**2. Brief overview of Viruses**

Viruses are ubiquitous infective materials composed of a genetic material generally protected by a proteinaceous coat. Only an electron microscope can visualize them. Viruses are obligatory parasites, which require a live cell to multiply. They cannot proliferate outside of the cell because they need the cell machinery to multiply their genetic material and to produce their own proteins always depending on the machinery of the host.

Generally, viruses infect by introducing their genetic material into the host cell. Then, the viral genetic material hijacks the host systems and the host starts to produce the viral proteins as well as the genetic material. At the end of the process, the viruses opt for staying inside the host cell or to rupture it and disseminate.

In order to use the host machinery, the genetic material of the virus codes for a few specific proteins able to interact with the host proteins. Although a small number of viral proteins are produced by the host, they have a high affinity for the host proteins. This is the reason why viruses are very specific to their host and very rarely viruses can infect different species.

## **3. Antibiotics**

Antibiotics are molecules able to inhibit the growth of bacteria. In nature, antibiotics are produced as secondary metabolites by specific groups of bacteria and fungi. The definition of secondary metabolites means that they are not involved in essential metabolic reactions in the cell. Then, if the genes responsible for their production are deleted from the bacterial DNA, they still can proliferate. Instead, it looks that antibiotics are produced in order to compete for nutritional sources by inhibiting or stopping the development of other bacterial competitors.

Penicillin was the first antibiotic discovered in 1928 by Alexander Fleming and started to be used to combat infections in 1942. Since then, new antibiotics were approved with a concomitant decrease over the last decades. The reasons for this decrease are discussed below.

When discussing the development of new antibiotic targets, it should be taken into consideration the bacterial target. Many metabolic pathways and enzymes in bacteria are highly conserved across living organisms. Therefore, these pathways and enzymes are not useful as targets because it will inflict similar damage(s) to human cells. Thus, the antibiotic targets should be directed to any bacterial target (e.g., protein, biosynthetic pathway, etc.) that does not have any similarity in human. Examples of antibiotics targeting bacteria and mechanism of resistance are depicted in **Figure 2**.

#### **3.1 Bacterial variation and development of resistance**

Bacteria multiply by binary fission, which means that the parental cell divides into two daughters. Each daughter is considered a clone or genetically identical

**3**

**Figure 2.**

*bacterial resistance [2].*

*DOI: http://dx.doi.org/10.5772/intechopen.83691*

offspring generated by vegetative multiplication. As mentioned before, bacteria multiply exponentially very fast with a generation time between 20 and 60 min, depending on the species. Thus, in a bacterial culture, although originated from a single cell, a prolonged growth may generate a residual change as a result of an adaptive process, resulting in spontaneous mutations. If we calculate the number of mutations (at a rate of 10<sup>−</sup>10 mutations per nucleotide base) in the genome of the bacteria *Staphylococcus aureus*, which contains 2.8 million nucleotide base pairs in its genome, an astonishing number of 300 mutations will be

*Mechanisms of bacterial resistance to selected antibiotics. (A) Antibiotic mechanisms. (B) Mechanisms of* 

*A New Era without Antibiotics*
