Why is freezing bacteria not a problem?

I have a follow up question to this question: Does freezing microorganisms such as probiotics kill them?

If freezing bacteria kills some of them, then you are effectively putting selective pressure on bacteria when you freeze them (the bacteria more resistant to low temperatures survive, bacteria less resistant die). Why then is it a good idea to freeze bacteria?

The ice crystals that form tear and shear the bacterial cells, unless you add a cryoprotectant (for example, glycerol) to the cells beforehand.

Remember, when water freezes its volume increases. This is why a closed soft drink can that you put into your freezer to chill faster, will burst if you leave it in until the contents are frozen.

Exactly the same thing happens to cells (animal cells, plant cells, yeast cells, etc.) if they are frozen without a cryoprotectant.

The long-term effectiveness of freezing or incinerating bacteria is rooted in how brutal and difficult to withstand such actions are.

Freezing generates ice crystals which, as mdperry points out, "tear and shear the bacterial cells." In theory this could create a selective pressure to survive these temperatures, and indeed akinetes are known to survive the freezing process. However, that is a non-trivial evolutionary step. To fit such a state into the lifespan of a bacteria would call for a substantial mutation. Freezing bacteria for a few hundred million years will likely be sufficient to "teach" them how to survive! Without such a step, it's just not reasonable to survive the abuse.

The same goes for incinerating. Once you get hot enough, the chemistry of organic molecules starts to become a permanent terminal issue. Evolving to beat this is simply really hard. In fact, an autoclave is enough to kill virtually everything, though one strain of Pyrolobus fumarii did survive at 121C in an autoclave.

Why is it that we can flash-freeze bacteria indefinitely but not humans or similar organisms?

Working in a biochem research lab for a while and I got wondering why is it that I can flash-freeze a glycerol stock of bacteria, store it at -80°C, and thaw them out months later with no apparent problems in growth/function but not in say a human. I am not very familiar with the current state of cryogenics, but my impression is that a human would die instantly if flash frozen in liquid nitrogen. Is it simply just an example of the simplicity/hardiness of bacterial systems compared to humans?

Your question includes your answer: it's the glycerol. The glycerol prevents formation of ice crystals which are what destroy the cells and the reason we can't simply freeze and thaw real humans.

Because the bacteria are very small single cell organisms the glycerol is distributed evenly within the cells while simply administering glycerol to humans would not result in sufficient distribution of glycerol in every cell of the body (there are other issues as well, our bodies simply aren't designed to be frozen).

Remember also that it isn't necessarily every cell in the stock that survives the freezing, but because there are so many and they grow independently it's not really an issue when you thaw them again.

Some animals (particularly certain species of frogs) produce their own anti-icecrystal proteins and are able to survive being frozen because of this.

Do turtles have a similar function to frogs in that manner? I remember having a friend when I was a kid who had a lot of animals. And one winter he left his turtles outside and they froze in their tanks, but were fine when they thawed and lived another many many years.

This absolutely works for human cells, just not for entire organisms. Most biology labs keep cell culture cell lines in freezers too, and these originate from almost any species: mouse, rat and even human (HEK293, see "The Immortal Life of Henrietta Lacks").

Multicellular organisms are much more complicated because they have extracellular structures that could be damaged and their intracellular architecture is much more complicated.

There's an issue with surface area:volume ratio. Bacteria and other single celled organisms have a huge amount of surface area per unit of volume. In humans, it's much smaller. This means that there is relatively less surface to lose the heat energy through, so humans would cool much more slowly. This then causes the size of the ice crystals to be wrong, but I can't remember how or why!

Not an expert but your questions raises in my mind this additional question, how would you flash freeze the entire human at the same time? It will take some time for the temperature to penetrate into the body's core. A bacterium has a much higher surface are to volume ratio.

It can now be done. "Sushi is similarly tricky to freeze -- ice crystals destroy the texture of the rice in the same way. So a method of supercooling the bite-size Japanese food was developed. It relies on magnets, which vibrate the water using magnetic fields and allow them to reach -10 celsius without freezing. Once the magnetic field is turned off, the water instantly freezes, without any time for ice growth. Once thawed, the sushi is perfectly edible.
Here is a video demo.

I'm guessing here too, but since the human body is made up of a variety of different cells, Iɽ wager that some of them react differently to being frozen than others do, and that being frozen could damage the connections between the cells themselves. Only being a single cell would be advantageous here.

I am an undergrad at a research university, and I basically just asked a professor if I could work with them. I dont get paid from them directly so its more of a volunteer thing (though I have gotten stipends through undergraduate research programs). I have also been involved in summer research programs (check out the Amgen Scholars program or NSF REUs).

Iɽ say that it is because of bacteria being simple and hardy, as opposed to a human being an extremely complex system with many moving parts.

In the case of humans, flash freezing causes water present to expand faster than the cell's ability to contain it, which makes the cell explode.

To avoid this, you can either introduce chemicals to stop this from happening (which is what happens at current human cryogenic facilities) or you can freeze the person slowly. Unfortunately, both these 'solutions' would kill a living person.

However, many bacteria have the ability to enter a state of 'suspended animation' which allows them to survive lack of food or other such conditions in which they could not normally survive (as an extreme example, see

I suppose that it boils down to bacteria being single, independent cells, and humans being multicellular organisms. Human cells depend on other parts of the body to survive (they need bloodstream for example).

12 Investigation ideas with a biological twist.

Germination experiments and simple osmosis labs are seen all too often by moderators. If you want to show real personal engagement then your investigation has to have a 'twist' that shows you have really thought about some biology. These ideas provide suggestions for this sort of enhancement of some simple lab ideas to help you show your personal engagement.

Spring, summer or autumn germination of seeds? Does light affect seeds differently according to their normal season of germination?

The effect of abiotic factors on the germination of seeds is a bit of a standard lab, the methods are very easy and doesn't show much personal engagement unless students can put an interesting spin on the investigation. This idea is one way to do just that. Research is necessary on the choice of seeds for the experiment. Students find out about the normal germination time of the seeds, the method the seeds use to control this germination time and then test testing the specific effect of light on germination. The colour of light (red / far-red, etc.), the intensity of light, or the day/night length could all be interesting factors, as could other abiotic factors appropriate to the season. Biodynamics agricultural ideas could be a rich source of possible hypotheses to test too.

Investigation of a method of scarification on the germination of seeds.

Scarification involves weakening, opening, or otherwise altering the coat of a seed to encourage germination. Scarification is often done mechanically, thermally, or chemically. Some mechanical methods involve nicking, sanding, or clipping off part of the seed's shell so water can get to the inside part to activate germination. Chemical methods can use sulfuric acid, hydrogen peroxide or alcohol. Thermal scarification is covered in detail in the next idea. You can search YouTube to see a selection of videos on this topic.

The effect of freezing or winter cold on germination of perennial seeds.

Some seeds (perennials) require a period of moist cold before they germinate. They get this naturally in the wild over winter but plants relying on natural cold stratification usually make many seeds, because the process in nature results in the death of many seeds. Gardeners have developed ways of creating these conditions to promote germination more safely, called cold moist stratification. Any of the following methods would make interesting investigations: How long should this freezing treatment last? What minimum temperature is required to have the 'freezing effect'?

Cold water soaking: Put seeds into a jar in the fridge and change the water frequently. This is supposed to be like snowmelt, it is supposed to wash away germination inhibitors from the seeds.

Refrigeration: Mix seeds into a little clean sand, sterile soil or paper towels. Put this into a resealable plastic bag in the fridge. Three months is a good time, but sometimes you can get away with less, even 3 weeks.

Autumn / fall planting: Plant the seeds in pots before it gets too cold and cover the pot with something which will allow a little water to get in but not too much and also insulate them from extremes of cold.

Any of these would be an improvement on the all too common 'factors which affect germination' experiments. Deliberately investigating a specific biological mechanism is better than simply describing an observed effect, without an understanding of why this happens.

This book, Norman Deno "Seed Germination Theory and Practice" (1993) is well known by gardeners in the USA. It describes many reasons and mechanisms for delaying germination used by seeds. Well worth looking at.

The effect of water temperature, or some other aquarium conditions on the gender distribution of livebearer offspring.

It is known that egg incubation temperatures can influence the proportion of male and female chicks which hatch, but could this same factor influence the gender of guppies, or other fish which can be easily kept and bread in an aquarium. Guppies are ideal because the male and female fish are so distinct and they readily breed. This experiment would need to be carefully carried out to comply with the guidelines on animal experimentation, but so lond as the conditions were within the natural ranges of the fish this would be fine.

The biggest challenge would be the length of time the fish take to breed, and also managing the conditions so the fish actually do reproduce. Predation of the fry might also be a challenge if there are other fish in the tank, although plenty of vegetation can reduce this. This is perhaps an investigation idea for a student who already keeps tropical fish, or who is in a school with a lab aquarium.

Does your blood group affect the frequency which you get insect bites, or the severity of the skin's reaction?

This is another investigation idea with which students must be careful to comply with the IB animal experiment guidelines. You must not plan to deliberately cause discomfort to participants, or mosquitoes. But I could imagine that the data might be collected after a evening event where mosquitoes were an irritation and where many of the participants knew their blood groups already. That means that it's not an idea for everyone. The principal of the methodology is interesting, although it may be that students with some means of access to a group of medical professionals in an after work barbeque could be the only ones who might try.
It could be a data-base type investigation if the right data cold be found. There is much research on malaria which may prove useful. Remember too that a hybrid study is also possible, where a small sample of experimental can be supplemented with some secondary data.

The effect of body position on heart rate and blood pressure.

The baroreceptor reflex helps to maintain blood pressure at nearly constant levels. The baroreflex uses negative feedback, an increase in blood pressure causes the heart rate to decrease and also causes blood pressure to decrease. Could the position of the body affect blood pressure and thus heart rate? This might make an interesting investigation. Of course controlling other factors which are well known to affect heart rate will be one of the challenges of an investigation into this topic.

Data analysis of body temperature in different groups of people.

If you can find a good data base which provides enough data so that you can choose a part of it (and thus control some variables) it could make a good IA. A hybrid IA could compare experimental data with a data set published in the biological literature. Here is a nice example of a set of data, Body temperature data and there are many more suggestions of data sets here: Temperature of a Healthy Human. An interesting option might be to test whether temperatures of a group of people in your school community measured in an experiment are the same as data selected from one of these data sets. Notice that there are interesting differences in 'oral' and 'tympanic' measurements as well as young and old people, males and females, and who knows more. There are plenty of options to investigate BMI and temperature, mealtimes, time of day, etc.

Testing the effectiveness of a shampoo, anti-tangle product or conditioner on a property of hair.

There are many claims made from producers of hair products which could be tested in an IA. Does conditioner really make your hair stronger, does a tangle easing product really prevent knots, or reduce friction between hairs? While it is not permitted to use body fluids in IAs, testing your own hair would be acceptable. To meet the animal experimentation guidelines the hair would have to be collected ethically, without causing pain! Research into the structure of hair and use of a microscope to study changes in the hairs might also be a useful part of this experiment.

Are the limits of manufacturer's instructions reliable? Eg. the IKEA Växer hydroponic growing system.

Testing the manufacturer's instructions for a growing system could make an interesting IA. Why does it advise specific types of LED lights? What is the ideal daily light duration? Does changing the chemicals dissolved in the water for the hydrponic system affect plant growth. The challenge of this investigation would be to find a biological reason for a changed variable to have the effect predicted. Obvious one might be light intensity affecting photosynthesis, temperature affeting transpiration or enzymes, or location, fertilizer, type of water used.

What happens after the sell-by date on yoghurt, and other dairy foods?

A simple question in appearance but quite a complex problem. The first aspect to decide would be for which reason are the products past their best. It could be growth of spoilage bacteria, release of enzymes naturally part of a ripening process, chemical changes from another cause. Of course a specific research question would be needed, but this would probably arise out the choice of food and the background research quite naturally.

Researchers clearly image internal and external structure of bacteria

Credit: Leiden University

Freezing bacteria super fast to gain a true-to-nature image of the internal and external structure. Ariane Briegel Professor of Ultrastructural Biology came to Leiden specially to carry out this research. Leiden University is one of the few institutes in the world to have the necessary equipment. Inaugural lecture 13 January.

"Microbes like bacteria are all around us, but they are too small to see. That's why we know so little about them," Briegel explains. Her field, ultrastructural biology, focuses on studying biological specimens with high-resolution imaging techniques. Briegel works mainly with highly advanced cryo-electron tomography. "We freeze the bacteria super fast - a true flash-freeze - so that the specimen is contained in perfectly clear ice." It's so fast that the water in the cell has no time to crystallise. "The bacteria remains completely intact and there are no defects in the image."

Bacteria in their natural state

The specimen is then placed under an advanced electron microscope, where it is spun round. Briegel: "Then we make 2D recordings from all angles, from inside and outside, like stickers. By putting all these together we can build up a 3-D image of the bacterium. The process is called tomography, the same technique that's used in hospitals, when patients are put into a scanner." The combination of these techniques offers a completely new vision of the world of microbes. "Cryo-electron tomography is the first technique for looking at bacteria in their natural state, exactly how they normally look."

Leiden University has two of the most advanced microscopes for electron tomography, in the Netherlands Center for Electron Nanoscopy (NeCEN) that is part of the Institute for Biology. It is the only facility for this technique in the Netherlands, and there are just seven institutes in the world that have this equipment. This was a unique opportunity and a very important reason for Briegel to come to Leiden University. "This is the first method in the field of biology that allows us to see the structure, the exterior and all the internal workings of microbes. We hope it will help us find out more about bacteria: how they grow, how they reproduce, how they move," Briegel comments.

One of the first subjects Briegel will focus on is the movement of microbes. Bacteria use signals from their environment, like the presence or absence of particular chemical substances, to determine where they need to go, for food, for example. "We've known about this process, called chemotaxis, for decades," she explains. "What we don't know is which structures the bacteria use to detect the signals. Or: what does the 'nose' of bacteria look like?" It's made up of thousands of very sensitive receptors arranged in a herringbone pattern on the outside of the bacterium. "Now we also want to know what the 'nose' of pathogenic bacteria, like the cholera bacterium, looks like. Once we have a clear idea of how bacteria 'sense' where they need to go, that will probably give us some cues for how to prevent infections."

In her inaugural lecture Briegel also makes a firm stand for fundamental research. "The tendency is primarily to do or finance research that will generate immediate results, like medicines or industrial applications. I do understand why the focus is on these things, but fundamental research shouldn't suffer as a result." She relates it to her own work. By gathering all kinds of basic information about bacteria - what they look like, how they move - it may be possible to develop new ways of preventing infections. Highly useful, given the increase in antibiotic-resistant bacteria and the fact that the opportunities for further developing current antibiotics are almost exhausted. "But it is possible if we have an image of the basic component, the bacterium. If research funding only goes to research that is expected to deliver immediate results, that's like putting blinkers on researchers. Then you never know what you are missing."

Freeze-Thaw Cycles and Why We Shouldn’t Do It

Freeze-thaw—you know it’s bad for your samples, don’t you? While working in the lab, you have most likely heard someone say ‘aliquot your protein/cells/DNA/RNA to avoid too many freeze-thaw cycles.’ But do you actually understand why?

You probably thought that avoiding freeze-thaw cycles had something to do with damaging cell structure as well as proteins or DNA/RNA—and you would be right. But you might be surprised to know that freeze-thaw cycles can damage your samples in several other ways, and we don’t quite understand how all of them work.

Damaging your samples during freeze-thaw cycles can cause problems with downstream processes. For example, multiple rounds of freezing and thawing can damage protein structures, which can interfere with study protein kinetics using surface plasmon resonance. Even minor DNA damage can result in uninterpretable data from PCR.

Your samples are not the only concern when it comes to freeze-thaw cycles. At the moment, a lot of research is going into the cryopreservation of embryos and gametes. Humans aren’t the only ones using assisted reproductive technology. Animal husbandry professionals, zoologists, and others are using assisted reproductive technology to increase farming production or aid in the preservation of endangered species. Many studies have shown that the freeze-thaw process can affect DNA integrity in sperm and also hinder embryo development.

Through this research and years of experimentation in the lab, we’ve learned that a variety of factors are responsible for damage caused by freeze-thaw cycles.
Different Mechanisms Cause Instability During Freeze-Thaw Cycles
Ice Crystals

Ice crystals that are formed during the freeze-thaw process can cause cell membranes to rupture. Rapid freezing results in ice crystal formation in the outer parts of cells, which causes the interior of the cells to expand, pushing against the plasma membrane until the cell bursts. While slow cooling allows water to leach out and reduce ice crystal formation, slow cooling still leads to cell rupture due to an imbalance in osmotic pressure. If you are freezing live cells or microorganisms, both of these processes can greatly decrease viability.
Freeze Concentration

In addition to mechanically damaging cells, ice crystals can also cause the salts and proteins in the buffer to become concentrated. This problem is known as freeze concentration and can cause significant stress on the stability of proteins. Although the exact mechanism of ice-induced protein denaturation is not fully understood we do know that changes in the physical environment of the protein lead to stresses that can impact stability. For example, freeze concentration has been shown to cause protein unfolding at the ice:aqueous interface for several proteins, including, azurin, liver alcohol dehydrogenase and alkaline phosphatase.
Oxidative stress

Another common problem seen as a result of multiple freeze-thaw cycles is oxidative stress, which may be generated through different mechanisms. Ice crystal-induced damage to organelle structures could lead to activation of rescue systems that are associated with energy generation. This results in a subsequent increase in oxidative stress and production of reactive oxygen species (ROS, free radicals produced as by-products of reduction-oxidation, or redox, reactions). When the balance between ROS and antioxidants is lost, oxidative stress results in molecular damage to DNA, proteins, and lipids in the cell. Some studies have shown that thawed cells contain an increase in phosphorylated H2AX,a marker of double strand breaks in DNA.
Tips to Minimize the Damage Caused by Freeze-Thaw Cycles

There are two main ways to avoid the changes seen after freeze-thaw cycles:

Don’t do it. The easiest and most obvious solution is to prevent freeze-thaw cycles. As I said at the very beginning, one of the best ways to avoid multiple freeze-thaw cycles is to aliquot everything – your samples, your antibodies, your cells, and anything else you can think of.
Use cryoprotectants. In addition to aliquoting, add a cryprotectant to your cells/samples/etc. to help prevent the stresses caused by freezing.

Cryoprotectants, which are an important addition to samples on their freezing journey, were first discovered in the UK by Christopher Polge in 1949. He inadvertently supplemented an experimental freezing solution with glycerol, resulting in the unexpected survival of his experimentally frozen cells. I’m sure you’ve all used something in the lab that has had glycerol added for this purpose (e.g., antibodies or RNase). Even the antifreeze for your car has glycerol added.

There are two main classes of cryoprotectants:

Intracellular agents.These agents penetrate the cell to prevent the formation of ice crystals and, thereby, membrane rupture. There are several common reagents, including dimethylsulfoxide (DMSO), glycerol, and ethylene glycol. The most common agent used in the lab is DMSO, which provides a high rate of cell survival. However, some groups have shown that it can promote stem cell differentiation in neuronal cells. Also, DMSO can be cytotoxic at room temperature. Even though DMSO may have some drawbacks, it works well enough for most applications.
Extracellular agents. These agents do not penetrate the cell membrane but act by reducing the hyperosmotic effect in the freezing procedure. Common extracellular agents include sucrose, dextrose, and polyvinylpyrrolidone. Cells preserved in extracellular agents (e.g., sucrose) tend to have a lower viability after thawing than DMSO-preserved cells. This may be because extracellular agents don’t prevent ice crystal formation.

Whatever method you use, you should always be mindful of the changes you are causing in your samples during freezing and thawing. Especially those that cannot be seen—they could affect your results.

Plants require more than just water and sunlight to grow. They also require many nutrients found in the soil. One of the most important nutrients required for plant growth is nitrogen. Nitrogen is used to build plant proteins and nucleic acids, including DNA.

Nitrogen is found naturally in the atmosphere. In fact, it makes up about 78% of the atmosphere! But this form of nitrogen (N2) cannot be used by plants. Nitrogen can be combined chemically with oxygen or hydrogen to form types of nitrogen compounds that plants can use. These nitrogen compounds can be added to the soil in the form of ammonium (NH4 + ) and nitrate (NO3 + ) fertilizer. Plants grow well when fertilizer containing nitrogen is added to the soil, but this method can be expensive and has to be repeated each time the nitrogen in the soil is used up.

Bacteria to the rescue! Bacteria are small, single-celled organisms that live in nearly every environment on Earth. From the freezing cold of Antarctica to the boiling heat of hot springs in Yellowstone National Park, some of these organisms are able to live in extreme environments and have many amazing capabilities. Some species of bacteria are able to turn milk into cheese while others can reproduce in less than twenty-four hours. Rhizobia, the type of bacteria that you will study in this experiment, can turn the nitrogen in the soil into usable nitrogen compounds like ammonium and nitrate ions. This is called nitrogen-fixation. These bacteria can attach themselves to the roots of some plants, forming little growths called nodules. The rhizobia receive nutrients and protection from the plant roots and the plants get their fill of nitrogen. This type of mutually beneficial relationship is called a symbiotic relationship. Legumes, and clover in particular, readily form this symbiotic relationship with rhizobia.

In this experiment, you will grow clover plants in soil with no nitrogen added, in soil with nitrogen fertilizer added, and in soil containing nitrogen-fixing bacteria (in this case, a species of rhizobia called Rhizobium legominosarium, or R. legominosarium). You will monitor the nitrogen levels in each type of soil using a nitrogen testing kit. You will observe the effects of nitrogen on the health of the clover plants by measuring the increase in biomass of each plant during the experiment.

But is it safe?

There is no evidence that not washing jeans is hazardous to your health, said Bernhard Redl, an associate professor in the molecular biology department at the University of Innsbruck in Austria. That is, when they are worn under normal conditions, such as everyday street wear.

Bacteria, skin cells, and sweat are transferred to our pants from our own body but “skin microorganisms are generally not hazardous to ourselves,” said Rachel McQueen, a professor of human ecology at the University of Alberta in Canada. McQueen’s research focuses on the development and retention of odours in textiles.

“There are environments where having sterile clothing is important,” McQueen said. That would be true in a hospital, for example, where clothing can transfer infections. “However,” she added, “sanitizing your hands is going to be a more important issue here than washing your jeans.”

A few years ago, one of McQueen’s students wore his jeans for 15 months straight without a single wash and then tested the level of bacteria on them. The student-teacher team was surprised to find that the unwashed jeans carried nearly the same amount of bacteria as those same pants after they had been washed and then worn for another 13 days.

“What I found was just normal skin flora,” McQueen told The National Post in 2011. “The counts were really, really similar. The bacteria load from the swabbed areas were pretty much the same.”

Fight against antibiotic-resistant bacteria has a glowing new weapon

A new chemical probe glows in the presence of a bacterial enzyme that contributes to antibiotic resistance. Credit: The University of Texas at Austin

In the perpetual arms races between bacteria and human-made antibiotics, there is a new tool to give human medicine the edge, in part by revealing bacterial weaknesses and potentially by leading to more targeted or new treatments for bacterial infections.

A research team led by scientists at The University of Texas at Austin has developed chemical probes to help identify an enzyme, produced by some types of E. coli and pneumococcal bacteria, known to break down several common types of antibiotics, making these bacteria dangerously resistant to treatment.

"In response to antibiotic treatment, bacteria have evolved various mechanisms to resist that treatment, and one of those is to make enzymes that basically chew up the antibiotics before they can do their job," said Emily Que, assistant professor of chemistry and one of the leading researchers on the team. "The type of tool we developed gives us critical information that could keep us one step ahead of deadly bacteria."

In a paper published online yesterday in the Journal of the American Chemical Society, the researchers zeroed in on the threat posed by the bacterial enzyme called New Delhi metallo-beta-lactamase (NDM). They set out to create a molecule that glows when it comes into contact with the NDM enzyme. When these chemical probes are added to a test tube, they bind to the enzyme and glow. Such a tool could be used to alert doctors to what kind of bacterial threat is affecting their patients and tell them which antibiotics to use.

NDM breaks down antibiotics in the penicillin, cephalosporin and carbapenem classes, which are some of the safest and most effective treatments for bacterial infections. Other classes of antibiotics exist, but they may carry more side effects, have more drug interactions and may be less available in some parts of the world.

In addition to indicating the presence of the NDM enzyme, the florescent chemical probe developed by Que and Walt Fast, a professor of chemical biology and medicinal chemistry, may help find a different way to combat these resistant bacteria. One treatment option that doctors use with resistant bacteria is to combine common antibiotics and an inhibitor. Although there is no known clinically effective inhibitor for NDM-producing bacteria, Que's probe could help find one.

Once the probe has bound to the enzyme and begun to glow, if an effective inhibitor is introduced, it will knock the probe loose and the glow would stop. This allows scientists to test a high volume of potential drugs very quickly—research Que and Fast hope to continue in the future.

"This allows us to work towards developing therapies and eventually understanding evolutionary characteristics of such proteins," said Radhika Mehta, a recent UT Austin doctoral graduate and lead author on the paper. Mehta is currently a postdoctoral fellow in the Merchant Lab at the University of California, Berkeley.

The study also examined a process called nutritional immunity, which comes from the human body's production of proteins in response to an infection. The proteins snatch up all the available metals in the body, such as the zinc required to make NDM, rendering the bacteria more susceptible to attack.

"The evolution of this bacteria since its discovery in 2008 indicates that not only is it developing antibiotic resistance, it's attempting to combat this natural human immune process. That's particularly scary," Que said.

Que's probe can also be used to study nutritional immunity and NDM because it will glow only in the presence of the zinc needed to form the enzyme.

If heat kills bacteria, why can't you simply reheat all food, no matter how old?

If heat kills bacteria, then why are there so many guidelines for food safety? Couldn't you just reheat any food and kill that bacteria?

(obv this might impact taste, but it seems simpler than the complex food safety laws)

Sufficient heat will kill live bacteria, sterilizing the food as far as infection risk, however many food-borne pathogens create toxins as part of their metabolism and those toxins will remain even after killing the bacteria.

For example, the bacteria Clostridium botulinum produces botulinum toxin which is capable of blocking the release of acetylcholine, functioning as a neurotoxin [Nigam & Nigam, 2010].

Similarly, E. coli is capable of causing infectious disease, but also may produce Shiga toxin, which can halt protein synthesis, killing or damaging cells even in the absence of live bacteria [Pacheco & Sperandio]

Also important, even if the toxin isn't particularly dangerous, it can still make your food taste pretty awful.

Both botulinum toxin and shiga toxins are easily destroyed by heat.

However certain strains of E.coli produce a group of "Heat-stable Enterotoxins."

Certain strains of Staphylococcus, for example MRSA, produce a heat stable toxin. Staphylococcus spcs. don't grow easily under refrigeration or under acidic conditions. Therefore staphylococcal food poisoning tends to be occur in certain foods that are kept for several days at room temperature, but don't have excess acidity.

E. coli [. ] may produce Shiga toxin

But isn't that thing living in our guts? What's protecting us from their toxins?

Wrong. Both toxins you described are protein-based, and will be destroyed by heat. Endotoxins such as lipopolysaccharide or teichoic acid are the ones capable of surviving heat.

But you can denature botulinum toxin and some forms of shiga toxin by heating them moderately. Obviously very high temperatures will destroy both.

Per the article, a minute at 150F will kill active bacteria, but a further 10 minutes at full boil are required to inactivate any toxins. However, the article also notes the risk of error in this procedure, along with flavor and quality issues.

Afaik, some toxins, e.g. the cereulides formed by Bacillus Cereus, can withstand even prolonged boiling, and even 121 °C for 90 minutes (source). So, just prolonged boiling will probably not destroy all toxins, but it will destroy the vast majority.

"If they’re still hot, start the cooling on the countertop. When the container is no longer hot to the touch, put it in the refrigerator, and cover it once the food is good and cold."

Why do people think you shouldn't put hot food in the fridge? (Especially since the author doesn't even cover it until it's cold.) This is the weirdest wive's tale I know of.

Edit: Yes, I agree that ice-baths are much faster than fridge-cooling or counter-cooling. (Even better: put the food in an ice bath in the fridge, then you don't have to remember to move the food after it cools.)

Edit 2: Before someone mentions it, counter-cooling does make sense if you need to stack your hot food in the freezer with stuff you don't want temporarily thawed.

The incident with the Japanese family related in this article sounds like a rare case. I've frequently left rice out for even a couple days and have never gotten food poisoning from anything. Many cultures leave out foods all day or over night-- Before meat was refrigerated, people used to leave out fresh meat for a up to several days-- cutting off the parts that seemed to be turning green.

Obviously, a lot of this is probably based on tolerance, which could be attributed to genetics or lifestyle. Infants and elderyly will be far more susceptible to food-borne illnesses, as well as those who have lived a very sterile life. Like the article says, the FDA's food handling instructions are VERY conservative and if followed well will leave very little margin for food to make people sick. I think this is incredibly important in commercial establishments, particularly since there is so much food being prepared, and the clientele will have highly varied tolerances.

I could see it being the case that foods that some people are perfectly health with, could cause other people serious problems, though-- so it's best to be safe, obviously.

Watch the video: Πάγος u0026 Απόψυξη Του Ψυκτικού Κυκλώματος. Για Να Ξέρεις. (November 2021).