Plant Phenolics as an Alternative Source of Antimicrobial Compounds
Inamullah Hakeem Said1, *, Nikolai Kuhnert1, * 1 Department of life sciences and Chemistry, Jacobs University Bremen, 28759 Bremen, Germany
Abstract
The use and search for new medicines and dietary supplements derived from plants have enhanced in the modern era. Plants are rich in a wide variety of secondary metabolites, such as tannins, terpenoids, alkaloids, and flavonoids, which have been found in vitro to have antimicrobial properties. Polyphenols are natural compounds characterized by a high structural variety, and their existence in plants makes them essential components. Increasingly, resistant bacteria require the continuous need for new and effective antibiotics. This class of natural products is becoming the target of anti-infective research, and many groups have identified that phenolic compounds are possessing broad-spectrum antibacterial and antiviral activity. Owing to interest in their biological activities, both the chemical structure of the phenolic moiety and any attached chemical groups define whether the polyphenol is bioactive or not. Compared to other phenolics, flavonoids class have been abundantly used in anti-infective research.
This chapter summarizes the application of polyphenol tested by various groups against infectious diseases caused by bacteria and viruses as pathogens. The reported MIC values are presented in a concise table, which is conflicting among different groups, possibly due to assay parameters and strain variation. However, it is shown that phenolic compounds have great potentials, which requires future research for the development of novel antimicrobial or antiviral agents or aid as a synergistic agent in combination with conventional anti-infective drugs.
Keywords: Antibacterial, Antiviral, Plants, Polyphenols, Secondary metabolites.
* Corresponding authors Inamullah Hakeem Said & Nikolai Kuhnert: Department of life sciences and Chemistry, Jacobs University Bremen, 28759 Bremen, Germany; E-mails: [email protected] and n.kuhnert@ jacobs-university.de INTRODUCTION
According to the European Union estimation in 2007, approximately 25,000 patients per annum die due to infection with major types of drug-resistant bacteria, which are responsible for bloodstream infection [1].
Increasingly, resistant bacteria require a continuous need for new and effective antibiotics. However, in the last two-three decades, only two new classes of antimicrobials were launched in the market. Many drug developers have left the field. The medical interventions in the 21st century extended and improved our lives, such as anti-cancer drugs, cardiac surgery, transplantation, bone marrow transplantation and joint replacements. All mentioned interventions require strict control of infection. Although the current research could routinely add new medicines and therapies to address numerous other diseases, unfortunately, our grip on infection is loosening.
The agents (bacteria, viruses, fungi, and parasites) that cause infectious disease are continually trying to avoid and by-pass the drugs that are essential for combatting infection. In the past decades, scientists have countered the loss of potency of antibiotics resulting from resistance to the discovery and development of new ones. Over the past half-century, the strategy of introducing new antibiotics to the market to replace older ones, which lost their effectiveness has proven to be highly successful. Unfortunately, this state of equilibrium has now been shifted. Microbes have developed multiple drug resistance mechanisms, and no new antibiotic has been discovered [2].
Structural modification of existing antimicrobials to which bacteria have developed resistance such as the azoles [3], the non-nucleoside reverse transcriptase inhibitors [4], ß-lactams and quinolones [5] were effective means of extending the lifespan. However, with the selection of chemotherapeutics these days, the researchers are finding alternative ways to find new antimicrobials and do not follow the track of parent structure alterations. One of the recent development is to focus on different target sites to those in current use in finding novel antibiotics [6, 7].
In the past, natural products were also the major promising sources of chemical diversity for starting materials of drug discovery [8], for example, the penicillins, the tetracyclines and the glycopeptides between 1940 and 1955. In almost all therapeutic areas, many natural products or...