Back To CourseMicrobiology 101: Intro to Microbiology
20 chapters | 207 lessons
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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
It shouldn't be a surprise to learn that the cells in living organisms have to make or consume everything they need to survive. Since you evolved to need vitamin B12, you can rest assured that it must be getting produced somewhere. But not by you, your salad greens, or the beef in your steak. Nope, vitamin B12 is only produced by a few species of bacteria and archaea. That means that you have to eat these microbes or, better yet, just have a few living in your gut, if you want a healthy, vitamin B12-enhanced nervous system.
Then again, you could simply go buy vitamin B12 supplements and end your dependence on microbes. The problem with vitamin B12, though, is it's a large and complex molecule. Take a look at this beast:
Synthesizing a molecule with such bulk step by step can be time-consuming and expensive. Wouldn't it be nice if we could get those bacteria and archaea to do it for us? They've already evolved an efficient production process. All we have to do is harvest the finished product. In fact, that is exactly what happens.
We'll come back to vitamin B12 in a minute. First, let's discuss the concept of a microbial factory. We know what microbes are: tiny organisms including bacteria, archaea, fungi, and protozoa. We know what a factory is: a building where products are made. So, it follows that a microbial factory is using a microbe to make a product. In this case, the factory building is the microbial cell! And, the products we're referring to include antibiotics, vitamins, hormones, proteins, vaccines, enzymes, and countless other small, organic compounds. The potential products are almost limitless. And so are the advantages of using microbes.
Let's look at vitamin B12 again, seen above. This molecule is large with a very complex and specific structure. Change the placement of one atom or bond, and you destroy the functionality of the vitamin. In order to chemically produce vitamin B12, you would have to figure out every step individually, place them in a very specific order, and make sure you can purify the vitamin B12 out of all the chemical catalysts you added and away from all the unwanted byproducts. And some of those catalysts are invariably toxic, reactive, heavy metal compounds that you really don't want tagging along in your daily multivitamin.
The bacterium Pseudomonas denitrificans, our microbial factory, has already taken care of all these issues. This bacterium already knows how to make vitamin B12, without making any errors in the final structure. There are no toxic byproducts because that would harm the bacteria. There are no industrial strong acids or bases, solvents, or heavy metals required because these would have to be produced by the bacterium and would likely end up harming it. All we're left with is the most efficiently produced vitamin B12 the Pseudomonas is capable of.
But that's not the end. Ever hear of genetic engineering? If not, there is a great lesson entitled What is Genetic Engineering? you might want to watch. Simply put, genetic engineering is the process by which scientists modify the genome of an organism. Through genetic engineering, we can modify the genome of our Pseudomonas so it makes very large amounts of vitamin B12, often up to 20,000 times more than normal, or secretes the B12 out of the cell so it is easier to collect. You can see how using the bacteria to make the vitamin can make a very tedious process much easier, safer, more efficient, and, the magic word, cheaper.
This example of vitamin B12 is actually very straightforward. We found a safe bacterial species that already makes our desired product and exploited it. But it rarely works out that well. Not every bacterium produces the things we want; some even produce very deadly byproducts and toxins we definitely don't want. To top it off, there are some compounds that higher organisms, like humans, produce that bacteria don't. We need a way to customize our factory to make exactly what we need, make it correctly, and make it safe for human use.
The first step is to pick a host for our product. The host should be very well understood, easy to use, safe, and easily genetically modified. Possibly the two most studied organisms in history are the bacterium Escherichia coli and the yeast Saccharomyces cerevisiae. The vast majority of microbial factories are based on one of these two workhorses. They fit every criteria. Most strains are harmless to humans (excluding a few nasty food poisoning E. coli), they are easy to grow and grow very fast in a lab. Finally, and maybe most importantly, they are so well understood that genetic engineering with these two has become commonplace, predictable, and very successful.
Before you can get started making insulin, for example, there is a small problem: E. coli doesn't normally make insulin. Fortunately, with our vast knowledge of E. coli, this is only a small problem. All organisms have DNA encoding genes, making up the genome, which directs all processes inside the cell. In addition to one large chromosome, some bacteria have plasmids, which are small circles of DNA, separate from the main chromosome. These plasmids can naturally contain genes for sexual reproduction, antibiotic resistance, or toxin production. Or, they can contain genes for any product we want the E. coli to produce! Today, it is actually pretty easy to insert the gene coding for human insulin into a plasmid, insert that plasmid into E. coli, and allow the bacterium to produce large amounts of the hormone. Once that one E. coli has divided into billions, each pumping out insulin, all you need to do is purify the hormone to get rid of the E. coli cells and their metabolic waste products.
Insulin and vitamin B12 are just a couple of examples of compounds produced by microbial factories. There are many that could have a direct impact on your life almost daily. You're probably already aware that most antibiotics, like the cephalosporins and penicillins, come from bacteria and fungi. Did you know that several major food additives are produced by microbes? The fungi aspergillus and candida produce the majority of the citric acid added to foods. Next time you're in the grocery store, check a few food labels for xanthan. This food stabilizer is produced by the bacterium xanthomonas. Many growth hormones and steroids are produced by microbes. Genetically engineered yeasts are the main producers of the hepatitis B vaccine. The list goes on and on, and new compounds are added every day.
A microbial factory is using a microbe to make a product. The bacterium E. coli and the yeast Saccharomyces cerevisiae are the two most commonly used microbes. These species have the advantages of being very well understood, safe for human use, fast reproducers, and easily manipulated genetically.
Some species can naturally produce commercially desirable products, like our example of pseudomonas denitrificans and vitamin B12. Other compounds require genetic engineering, which is the process by which scientists modify the genome of an organism. Plasmids, small, circular pieces of DNA containing the genes needed to produce a compound, like insulin, are inserted into a host microbe. As the host bacterium grows and reproduces, it makes insulin. In this way, large amounts of insulin can be produced, purified, and used by diabetics.
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Back To CourseMicrobiology 101: Intro to Microbiology
20 chapters | 207 lessons