**3.1 Basic characteristics**

*The Universe of Escherichia coli*

**4**

**Figure 2.**

*represented under a and a*′*.*

Chalmers and became *Escherichia coli* [3].

demonstrated that these bacteria have fermentative ability. He also performed the Gram method of staining and revealed that these bacteria rapidly take color with all aniline dyes but lose the color after treatment with potassium iodide and alcohol [2]. Later, in 1919, the bacterium was renamed after its discoverer by Castellani and

*Escherich's drawing of the stool bacteria, as seen under light microscope [4]. Panel 1: Preparation of a meconium of a 27-hour-old infant. The E. coli as Bacterium coli commune is represented under d. Panel 2: Preparation of a stool of a 2-month-old healthy breast-fed child. The E. coli as Bacterium coli commune is* 

The bacterium *E. coli* (**Figure 3**) belongs into the family of *Enterobacteriaceae*. It is a Gram-negative rod-shaped bacterium, non-sporulating, nonmotile or motile by peritrichous flagella, chemoorganotrophic, facultative anaerobic, producing acid from glucose, catalase positive, oxidase negative, and mesophilic [5].

*E. coli* is a well-known commensal bacterium that is among the first colonizing bacteria of the gut after birth. It is a highly successful competitor in the human gut and is comprising the most abundant facultative anaerobe of the human intestinal microbiota [7]. As it is a facultative anaerobe, it survives when released to the environment and can be spread to new hosts. *E. coli* is thus an important component of the biosphere [8].

Even though *E. coli* is a well-known commensal bacterium, many pathogenic strains of *E. coli* do exist. Several highly adapted *E. coli* clones have acquired specific virulence factors, which confer an increased ability to adapt to new niches and allow them to cause a broad spectrum of disease, and intestinal and also extraintestinal infections [7].

**Figure 3.** *Scanning electron microscopy of a single bacterial E. coli cell adhering to 19-day-old Caco-2 cells [6].*
