(Photo credits: The New Yorker) Everyone knows bacteria are small microscopic organisms millions of times smaller than us. They can be helpful or harmful to us and they are everywhere. So have you ever wondered how such a tiny little thing can cause us to get sick or even cause death to some? Many think of bacteria as these simple small organisms that are incapable of communication and all do their own thing. This however is very wrong. Think about it like in society, no one person can achieve a giant feat by themselves. And in the case of the bacteria, no one bacteria can conquer a human millions of times larger than them by themselves. So now the most logical deduction is that bacteria work together to conquer this feat. We all know that bacteria can multiply really quickly, hence easily achieving large numbers. But if bacteria can't tell each other what to do, how do they achieve this? Well bacteria achieve this via a system known as quorum sensing (QS). QS is a system that only activates at high cell density; when there are a lot of bacteria. This is how bacteria can tell how many of themselves are there, or at least when there are many copies of itself around. This is why we call it the social media platform of bacteria. So when there are enough copies of themselves, the bacteria are now able to conduct a single activity using a collective effort such as causing disease in a human. I'm sure we are all relatively familiar with hormones, the chemical signals that sends messages to various parts of the body we all learnt in high school biology. Well bacteria don't exactly use hormones, but it is a similar system. Now this system is different in the two classifications of bacteria, Gram-negative(GM-) bacteria and Gram-positive(GM+) bacteria. We are going to talk about GM- bacteria first as it is a simpler system. Just like in a endocrine system(hormones system), chemical signals are produced to talk to one another. Here we see a diagram of how QS works essentially in GM- bacteria. Now don't be afraid of the complexity, it is actually a rather simple system. First let us identify the legend of the system. The red quarters are the chemical signals, the autoinducers. The yellow fractions of circles are the receptors. The autoinducers are able to be read by the receptors and the receptors is what tells the cell what the chemical signal means.
So now looking at the figure for low cell density, we can see that there are very little autoinducers visible. This is because the cell is alone, hence the amount of autoinducers produced in the environment is very little. This is similar to Facebook. When having a small social circle, the number of "likes" you get on your posts is small. The opposite is true when having a large online social circle. This is the basis of QS and is what is happening here in this diagram. Now looking at this analogy, look at the figure for high cell density. This cell is now not alone,many other cells are present. This allows for many more autoinducers to be produced in the surroundings, allow the recptors to pick this up, allowing the cell to realise it has many "likes on its posts". This allows the receptors to do its second function. This allows the receptors to activate genes to produce certain proteins. Now the bacteria is able to produce these genes activated by the receptor. This, when conducted by harmful disease-causing bacteria, is the cause for bacteria making harmful toxins in our body. And this is how this large social network of microorganisms can cause disease or a large orchestrated activity.
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