The protein-based toolkit that lets bacteria keep building in the low temperatures of your refrigerator
You can’t avoid sharing your food. Even if you pack up your fruit, sandwich or pasta in closed container in your fridge to hide it from pets, kids and insects, it will eventually get nibbled on by bacteria clinging to its surface. Putting food in a refrigerator does help though, as the cold can greatly inhibit bacteria’s growth, minimizing just how many microbes you’ll risk ingesting when you finally get to your leftovers. Of course, anyone who has cleaned out a fridge knows that this protection is temporary, which is sort of weird if you think about it. If the temperature doesn’t significantly change when food is in a fridge, how do bacteria overcome the chill to eventually take a slimy bite out of your old casserole?
Making sense of a cell’s instructions
The first step to understanding how bacteria can beat the cold is figure out how the cold slows the bacteria. Inside bacterial (and other organisms’) cells, DNA is read by proteins to create a sort of working copy of the cells instructions, called RNA. Proteins called enzymes then go through the RNA instructions to build whatever new proteins the cell needs to perform its task, such as split and multiply across your cottage cheese. Making those enzymes is a lot of work, and bacteria seem to put about half their resources into creating them, just so they can do the other work the cell needs completed.
Lowering the temperature messes up this process in two ways. First, enzymes just function more slowly when they’re cold, so every step of a cell’s operation slows down. The second issue is that the RNA that’s used by enzymes to build new, functional proteins gets crumpled up, or “structured,” when it’s cold. Like a tangled set of Christmas lights, structured RNA is much harder for an enzyme to make sense of, and so it needs to somehow be relaxed into a looser configuration before it can be used.
Keeping counter-measures close at hand
To make that RNA usable, bacteria employ another set of proteins called Cold Shock Proteins (Csps). These help untangle the RNA to make it available to enzymes, but that’s only helpful if the Csps are available when a bacterium needs them. To prepare for unexpected swings in temperature, bacteria actually carry a lot of RNA instructions for Csp production, ready to deploy at a moment’s notice. This way they don’t have to unpack those instructions out of DNA, then RNA, then production to start dealing with the cold.
The instructions for Csps are essentially always kept at the ready, saving the bacteria time when temperatures drop. If that weren’t enough, researchers have found that the RNA that is used in Csp production actually reacts to temperatures in reverse. Unlike most of the RNA that needs rescuing from low temperatures, these instructions are crumpled and structured when the cell is warm, but relaxes for use when things get cold.
These specializations help bacteria bounce back from temperature changes that would otherwise immobilize them. Once the Csp RNA is in use, a bacterial cell will likely invest as much as 25 percent of its protein-building energy into making Csps alone. And this isn’t even the only way bacteria respond to cold temperatures- in some cases, pieces of overly-structured RNA are destroyed altogether to allow enzymes to get on with their work. So even though we can relax a little when we put our food in the refrigerator, the bacteria on our leftovers are doing their best to spring into action, engaging in a slow-motion race that we only notice when the bacteria win.
Source: Refrigeration slows – but doesn’t stop – food rot. Now scientists know why by Melanie Silvis, Massive
