use for everyday life. You've probably heard about the recent contamination scandals in Taco Bell and Chipotle restaurants, and this will give you an idea of how E. coli can be dangerous when there is too much of it in your food.
So, to answer our question of how salt affected cell populations and their size,
we elaborated a protocol: we were going to place cells in 5 highly nutritious
environments(called Lysogeny Broth) and then add different amounts of salt to each culture. Finally, we let the cultures grow for 2 hours in an incubator at the
temperature of the human body(37°C), then we looked at the cells under the
microscope, took pictures to measure their size and used what scientists call the
standard CFU counting method to calculate the cell concentration of each culture.
The two graphs above will give you an idea of what our research yielded as results. The left graph shows how the logarithm(which represents the exponent needed to bring 10 to a given number) of the number of cells in one milliliter of culture varies according to the salt concentration in it. As you can see, there seems to be a trand in which the number of cells per mL falls as the amount of salt increases. However, the values seem to go up and down a lot, so it's a bit hard to tell if there's a clear relation between the two parameters.
On the right, you can see the size of the cells depending on the amount of salt
in their environment. Blue dots each represent on cell and the big red dots represent the average cell diameter for a given salt concentration. There were 30 cells measured in each culture, in order to get a good idea of what most of the cells in colony might look like. While there are many different cell diameters in all the cultures, it seems like the averages are all very close to each other. The highest average is less than 10 pixels above the lowest, showing how little salt seems to affect this parameter.
So, can we conclude from these results? It seems that salt has little to no effect
on the diameter of cells within a culture, but that it does limit the number of cells
inside it. While population falls in higher salt concentrations, we don't know if it's because salt has kept the cells from multiplying as fast or if it is killing them directly. It would seem that the way in which salt acts remains unclear, so more work is necessary to answer our question. What we can say is that salt does seem to be a good antimicrobial compound, it limits the number of cells in an environment but this study cannot tell you what the magic amount to use is.
If you want to know more:
References:
1. Centers for Disease Control and Prevention. "Multistate Outbreak of E. coli O157
Infections
Linked to Topp's Brand Ground Beef Patties." (2010).
2. Hajmeer, Maha, et al. "Impact of sodium chloride on Escherichia coli O157: H7
and
Staphylococcus aureus analysed using transmission electron microscopy." Food
microbiology 23.5(2006): 446-452.
3. Pearson-TheBiologyPlace :
http://www.phschool.com/science/biology_place/biocoach/biomembrane1/hypertonic
.html
4. Escherichia coli - Wikipedia: https://en.wikipedia.org/wiki/Escherichia_coli
5. And, Applied. Influence of Osmoregulation Processes on Starvation Survival of
(n.d.): n. pag.
Web.
By Daniel Loeb, CRI
For centuries, before we had refrigerators and other ways of conserving of our
food for long periods of time, people used salt. This chemical compound helped
people from falling sick because bacteria had grown all over their food and
contaminated it. Even today, this method is used for meats of various kinds and it is therefore important to make sure that salt really is as effective as popular belief suggests. We decided to look at how different amounts of salt in cultures of
Escherichia coli would affect the number of cells inside. While we do this, we can
also study how salt acts on the cells and why it has it's effects. We thought that as salt concentration increased in the cells' environment; eventually it would be higher outside of the cells than inside, in what is called hypertonic conditions. In order to change this, the cells would release water and shrivel up as their size decreased. We chose to work on E. coli because it is an organism that is easy to cultivate and study, and it is present in humans' everyday lives, in their intestines, their drinking water and their food, so our results could put to good use for everyday life. You've probably heard about the recent contamination scandals in Taco Bell and Chipotle restaurants, and this will give you an idea of how E. coli can be dangerous when there is too much of it in your food.
So, to answer our question of how salt affected cell populations and their size,
we elaborated a protocol: we were going to place cells in 5 highly nutritious
environments(called Lysogeny Broth) and then add different amounts of salt to each culture. Finally, we let the cultures grow for 2 hours in an incubator at the
temperature of the human body(37°C), then we looked at the cells under the
microscope, took pictures to measure their size and used what scientists call the
standard CFU counting method to calculate the cell concentration of each culture.
The two graphs above will give you an idea of what our research yielded as results. The left graph shows how the logarithm(which represents the exponent needed to bring 10 to a given number) of the number of cells in one milliliter of culture varies according to the salt concentration in it. As you can see, there seems to be a trand in which the number of cells per mL falls as the amount of salt increases. However, the values seem to go up and down a lot, so it's a bit hard to tell if there's a clear relation between the two parameters.
On the right, you can see the size of the cells depending on the amount of salt
in their environment. Blue dots each represent on cell and the big red dots represent the average cell diameter for a given salt concentration. There were 30 cells measured in each culture, in order to get a good idea of what most of the cells in colony might look like. While there are many different cell diameters in all the cultures, it seems like the averages are all very close to each other. The highest average is less than 10 pixels above the lowest, showing how little salt seems to affect this parameter.
So, can we conclude from these results? It seems that salt has little to no effect
on the diameter of cells within a culture, but that it does limit the number of cells
inside it. While population falls in higher salt concentrations, we don't know if it's because salt has kept the cells from multiplying as fast or if it is killing them directly. It would seem that the way in which salt acts remains unclear, so more work is necessary to answer our question. What we can say is that salt does seem to be a good antimicrobial compound, it limits the number of cells in an environment but this study cannot tell you what the magic amount to use is.
If you want to know more:
References:
1. Centers for Disease Control and Prevention. "Multistate Outbreak of E. coli O157
Infections
Linked to Topp's Brand Ground Beef Patties." (2010).
2. Hajmeer, Maha, et al. "Impact of sodium chloride on Escherichia coli O157: H7
and
Staphylococcus aureus analysed using transmission electron microscopy." Food
microbiology 23.5(2006): 446-452.
3. Pearson-TheBiologyPlace :
http://www.phschool.com/science/biology_place/biocoach/biomembrane1/hypertonic
.html
4. Escherichia coli - Wikipedia: https://en.wikipedia.org/wiki/Escherichia_coli
5. And, Applied. Influence of Osmoregulation Processes on Starvation Survival of
(n.d.): n. pag.
Web.
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