How is it that humans are able to turn milk into so many different foods, from butter and ice cream to yogurt and cheese? In the case of yogurt and cheese, we actually have a fair amount of help. These foods are modified through anaerobic respiration — the way bacteria convert sugar into energy in the absence of oxygen. A particular type of bacteria, Lactobacillus, converts glucose (a sugar) into lactic acid. This increases the acidity of the milk mixture, causing an important protein, casein, to solidify and produce the somewhat creamy substance we know as yogurt. If we isolate and compress the solid curds instead, we get cheese.
For as long as we have existed, humans have relied on other organisms to produce chemical compounds we use in everyday life. This process, known as biosynthesis, takes advantage of natural biological pathways to make anything from food to proteins and pharmaceuticals. While some of these processes are meant to produce specific chemicals that can be purified or refined, others seek to change the composition of a mixture.
One prevalent commercial use of biosynthesis is the production of ethanol. This reaction is also the result of anaerobic respiration, but instead of producing lactic acid, the yeast microorganisms ferment glucose into ethanol. Simply placing these organisms in a solution containing glucose will allow for this fermentation. However, by varying factors in the process, such as temperature, moisture, and pH, one can control how much glucose the bacteria ferment and, therefore, the chemical composition of the final solution. This solution can then be distilled and refined in a wide variety of ways to produce beer, wine, vodka, whiskey, and a variety of other spirits.
Scientists are still learning new ways to capitalize on biosynthesis. Take, for example, the case of opioids. These painkiller medications, such as oxycodone and morphine, were historically derived from the opium poppy, Papaver somniferum. This meant that the supply of these drugs was limited not only by the supply of poppies that could be grown, but also by the capacity to refine and purify crude opium.
Recently, however, a group of researchers at Stanford University produced bioengineered yeast that could synthesize the chemicals thebaine and hydrocodone, two major opioids, from basic sugars. This team first engineered yeast that would produce greater amounts of (S)-reticuline, a chemical precursor to many major opioids. Then, through the discovery and introduction of a wide variety of enzymes, the group was able to incite the yeast to complete the synthesis process.
The discovery of complete biosynthesis of opioids represents a major advance for medicine. Opioids are classified as essential medicines, and yet there is still a major shortage of these painkillers in developing countries. Using yeast to produce these compounds from start to finish could streamline the efficiency of the process. Furthermore, this type of synthesis may change a process that involves both biological and artificial synthesis into one that requires minimal human involvement.
It may be years before fully biosynthetic painkillers are widely available. While the Stanford study did prove that it is possible to complete such a synthesis, it did not show how to optimize and scale up the process to make it commercially viable. However, as our understanding of chemical biology grows, the opportunities to directly manipulate microorganisms may allow us to produce compounds that would be virtually impossible to create in a lab on large scales. In this way, the same techniques we used for thousands of years to make food and drink could be the key to solving major medical problems in the future.