Escherichia coli

Escherichia coli or just E. Coli, is a gram-negative rod-shaped bacteria named after Dr. Escherich Theodor, who discovered it in feces, thus concluding that it colonizes the colon.

Alright, now E. Coli is gram-negative because its cell wall has a thin peptidoglycan layer so it cannot retain the crystal violet stain, but instead, it stains pink with Safranin dye used during Gram staining.

So it looks like a little pink rod under the microscope.

Also, E. Coli is a catalase positive bacteria, and that means it produces an enzyme called catalase.

This can be tested by adding a few drops of hydrogen peroxide to a colony of bacteria, and catalase makes hydrogen peroxide dissociate into water and oxygen, making the mixture foam.

E. Coli is also a lactose fermenter, because it can produce an enzyme called beta B-galactosidase that cleaves lactose into glucose and galactose monomers.

To test this, E. Coli can be cultivated on lactose-containing media such as Phenol lactose, and as it ferments it, the fermentation results in the production of acids that turn the red of phenol to yellow.

It is also a facultative anaerobe, meaning it lives in environments with or without oxygen.

Now, taking a closer look to this bacteria, E. Coli is encapsulated, meaning it’s covered by a polysaccharide layer called a capsule.

E. Coli is a motile bacteria, because it has helical whip-like threads called flagella that it can use to move around.

When E coli is cultivated on eosin methylene blue agar, it grows into black colonies with a greenish-black metallic sheen.

Alright, most of E. Coli are harmless, and they can peacefully colonize the human gut without causing any trouble.

However, some strains of E. Coli are pathogenic, meaning they can cause illness. It starts with this bacteria using little thread-like extensions called fimbriae to attach to the host cell surface.

E coli has many different strains that can do that, and they cause different diseases. These strains can be classified by two systems.

The first system uses serotypes, and it groups E. Coli strains based on their antigens.

Antigens are elements that the host’s immunity considers foreign and mount an immune reaction as a response.

So, bacteria within a given serotype, trigger a similar immune response.

Alright, E. Coli has a number of antigens, and among them we have somatic antigens located just on the cell membrane, and these ones are abridged with the letter “O”.