**6. References**


In this chapter, we proposed a structural classification of antimicrobial peptide families considering the diversity of their structures and then we reviewed the traditional function and biological activities. Finally, we propose new insights into the functions of antimicrobial peptides that could provide a large body of research to create new classes of antimicrobial therapeutics. AMPs are widespread molecules throughout the animal and plant taxa, this fact suggest its relevance in the evolution of immune response. Traditionally, their basic molecular and biochemical nature is related to the disruption of membrane potential and/or structure with the ensuing cell death. However, the diversity in the structure and biological properties (above mentioned) of AMPs within and between species suggest that these

The immune system of living organisms is formed by a set of cells, molecules and reactions. All of these features are continuously evolving to resist (attack and eliminate) pathogen invasion and to limit the negative (in terms of host survival and reproduction, i.e. fitness) consequences of the infection (Hoffmann and Reichhart, 2002). On the other hand, pathogens success depends upon overcoming the selective immune pressures brought about by the host. As a consequence, both, host and pathogens, evolve traits and strategies to increase the fitness of each one. Van Valen (Van Valen, 1973) proposed this co-evolutionary arm races as an evolutionary theory called "The Red Queen Hypothesis". The theory was proposed citing Lewis Carrol's Red Queen, where it takes all the running somebody can do to keep in the same place. Given this situation, immunologically, there is never a "best" solution to pathogens infection. To understand this scenario, we must consider (1) pathogen's short generational cycles, that may provide enough time to adapt to the host's immune response. As an outcome there will be grounds for high variability in immune response. (2) Differences in the kind and burden of pathogens, where divergent hostpathogen interactions for each species are possible and can be reflected in the course of action of taxa immune response (Read and Taylor, 2001). (3) Virulence differences, where there can or cannot be a harm imposed on a host (for example *Bacillus anthracis* vs. commensal microbiota). As long Hypothesis, an immune effecter that has the ability to be produced under different infection circumstances could have a selective advantage, antimicrobial peptides could have this property, as the host has to deal with the problem of

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**4** 

*South Africa* 

**The Susceptibility of** *Staphylococcus aureus*

The emergence of multi-drug resistant organisms has increasingly become a global public health issue. Rational and appropriate uses of antibiotics as well as strict infection control measurements are recommended in order to reduce the emergence of antibiotic resistant bacteria (Tseng et al., 2011). The complexity in treating multi-drug resistant infections has led to an increase in the search for novel and effective antibiotics, especially structures originating from natural products. Promising molecules could serve as lead compounds to

This chapter aims to review the susceptibility of two of the most common micro-organisms that are often implicated in antibiotic resistant infections, namely the Gram-positive *Staphylococcus aureus* and Gram-negative *Klebsiella pneumoniae* against natural products, specifically plants. Numerous researchers have investigated the susceptibility of these bacteria to plant extracts as well as to the individual components thereof. Flavonoids as a group of compounds originating from natural products have been investigated against these bacteria. Flavonoids are diverse polyphenolic compounds which are widely distributed in the plant kingdom. They are abundantly found in natural sources like fruits, vegetables, seeds, nuts, flowers, tea, wine honey and propolis and therefore form part of the normal diet of humans (Cook & Samman, 1996). Many reports claim the usefulness of flavonoids in medical conditions, including anti-inflammatory, oestrogenic, antimicrobial, antioxidant and chelating, vascular and antitumour activities (Cook & Samman, 1996; Cushnie & Lamb, 2005). Flavonoids consist of a C15 skeleton composed of two phenolic rings, namely the A and B rings linked through a heterocyclic ring, C. They are classified according to their biosynthetic origin into major classes including flavones, flavonols, flavanones, chalcones, flavanols, anthocyanidins, isoflavones and dihydroflavonols. Substitution patterns vary and some flavonoids occur as glycosides which are hydrolysed in the human gut to the aglycones. Flavonoids also occur as monomers, dimers or

Many reports exist on the antimicrobial activity of flavonoids (Basile et al., 2010; Du Toit et al., 2009; Tanaka et al., 2011). Extracts as well as isolated compounds were tested against a

**1. Introduction**

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Corresponding Author

be developed and researched further.

oligomers (Cook & Samman, 1996; Cushnie & Lamb, 2005).

**and** *Klebsiella pneumoniae* **to Naturally Derived Selected Classes of Flavonoids** 

Johannes Bodenstein\* and Karen Du Toit

*University of KwaZulu-Natal (Westville Campus), Durban,* 

*Discipline of Pharmaceutical Sciences,* 

