Bacteria: all shapes, all colors

In the last article we talked about the two different types of cells that make up all living things and the variety of sizes we find in the microworld. In this article, however, we discuss only bacteria, and find out how to differentiate them, by shape and color.

Differences among bacteria: a matter of form…

Bacteria can have three basic shapes: spherical, cylindrical and curved. These are only three forms, but it is curious to observe how these are declined in many varieties, clustering together. For example: two round bacteria, called coccus, can stand in pairs. Then you call them diplococci. However, if we find them in more or less long chains, we speak of streptococci. I’m sure you’ve heard of these, haven’t you? Perhaps related to some frequent sore throats in children. Equally notorious, again for unkind things, are staphylococci. This category includes bacteria, again with a round shape, which this time stand together to form small “clusters” of cells. Completing the parade of spherical bacteria are the tetrads, which are groups of four cells, and the sarcines, which are packets of eight cells arranged in a cube shape.

Bacteria with a cylindrical shape, like short sticks, are called bacilli. And guess what we call them if they are found in pairs or in chains? That’s right: diplobacilli and streptobacilli. The ones that are a little undecided, which are somewhat cylindrical but not too much, on the other hand, are the coccobacilli: a middle ground, in short.

Curved bacteria, on the other hand, are of three types: if they are only slightly curved rods, they are called vibrio. If they are very curved, like a corkscrew, they are spirilli. On the other hand, if they have lots of coils, like an old telephone cord, they are spirochetes.

the shapes of bacteria

These you see in the above sketch are the main shapes but there are also square-shaped ones , star-shaped ones, or filamentous bacteria, bacteria with stalks, shape-changingbacteria…in short, the (micro)world is beautiful because it is varied!

…and color?

Bacteria tend not to be particularly colorful unless they contain pigments, such as chlorophylls or carotenoids. These colored compounds can be used to do photosynthesis (oh yes: not just plants do it!) or to protect the cell from radiation. Like sunscreen, in short. Since there are not so many colored bacteria, how can color be used to classify them?

Let’s go back to back to 1884. Danish physician Hans Christian Gram was in Berlin in the laboratory of microbiologist Carl Friedländer. He was working on pneumonia, which was still claiming many lives at the time and whose cause was still not entirely clear. In order to be able to see more clearly the bacteria that were the main suspects of the disease, Gram invented a new staining technique that allowed the bacteria to remain clearly visible, but decolorized the lung cells and everything else in the sample. This method of staining bacteria today is called Gram staining, in his honor.

What our dear Gram could not have imagined, however, is that this coloring of his would be used for a very long time to Divide the diverse world of bacteria into two groups: i Gram-positive bacteria and the Gram-negative bacteria. Gram-positive bacteria are stained with purple, and they retain their color even if you try to bleach them with alcohol. Gram-negative bacteria, on the other hand, completely lose their purple color upon passage in alcohol and return to colorless. To be seen, therefore, a second dye is usually used to make them pink. To recap: positive purple, negative pink.

The remarkable thing, and the most useful thing for those classifying bacteria, is that differences in staining actually reflect differences in the structure of the cell itself. In fact, Gram-positive bacteria are bounded by a single cell membrane, outside of which they have a thick bacterial wall. A kind of very heavy coat to wear on the coldest days. Gram-negative bacteria, on the other hand, dress in layers: they have two cell membranes and, in between them, a very thin bacterial wall. In the following figure I try to show this in a simple way.

struttura e colori batteri, gram-positivi e gram-negativi

Based on the characteristics of their outer coatings, therefore, the two categories of bacteria respond differently to dyes, and are therefore classified into two distinct groups.

In this photo, which I took under an optical microscope, you can see Gram-positive and Gram-negative bacteria taken from an environmental sample, magnified over 1,000 times. Can you recognize them?

bacteria color differences

All food-interest bacteria, such as lactic acid bacteria and probiotics that you find in our INgredients Town , are Gram-positive. Sometimes, Gram-negative bacteria are looked upon with distrust, as if they are more dangerous or more pathogenic than Gram-positive bacteria. Certainly, their outermost membrane contains a substance that triggers strong immune reactions in our bodies.

However, a fine example of coexistence between Gram-negative bacteria and humans is our intestines, where billions of Escherichia coli – Gram-negative par excellence – live, but without causing us harm. Similarly, some Gram-positives can also be dangerous pathogens, just think of the bacteria that cause anthrax, tetanus or botulism.

So, as always, even with bacteria it is better not to lump everything together, but to evaluate on a case-by-case basis!

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