If mouth-wash kills 99% of bacteria – are we genetically engineering superbugs?

Imagine a cell is like a car. The goal of antibacterials, antibiotics, alcohol, etc. is to disable that car. There are two basic ways to do it, delicate and brute force. You could, for example, cut the ignition wire so the car wont start, but a manufacturer can move the wire someplace so you cant see it. This is how antibiotics work, they target specific parts of a cell, and sometimes cells can hide these parts.

The other way to disable a car, however, is to just blow it the fuck up. Sure the manufacturer can put armor on it, but at the end of the day a big enough explosion is gonna get the job done, however, you’re going to have collateral damage. This is how alcohol and fire work, they obliterate everything, there is no way for a cell to adapt to them any more than you can adapt to surviving an explosion. For this same reason, however, we can’t use these methods internally because they cause too much collateral damage. Would injecting ethanol into your blood stream kill bacteria? Probably, but it will kill a whole lot of other important cells first.

Resistance against bacteriostatic and bactericidal agents is a serious concern facing today’s healthcare community (and the rest of us really). The mechanism of action of antibiotics versus alcohol has already been addressed at great length by other posts. But the heart of the question asks whether the routine use of a substance that kills bacteria in the oral cavity will select for bacteria with resistance and subsequently lead to a superinfection.

The short answer is no. If you would like to know why, then keep reading. Otherwise we can all go back to making Cam Newton memes.

Using a bacteriocidal mouthrise like Listerine is part of an oral hygiene regiment intended to lower the bacterial load in the oral cavity and maintain a healthy oral environment. Along with brushing and flossing, mouthrinses remove debris from the dentition, introduce fluoride, and kill bacteria on the teeth and soft tissues – the fleshy surfaces of the palate, tongue, inner cheeks, gingiva (gums), and the oropharynx (back of mouth/beginning of the throat).

The only trouble is (gee wiz) that the human mouth is full of microbes! The exact number of bacteria in the mouth is not easily quantified. But researchers have estimated that the mouth holds between 500 to 650 different species of microbes numbering over 20 billion at any given time. In fact, the average person swallows billions of bacteria in the saliva every day. Luckily most of the bacteria is a natural part of the dentition and not harmful under normal conditions.

To make a short story long, the reason why it’s not likely to select for resistant microbes by using mouth rinse is that, even with perfect oral hygiene and regular mouthrinsing, it’s virtually impossible to kill all of the normal bacterial flora of the oral cavity. The anatomy of the periodontium (gingiva and bone around the teeth) allows for areas where microbes are shielded from even the most enthusiastic and meticulous oral hygiene endeavors. Therefore, while mouthrinsing does lower the bacterial load for a short time, it doesn’t fundamentally change colony dynamics of the present bacteria.

BUT… because there is always a but. That’s not to say that the intraoral environment CANNOT change to favor one type of bacteria over another. For instance, frequent consumption of sugary and/or acidic foods/drinks, in conjunction with lack of adequate cleansing of the teeth, DOES change in environment in the mouth to favor bacteria that wreak havoc on the teeth and gums. Also, systemic conditions that impact the immune system, quantity or quality of the saliva, or cause autoimmune disturbances can do the same.

Edit: Since people are asking questions;

I guess I didn’t directly answer the original question and in short, no you’re not. The 1% of bacteria that survive such treatments do so because they hide in little cracks and crevices in your mouth so they aren’t exposed to the Listerine rather than they have evolved some resistance to it. I would, however, like to point out some bacteria have evolved resistances to low levels of ethanol (yeast for example) and other antibiotics, but none can tolerate higher levels just like an armored vehicle can survive a small explosion but not a large one.

To stick with our car metaphor alcohol kills cells partially by denaturing proteins, which is equivalent to taking apart the engine, and by solvating the cell membrane, which is the same as removing the fame of the car (although technically that is a cytoskeleton in a cell, maybe doors and windows is a better parallel, but you could still drive your car then). Good luck getting anywhere without those.

The reason the other cells in your mouth aren’t killed (and many actually are) is your cells are part of an organism as opposed to an individual bacterium. This has the benefit of reducing the exposure/surface area cells have to the environment (the listerine) as well as cells not exposed can help out cells that are maintain their internal chemistry.

And finally, I guess there are treatments where ethanol is injected directly into the blood stream. I learned something too. Woo.

Edit 2: A few people have asked me why hand soap and other sanitizes are different and as far as I am aware they aren’t (although living an extremely clean lifestyle will reduce your immunity to things). I’d like to note I haven’t seen a study linking the use of hand soap to an increase in the number of harmful bacteria, just anecdotal evidence and some theories, but I have seen one that says normal soap is as good at getting rid of bacteria as antibacterial soap so that’s something to think about.

I believe the confusion stems from what I consider ultimate antibacterial properties and safe ones (I made these terms up they’re not scientific). That is to say ultimate antibacterials kill everything, good cells, bad cells, whatever and alcohol is both an ultimate and average antibacterial depending on its concentration. When people talk about bacteria evolving resistances to antibacterials they mean at levels which are safe for good cells, our cells and naturally occurring bacteria rather than a complete and total resistance as can be developed to something like penicillin. Penicillin marks a very specif portion of very specific kinds of cells so your body will attack them. Bacteria are getting really good at hiding that part of their cell.

To put it another way, a cell may evolve a resistance to moderate heat but will never to extreme heat because at certain temperatures molecular bonds simply break down. There are bacteria which have evolved to live in extreme ranges of temperature, hundreds of degrees on any scale, but even these have their limits. No bacterium we are aware of can live on the surface of the sun, or even evolve to, but that doesn’t help keep us kill them since using the surface of the sun to kill the bad cells would also kill the good ones.

I’m not saying it’s impossible for bacteria to acquire resistance to antiseptics, these bacteria exist and have a big thick “shell” surrounding them, but this would be completely incompatible with it’s function of living in your mouth (the whole reason it was there to begin with).

You may be wondering now why antiseptic doesn’t kill your own cells, the ones lining the mucous in your mouth. In fact, they do. When antiseptic is applied to the skin your cells are protected by the keratin layer lying on top of your skin. When applied to mucous in the mouth the antiseptic can penetrate easier into your tissue (no keratin layer), that’s why the concentration of antiseptic in Listerine is a lot lower than that of antiseptics you would use on your skin. Also, if you leave Listerine in your mouth for more than a few seconds it starts to burn, that’s the antiseptic starting to damage your cells.

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