Microorganisms: let’s learn about them (and recognize them)

Tiny little ones, but very different. We can hardly ever see them with the naked eye, so we tend to group them under the term “microorganisms.” Yet within this category lurks an impressive variety of extremely diverse little beings that have very little in common. Sometimes just the size.

With the previous articles, we realized how important definitions are. Knowing how to name something is the first step to knowing it better and being able to study it more deeply. That is why today we are going to name the different microscopic inhabitants we find all around us. But also at us. E inside of us!

Microorganisms: an initial classification

To find order within the huge variety of microorganisms around us, let’s start by looking at what they all have in common: the cell. To be called “micro-organisms,” they must as a first step be “organisms,” that is, living beings composed of at least one cell. All living things known so far, divided into three domains, have cells that belong to two categories: prokaryotic and eukaryotic.

Prokaryotic cells are the simplest, consisting of a membrane separating the inner environment (the cytoplasm) from the outer environment, a storehouse of genetic information (DNA) and little more. This cell type is typical of prokaryotes, that is, bacteria ( Bacteria domain) and archaea ( Archaea domain). The Eukaria domain, on the other hand, to which we also belong, includes living things equipped with eukaryotic cells.

Eukaryotic cells are more complex than prokaryotic cells. In the scientific world, the word “complex” means “composed of multiple, interconnected, cooperating and interacting parts.” And indeed within eukaryotic cells we find many parts that coordinate their activities to ensure the life of the organism, whether it consists of a single cell or multiple cells.

I like to compare the eukaryotic cell to a house; within it we find many “rooms,” many environments, in which different activities take place throughout the day. These rooms are called “organelles,” are bounded by one or more membranes, and are present only in eukaryotes. Of these, the nucleus was the first to be discovered and used to distinguish cells into the two categories we have described. In eukaryotes, DNA remains stored in the nucleus, which protects this very important booklet of genetic instructions.


Yeasts: the microorganisms most loved by bakers and makers of alcoholic beverages!

Yeasts are unicellular eukaryotic organisms (i.e., formed by a single cell, which lives independently of the others, constituting a separate organism). They are generally very small compared to other eukaryotic cells. Despite being so small (generally under 10 μm, or a hundredth of a millimeter), these little guys contain within them an actual nucleus and all those organelles that organize the cell into specialized rooms.

The most famous yeast of all is surely Saccharomyces cerevisiae, which is the big boss of baking and wine and beer production. Because of its ability to eat sugars and ferment them, this yeast produces ethyl alcohol (of interest to producers of alcoholic beverages) and carbon dioxide (of interest to bakers to make dough swell and make it fluffy).

I’m sure many of you are already itching your hands, so now I’ll specify: to make bread, pizza, beer, and wine, you don’t just use Saccharomyces cerevisiae. Many other yeasts, bacteria, or consortia of microorganisms can be used for these fermentations. In the photo below, which I took, you can see the Saccharomyces cerevisiae cells viewed under a light microscope, magnified more than 1,000 times (heat fixation and staining with methylene blue).

yeasts - microscope image


Bacteria: tiny microorganisms allied to food fermentation

We now come to bacteria. They too play a very important role as “ferments,” that is, as microorganisms capable of fermenting and producing substances of food interest (e.g., lactic acid that acidifies our yogurt or other acids that give the characteristic aroma to cheeses or vinegar). Bacteria are single-celled prokaryotes.

If yeasts already seemed small to you, bacteria are even smaller! In fact, most of them measure less than 2 thousandths of a millimeter (typically between 0.2 and 2 μm, although there are exceptions, with “giant” bacteria). There are no membrane-bounded environments in their cells. It is all one big “open space,” in fact. But don’t be fooled by the simplicity of their cell-these tiny little beings are actually tough guys. Some of them are capable of living in conditions that we would find terribly inhospitable: hot environments, cold environments, salty waters–not to mention archaea, which are true superheroes in terms of ability to survive in extreme conditions! (Before microbiologists stone me to death, I again specify that archaea are NOT bacteria: although in some ways they seem to resemble bacteria, they are actually part of two different domains of living things).

Below, in the photomicrograph I have prepared for you, you see a “collection” of bacteria of many different species from a sample of standing water I took in an irrigation canal. The bacteria appear as blue or blue dots, either isolated or in clusters of two or more. Magnification is always greater than 1000✕ (hot fixation and staining with methylene blue).

Bacteria: microscope image

The different sizes of bacteria, yeast and human cells

To better give an idea of the different sizes of bacteria, yeast and animal eukaryotic cells, I prepared this last slide. You can recognize there the yeasts and bacteria I showed you earlier, side by side with each other, and even a few human cells, taken directly from my cheek and sacrificed for the sake of science. To make the microorganisms and the nucleus of my (eukaryotic) cells more obvious, I added a blue dye. Notice how gigantic the human cell is compared to the other two. In the image to the right, I highlighted the various subjects with a black outline and put the labels (1000✕, methylene blue).

the different sizes of bacteria, yeasts and human cells - microscope images

In the next article we are going to take a closer look at bacteria, to see the wide variety of different forms we can find. Don’t miss it!

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