**1. Brief overview of Bacteria**

Bacteria are ubiquitous unicellular organisms able to adapt to the environmental changes in a very fast way. The double time of bacterial cells varies between 20 and 60 min.

To visualize bacteria, a microscope is required. However, as a result of their transparency, their visualization is impaired unless a stain is used. In 1884, the Danish bacteriologist Hans C. Gram published a technique by which, bacterial cells can be divided into two groups according to their color after the staining [1]. Based on the staining remnant, bacteria are classified as Gram-positive (purple) and Gramnegative (pink). This separation is based on the ability of the Gram-positive bacteria to retain the dye crystal violet, according to their cell wall composition (**Figure 1**).

**Figure 1.** *Cell wall comparison according to Gram staining [2].*

Not all of the bacteria are classified in these groups. For instance, mycobacteria species do not respond to Gram staining as the result of a lipidic cell wall resistant to the stains. However, the Ziehl-Neelsen stain or acid-fast staining was developed *and these species are visualized as a bright red color.* The classification of bacteria in the Gram-positive and Gram-negative group is important to understand the activity of antibiotics, which will be described later on. In this regard, antibiotics can be specific to treat either Gram-positive, Gram-negative, or both of them and then defined as broad-spectrum antibiotics.
