Antioxidants: Natural Antibiotics DOI: http://dx.doi.org/10.5772/intechopen.84864


#### Table 1.

5. Natural antioxidants as antibiotics

makeup bacteria cannot be copied.

curcumin, etc.

Antioxidants

(LPS) [6].

354

6. Mechanisms of action

The increased resistance of pathogenic microorganism against antibiotic becomes a major issue around the globe from the last decade. To overcome this serious problem, it is necessary to discover a new world of antimicrobials, which are not only beneficial in bacterial infection but show long-lasting effect by boosting the immunity of the body. However, we do not skip the usage of previously practiced antibiotics as some of them show a very effective result in bacterial infection, but there is a need of advanced or may say strong antibacterials whose chemical

Plant synthesizes a variety of secondary metabolites (phytochemicals) which are involved in plant defense mechanism, and it is recognized that major classes of these molecules have beneficial effects on health including antioxidants and antimicrobial. The attractive antioxidant as well as antibacterial activity of phytochemicals seeks attention as it may replace the synthetic antioxidants, which cause deleterious effect on human health such as cancer. The plant

kingdom is rich in various phytochemicals like phenolic acid, flavonoids, gingerol,

Phenolic acids and flavonoids are a very important class of antioxidants as it directly affects bacterial growth and causes hindrance in their pathogenic activity. The mechanism of action of antioxidants as antibacterial is still not fully understood, but some researches reveal that the attributable antibacterial activity involves three basic mechanisms: outer membrane permeability, cytoplasm leakage, and inhibition of nucleic acid formation. The interaction of polyphenols with nonspecific forces like hydrogen bonding and hydrophobic effect lipophilic forces as well as by covalent bond formation was related to microbial adhesion and enzyme and cell envelope transport protein. The antibacterial activity of polyphenols may also due to the capacity of these compounds to chelate iron, vital for the survival of almost all bacteria. Polyphenols rupture the wall,

increase the permeability of cytoplasm membrane, and release lipopolysaccharides

The cell wall composition of Gram-positive and Gram-negative differs significantly as Gram-positive bacteria have thick layer of peptidoglycan along with lipoteichoic acid but lack of outer membrane. Gram-negative bacterial outer membrane consists of phospholipid, protein, and LPS and a thin layer of peptidoglycan. Both Gram-negative and Gram-positive bacterial cell walls play a very important role in osmotic protection of cell. Any damage to cell wall will decrease the tolerance of cell against osmotic pressure and ionic strength. Many researchers have demonstrated that the interaction of polyphenols with bacterial cell wall is different for Gram-negative and Gram-positive bacteria. Different interaction cites for antibacterial agents in variety of bacterial strains are shown in (Tables 1–4).

The activity of antioxidants against bacterial inflammations is being progressively recognized. They also work synergistically with current antibacterial agents against the resistant strains of bacteria. The diversity in the structure of natural products makes it impossible for bacteria to copy its functional moieties, unlike the synthetic agents. The structure of antioxidants holds the key role in determining the antibacterial activity. Different groups of researchers investigating the relationship between flavonoid structure and their antibacterial activity generalized that active compounds share common structural features. Moreover, the unique structural

Natural antioxidants and their role in inhibiting bacterial growth in living system showing interaction between the bacterial cell wall and with cell membrane.


#### Table 2.

Natural antioxidants and their role in inhibiting bacterial growth in living system showing metal ion deficiency due to chelating ability, microbial enzyme inhibition and substrate deprivation and other inhibition mechanism.

features may be essential for flavonoids to gain adjacency or uptake into the bacterial cell. Like, polyhydroxylated flavonoids show more pronounced antibacterial activity than mono-hydroxylated or non-hydroxylated flavonoids. Structural


#### Table 3.

Purified antioxidants extracted from plant sources and their antibacterial potential: phenolic acid, flavones, flavanonols, catechins, and vitamins.

similarity among flavonoids is too dominant that there are three probable hypotheses regarding their mechanism of action:

> bacterial population which reduce the oxygen consumption by bacteria (interruption respiratory chains). Reduction in surface area of cells decrease the nutritional uptake like uridine and thymidine (specify nucleic acid inhibition). Moreover, the prospect of baffling and the cause and effect of mechanisms of actions exist. For example, the interruption of membrane integrity by an antibacterial agent will impart negative effects on proton-motive force that directly influence the synthesis of ATP and solute transport into the bacterial cell. The deterioration of bacterial capability to produce energy and to attain nutrients results in declining capability of bacterial cell to make DNA and peptidoglycan. So, one mechanism of action may be

> Purified antioxidants extracted from plant sources and their antibacterial potential: chalcone and flavanone.

Correspondingly, if any enzyme of bacteria-like DNA gyrase is obstructed by an antibacterial agent, then this swift automated cell death and lysis. Likewise, the

misunderstood as multiple.

Antioxidants: Natural Antibiotics

DOI: http://dx.doi.org/10.5772/intechopen.84864

Table 4.

357


According to a recent development, the study of mechanism of actions is not that reliable as it was assumed earlier. Like epigallocatechin gallate only induce clumping of FabG enzyme and have no such effect on other enzymes. Another such development is that flavonoids cause aggregation of bacterial cells. Clumping of bacterial cells on treatment with flavonoids causes reduction in surface area of


#### Table 4.

similarity among flavonoids is too dominant that there are three probable hypothe-

Purified antioxidants extracted from plant sources and their antibacterial potential: phenolic acid, flavones,

According to a recent development, the study of mechanism of actions is not that reliable as it was assumed earlier. Like epigallocatechin gallate only induce clumping of FabG enzyme and have no such effect on other enzymes. Another such development is that flavonoids cause aggregation of bacterial cells. Clumping of bacterial cells on treatment with flavonoids causes reduction in surface area of

ses regarding their mechanism of action:

flavanonols, catechins, and vitamins.

Table 3.

Antioxidants

356

a. Flavonoids of same structure take same mechanism.

b.All flavonoids follow multiple mechanisms of actions.

c. All flavonoids have same sole mechanism of action.

Purified antioxidants extracted from plant sources and their antibacterial potential: chalcone and flavanone.

bacterial population which reduce the oxygen consumption by bacteria (interruption respiratory chains). Reduction in surface area of cells decrease the nutritional uptake like uridine and thymidine (specify nucleic acid inhibition). Moreover, the prospect of baffling and the cause and effect of mechanisms of actions exist. For example, the interruption of membrane integrity by an antibacterial agent will impart negative effects on proton-motive force that directly influence the synthesis of ATP and solute transport into the bacterial cell. The deterioration of bacterial capability to produce energy and to attain nutrients results in declining capability of bacterial cell to make DNA and peptidoglycan. So, one mechanism of action may be misunderstood as multiple.

Correspondingly, if any enzyme of bacteria-like DNA gyrase is obstructed by an antibacterial agent, then this swift automated cell death and lysis. Likewise, the

enhances the supercoiling of DNA of bacteria, and its inactivation leads to bacterial death. Estimation of DNA supercoiling is the important parameter in assessment of flavonoids activity to inhibit DNA gyrase. It has two subunits, gyrase A that takes part in DNA breakage-resealing and gyrase B that is involved in the hydrolysis of ATP, the driving force for the DNA supercoiling. The topoisomerase (DNA gyrase) inhibitors form a cleavable complex of agent-topoisomerase-DNA or interfere with

Kaempferol show the strongest antibacterial activity (MIC50 = 25 μg/ml) against E. coli DNA gyrase. It inhibits the activity of gyrase enzyme that holds the key role

Structure of Polymethoxylated flavones Polymethoxylated flavones (shown in structure) usually found in citrus peel possess broad spectrum antimicrobial activity. It shows antibacterial activity against E. coli and S. aureus with IC50 values ranging from 1.45 to 1.89 mg/ml [8] (see

Quercetin is one of the ubiquitous flavonoids, impedes the DNA supercoiling, and causes DNA to cleave. Quercetin encourages DNA scission by forming gyrase-DNA-quercetin cleavable complex. Cleavage of DNA was promoted at quercetin concentration above 80 μM in the presence of gyrase, and at 640 μM the maximum cleavage was obtained [9]. MIC values of quercetin are listed in Table 3. Quercetin obtained from yellow onion skin has inhibitory effect on antibiotic-resistant bacteria H. pylori. Sulfur and quercetin have synergistic growth inhibitory effect with

Structure of quercetin

First inhibition pathway involves rivalry at binding site of ATP at gyrase B that prevents DNA supercoiling. The second mechanism involves binding to DNA that

Mode of action of quercetin inhibition includes two mechanisms:

the gyrase binding to DNA (see Table 2).

DOI: http://dx.doi.org/10.5772/intechopen.84864

in DNA supercoiling and bacterial growth.

beta lactam, a very functional antibiotic [10].

6.1.1 Kaempferol and PMFs

Antioxidants: Natural Antibiotics

Table 3 for MIC values).

6.1.2 Quercetin

359

Figure 2.

The schematic layout demonstrating the natural antioxidant role as antibacterial: (a) representing inhibition of energy metabolism; (b) representing disruption of membranes; and (c) representing interruption in nucleic acid synthesis.

antibacterial agent that impedes the synthesis of nucleic acid may be misinterpreted as the agent that alters the cytoplasmic membrane functions [7].

The following mechanisms of actions are attributed to the antibacterial action of flavonoids as reported by different groups of researchers:

	- a. Alteration of cytoplasmic membrane fluidity
	- b. Inhibition of cell wall formation
	- c. Inhibition of cell membrane formation

The mechanisms of action of antioxidants as an antibacterial are shown in Figure 2.

#### 6.1 Inhibition of nucleic acid synthesis

Among the classes of antioxidants, flavonoids significantly show inhibitory activity against nucleic acid synthesis. Interaction of flavonoids with DNA- or with ATP-binding site of gyrase finally leads to the inhibition of nucleic acid synthesis as shown in Figure 2. Metabolism of DNA in bacteria comprises transcription, recombination, DNA replication, and transport of genetic information. A vital enzyme to control vigorous changes of nucleic acid is DNA gyrase. Gyrases, characteristic and crucial bacterial enzymes that change the topology of DNA, are the amiable aim to hit for the antibacterial agents. DNA gyrase, in a reaction that depends on ATP,

#### Antioxidants: Natural Antibiotics DOI: http://dx.doi.org/10.5772/intechopen.84864

enhances the supercoiling of DNA of bacteria, and its inactivation leads to bacterial death. Estimation of DNA supercoiling is the important parameter in assessment of flavonoids activity to inhibit DNA gyrase. It has two subunits, gyrase A that takes part in DNA breakage-resealing and gyrase B that is involved in the hydrolysis of ATP, the driving force for the DNA supercoiling. The topoisomerase (DNA gyrase) inhibitors form a cleavable complex of agent-topoisomerase-DNA or interfere with the gyrase binding to DNA (see Table 2).
