The Language of Bacteria: Chemicals and Communication
In the last essay, we covered how bacteria share helpful genes with each other, and what that means for the spread of antibiotic resistant bacteria (“superbugs”).
Another aspect of the bacterial world is that of chemicals and molecules. When you live in the world as a single cell, you cannot eat the kind of food we do, and you cannot interact with your environment the way we do.
When we eat, our body has to break our food into molecules for our cells. Cells don’t eat hamburgers and salads; they take in glucose (a carbohydrate, or sugar), lysine (an amino acid from protein sources like beef), and retinal (an aldehyde molecule that is commonly known as Vitamin A). Bacteria are the same way. If they ingest something big (for example, another bacterium), they have to break it down into its molecular parts. They’ll take from it sugar (energy!), amino acids (for building proteins), lipids (or fats, which make up their phospholipid membranes), and vitamins and minerals.
Furthermore, when we interact with our environment, we can taste things, touch things, see things. It is a complicated process for those signals to change into chemical signals your cells can understand. Bacteria, however, do not have eyes and ears and fingers and noses. They have receptors. And these receptors work by interacting with the environment, one molecule, one stimulus, at a time.
Receptors are structures anchored through the cells’ membranes. Some receptors are sensitive to non-chemical things, like heat or light. Others detect for acidity, which depends on the chemical nature of the environment the bacteria live in. Still others look for food. But another very important part of their environment is: what are the chemicals that other cells around them are producing?
That’s right, the bacteria are all producing organic molecules and sending them out into the world, to be received and read by their cellular neighbors. Each molecule they send out has its own individual receptor, shaped to fit it perfectly. When a molecule comes into contact with the receptor, the receptor latches on to it on the outside of the cell membrane. The receptor is anchored throughout the membrane, however, with an outer portion and an inner portion. So outside the bacterium, above its cell membrane, the receptor is holding the molecule. Under the cell membrane, it is sending out a signal, telling the cell whatever message that molecule holds.
Bacteria of different species produce different molecules. What’s more, all cells – bacterium or not – send out molecules of their own. For example, in your human cells, if one of them gets infected with a bad bacterium, that cell will make a warning signal by producing a particular molecule and sending it out into your body. Eventually that signal will reach the bloodstream, which has White Blood Cells – soldiers that patrol your cells and protect them from harm. If the infection is bad enough for your brain to start organizing an advanced search-and-destroy mission, it has cells organized in glands that can send out hormones. These hormones travel through your bloodstream and to the infected area. Hormones are actually just really fancy molecules, so your cells have receptors that can read them, too. Inflammation results.
So, the bacteria have their language – chemicals that bacteria produce and send to other bacteria. And the human body’s cells have another language – chemicals that human cells produce and send to other human cells. But here’s the thing: bacteria actually know when your body starts organizing an attack against them. This is because bacteria have receptors that can read your body’s chemical signals, too.
Not only are bacteria producing and reading chemicals unique to their own species, they are reading chemicals from bacteria species different from their own. AND they are reading signals from a completely different – and astoundingly more complicated* – organism. This gives bacteria the ability to “talk.” They can work together in an infection, and together they eavesdrop on the conversation of human cells. It is then that they can continue working together to fight back.
Now, in what ways do bacteria actually use their communication skills to “fight back” at your natural immune response? Well for one thing, it’s important to realize that if you get just one pathogenic bacterium inside you, it’s not really going to do much of anything. It knows that as one bacterial cell against all of you, it doesn’t really stand a chance. So it will stay quiet and start dividing (remember, bacteria can reproduce rapidly).
When this bad bacterium has reached a certain population size, they communicate together so that, all at once, the bacteria release their toxins and make you sick. All by itself, one bacterium can’t do much. But a large enough population can cause a noticeable sickness.
This kind of thing was first observed in marine microbes. There is a species of octopus that glows in the dark. But it only glows because it has a symbiotic relationship with bioluminescent bacteria. That is to say, the octopus provides a home for glow-in-the-dark bacteria, and the bacteria provide light for the octopus’s hunting/defense needs. However, the octopus is a shy creature who doesn’t want to glow during the day – only the night. So every morning, the octopus spits out a bunch of the bacteria. With only a small amount of bacteria inside him, they stop glowing. The bacteria then spend the whole day dividing and growing in numbers. By the time night comes around again, a “threshold number” of bacteria is reached, and all at once – they start to glow again! In the same way, pathogenic bacteria in the human body remain silent until they sense a threshold number of other pathogenic bacteria. Once they hit that number, the bacteria release their pathogens and together, they make you sick.
After this initial attack, your immune system starts its defense response. But the bacteria can communicate with your immune system, so they see it coming. Their next strategy is to aggregate together in a clump called a biofilm. The outer bacteria in the biofilm secrete a substance that protects them. This biofilm coating keeps out the battle weapons of your immune response – and antibiotics. So when it comes to the modern medicine of antibiotics, bacteria have two tricks up their sleeve: they can pass around genes for antibiotic resistance, or (even if they don’t have those genes), they can build up these biofilms to stop the antibiotics from ever reaching them.
*The fact that bacteria can communicate with humans is, in and of itself, a remarkable fact. According to evolutionary theory, bacteria and humans diverged billions of years ago. This is a serious case of "coevolution," and one that shows the complexity of even the tiniest of organisms.