Bacteria Size Hypothesis

Factual Questions
1

I have always found it to be a cruel trick of nature/God that bacteria and single-celled organisms that are responsible for infectious diseases are both:
-too small to be visible to the human (naked) eye individually, and are also -too light for us to be able to feel them as they are on on our body, inside our nose and mouth, etc.

From my AP Biology class, I remember that the reason why bacteria tend to be small is that single-celled organisms have to make a set of steep trade-offs in terms of their volume-to-surface area ratio; specifically, that as the radius of a microbe increases, its volume, which corresponds to how much nutrition the bacteria requires to maintain its life processes, increases as its cube whereas its surface area, through which all nutrients must pass, increases only as its square. It makes a lot of sense that if a bacteria gets too big, then it’s surface area won’t increase “fast enough” for it to be able to survive. A mutation that increases the size of a bacterium of a particular species will tend to be selected against because it has a lower likelihood, all other things being equal, of accessing the necessary sources of food required to survive, and hence reproduce (since survival is necessary although not sufficient for reproduction).

I was wondering: could there be another reason other than the “geometrical” reason above? Specifically:

Is it possible that the size of bacteria that cause infectious diseases are also limited by the fact that if they could become large enough for us to be able to see or feel an individual pathogen through triggering our senses of sight or touch (mechanoreception), respectively, then we would kill them before they had the opportunity to get inside of us and reproduce at a massive level?

I thought it was strange that bacteria have so many different sizes below a certain ceiling; namely, that no bacteria are above some size (as far as we know), but there are many species of bacteria that fall within both the 1-2 um range, and the 10-20 um range. Wouldn’t it make more sense, if the volume-surface area ratio was the key determinant of a microorganism’s size, that there would be a certain optimal size that, ceteris paribus (which is of course not really ever the case in the “real world”) maximized the fitness level of a bacterium so that we would see that the size of bacteria fall within a fairly narrow range centered around that optimal size?

In conclusion, do you think that if genetically modified all animals (humans being included) on Earth to have vision that could easily see 10% of the bacteria species that presently cause infectious diseases in those animals, that these 10% that fall within animal/human naked eye visibility would be selected against and killed off before they were able to pass from “individual” to “individual,” in a statistically significant way compared to our present state, where neither animal nor human is able to see them?

Could it be that the improvement and development of our senses was, in addition to detecting predators/prey from afar, detecting microorganisms up close, and that once upon a time, when our senses could have been much worse, and that at that time, bacteria were much bigger than they are today as an average?

2

No. The difference between “large enough for us to be able to see” and “largest bacteria” is several orders of magnitude. If your hypotheis had any merit there should be plenty of bacteria touching the line between visible and invisible, and lots of them should be large enough to be visible too. It’s not like we’re able to police everything we touch or take in as it is, and we’re not the only targets for infectious diseases either. Supersized bacteria would happily be infecting every critter out there which didn’t thoroughly clean all its food from every speck of dirt and dust.

3

No, there would be no optimal size.

Every species of bacteria has different biology and different niche environments. These differences are enough to explain the different size phenotypes as each species of bacteria would have a different optimum based on their biology and environment. Also, I am not a microbiologist, but I believe bacteria of the same SPECIES do in fact have a very tight distribution regarding their ‘adult’ size. This would further indicate that ‘optimal’ is defined on a per species basis.

The geometric reason you list above is likely the driving reason as to why you never see bacteria above a certain size. But it’s certainly not the only reason.

Besides, you have to remember that humans have only been around for a few million years. Bacteria have been around a few billion. They evolved their optimal biology far far far before we even existed.

4

Perhaps more significantly, macroscopic multi-cellular parasites exist and are successful enough even when we can see them. Worms, ticks, fleas, lice, mosquitoes, leeches, etc. are all doing just fine despite our best efforts.

5

Actually… Thiomargarita namibiensis is large enough to be seen with the naked eye.

6

Also, while not a bacterium, the biggest single-celled organism is 8 inches in diameter, so size is less of an issue than the OP suggests (from the description, it isn’t a big blob, but is made of tubular structures, which increases its surface area).

7

Not really. Infectious bacteria make up a tiny fraction (say one millionth) of the total number of bacterial “species.” If avoiding immune responses was one of the selective pressures keeping bacteria small, we would expect infectious bacteria to be smaller than non-infectious (the vast majority) bacteria. This is not the case. Also, there are plenty of parasites that are many orders of magnitude larger than bacteria, yet still too small for you to see or feel them. You likely have some mites living on your eyelashes right now that are about 100 times longer and several thousand times more massive than a typical bacteria, yet you are powerless to kill them off. For our senses of sight and touch to come into play, a bacteria would have to become as large as a gnat or tick.

They sort of do. The vast majority of bacteria fall within .5-50 microns. This is a relatively narrow range, encompassing only 2 orders of magnitude. Think of the size difference between a blade of grass and a tree, or a mouse and a whale, or a dust mite and a king crab. Volume/surface ratios are the main reason for this small range. Any larger and it’s difficult to exchange material with the outside environment fast enough, any smaller and it’s difficult to fit the minimum amount of metabolic and genomic machinery necessary for life (the current theoretical limit is a little under .2 microns).

The variation in size within this range is dictated by a huge variety of things, including the vast metabolic diversity of bacteria (different bacteria consume different things at different rates), and the incredible diversity of environments (temperature, pressure, osmotic stress, pH, nutrient concentrations, concentrations of various electron acceptors, etc).

8

Just a bit too late. I have a degree in microbiology, for what it’s worth, and I wholeheartedly endorse wheresmymind’s post. It’s almost word for word what I was planning on saying.