**1.2 Problematic of microbial infections nowadays: drug resistance and climate change**

According to WHO microbial infections are said to be the second cause of death globally, with low- and middle-income countries bearing the greatest burden. They include bacteria or fungi but viruses and protozoa diseases are also listed in this category (https://www.who.int/news-room/fact-sheets/detail/antimicrobial-resista nce). Their origin preceded that of human life on earth [15]. In fact, human comes from successive mutations and evolution of bacteria [15]. Our body is made up of more than 100 trillion bacteria [16–18]. Some of them are useful in human life where they played a critical role in metabolism. However, greater percentage of them has been found to be harmful. By analogy to what is being said by believers, "you are dust and will return to dust," one can also argue that "you are bacteria and will return bacteria."

A lot of concerted effort has been put forward since the existence of mankind in trying to understand the biology of infective pathogens and their control. Some success has been achieved although there still more room for further research on this area. Through our constant manipulation and uses of these pathogens together with huge amount of chemicals, including drugs, we end up developing "new organisms" with different properties compared to their natural counterparts. In fact, the original pathogens start developing resistance to the drugs that were previously used for their eradication, making the problem worse [19–22].

The question of resistance of pathogens to commercialized drugs relies on the living environment of these small organisms. A misconception of bacteria considered that they exist as individual organisms [23]. However, things are different. Bacteria accumulated in colonies to live. They generally stick on a surface and gathered to survive together [23, 24]. Such a constitution known as biofilm is made up of bacteria somehow wrapped in a certain liquid (extracellular matrix) with strange properties. The entire constitution acts as a safety membrane for bacteria. The so-described making-up of this living organism constitutes the first barrier to bacteria and therefore the first stage of resistance [25, 26]. Biofilm are quite distributed in hospitals and nursing homes. They are claimed in household and industrial pipes, biomaterials such as contact lenses, medical devices including implants and urinary catheters, as well as plant and animal tissues. Cases of bacteria resistance have exploded this last decade especially in those zones [23–26]. This includes bacteria like *Acinetobacter*, *Pseudomonas* and various *Enterobacteriaceae* (*Klebsiella*, *E. coli*, *Serratia* and *Proteus*) which cause severe and often deadly infections such as bloodstream infections and pneumonia. Bacteria are carried over by devices such as ventilator systems and blood stream catheters. In high-income countries, 7% of all hospitalized people will contract some form of infection, including one in three people in intensive care units. In low- and middle-income countries, this figure rises to at least 10% of hospitalized people, and up to half of people in intensive care units, said the WHO (https://www.who.int/news-room/fact-sheets/detail/antimic robial-resistance).

The World is currently facing an unprecedented rising of temperature, leading to certain change in habits and behavior. Discussion are mainly focused on how it could have an impact on human life, as it's getting colder and colder when it is the cold season or more and more hot when it is the hot season or even hot when it is supposed to be cold and vice versa. Politics in every country recommend adopting new habits to stop the rising up of the earth temperature. In the meantime, it seems that no one is caring about changes occurring at microorganism scale. It comes out that changes occurring in microorganisms due to climate change are not so important for us. And yet we should beware at least, the occurrences of the new viruses Ebola and Corona these last years should make us change our mind on how the World is changing. New microorganisms would be discovered, and the existing microorganisms could mutate to more harmful organisms. Fortunately, almost the same changes are also expected in plant kingdom even though the global warming and deforestation contribute also to plant extinctions. It is also awaited that new metabolites are being made as a result of new biosynthetical routes. These metabolites could either be directly active against pathogenic microorganisms or inspire new synthetical routes in laboratories to reach new drugs and medicines.

#### **1.3 Standard antibacterial and anticancer cutoff points**

Discussions are ongoing in the literature between scientists to clearly established standards for a substance to be considered for further steps in drugs development. Established standards are rare or inaccessible. The Kuete's group proposed standards of evaluating antimicrobial or anticancer properties of secondary metabolites derived from plants. They established that the antimicrobial activity of a crude extract can be considered significant when it MIC is below 100 μg/mL, moderate when between 100 and 625 μg/mL and low when more than 625 μg/mL. For pure compounds, the activity is considered significant when the MIC is below 10 μg/mL, moderate when between 10 μg/mL < MIC<100 μg/mL or low when greater than 100 μg/mL. One can notice that these standards are not considering the MIC of standard commercial drug references. We are not saying that the comparison of obtained MIC with those of the commercial drug is not necessary but the abovementioned cut-off points precise the ranges for a plant substance to be considered as valuable in drug discovery. Likewise, an extract is said to possess a good cytotoxicity if the IC50 values are below 4 μg/mL, modererate when 4 < IC50 < 20 μg/mL/10 < IC50 < 50 μM and low with IC50 above 100 μg/mL (250 μM) [14, 15]. While for cancer cell lines the activity of the pure compound is consider strong when IC50 < 10 μM [20, 21].

Another point of constant intensive discussion remains the relative low activity of most plant extracts and related constituents against microbial and cancer strains. Their activities are sometimes hundred- or thousand-fold less than the sensitivity of existing drugs. Some scientists find these activities not significant enough to be considered for clinical trials as a phytochemical substance should show comparable sensitivity with the commercial drugs. However, knowing that most cancer treatments are based on chemotherapy which is, as known, as harmful as the disease, can natural substances replace synthetic compounds? Likewise, one can also question infectious disease treatments in the same words. In what extent can we clearly consider a plant based products in drug development with respect to their activity against strains of bacteria or tumor cell lines? As mentioned above, the objective is not to compete with existing drugs developed with many expenditures but simply to select valuable extracts and phytoconstituents which could be used alongside actual treatments because of their safety and availability status especially for less developing countries.

#### **1.4 World sustainable development goals related to the field**

In September 2015, the UN adopted a list of 17 goals for a better life on the planet with emphasis on the quality of life for posterity (https://sustainabledevelopment.

#### *Plant-Derived Compounds against Microbial Infections and Cancers DOI: http://dx.doi.org/10.5772/intechopen.92596*

un.org/?menu=1300). Globally it is recognized that ending poverty and other deprivations must go hand-in-hand with strategies that improve health and education, reduce inequality, and spur economic growth, by tackling climate change and working to preserve our oceans and forests are essential for our future as human beings. In the health sector, people should commit themselves to promote healthy lively hood and well-being for all at all ages. These are objectives stated in the Sustainable Development Goal number 3 of the list. Targets within this goal include ending the preventable deaths of newborns and children and ensuring access to effective medicines to all.

However, we are still living in a place where basic infections (malaria, typhoid, diarrhea, cholera and others) can cause death; a World where medicines are too expensive and inaccessible to everyone; a region where people have to walk more than 10 Km to expect treatment in a hospital or a World with increasing political and economic crisis. Nowadays, one should also highlight the increasing resistance of microbes and other pathogens to existing drugs and the occurrence of new strains of bacteria and virus. The former has been related to the overuse and misuse of drugs which modify the living pathogens environment making them used to it, thus developing tolerances to the used drugs.

One of the alternatives to tackle these challenging issues remain natural remedies and drugs. Many sources are being investigated but plants remain the most exploited. Substances from plants are quantitative, affordable, reachable and biologically recognized and easily metabolized by other organisms. They are environmentally friendly and can thus be promoted ever. Numbers of reports are available in the literature, highlighting the antimicrobial and anticancer properties of phytoextracts and products. Extracts can then be standardized and proposed to our fellow population to alleviate the cost of various and diverse drugs available in the markets.

#### **1.5 Rationale of this survey**

The present research literature aims to review recent plant compounds reported for their anticancer or antimicrobial properties which constitute valuable candidates to drug development. Our survey covers research reported from 2010. We only listed compounds with MICs or IC50s < 10 μg/mL for a molarity scale ranging from 10<sup>6</sup>–20 μM. Activities of extracts were not highlighted herein. Both sensitive and resistant strains were checked out without restriction.

#### **2. Plant-based secondary compounds with antimicrobial properties**

Infective diseases are one of the most common illnesses in the World. They are currently the main concern on earth due to the ongoing Coronavirus (Covid-19) outbreak. Some pathogens spread out in animals and much of them are not known so far. But, at one moment or at another, due to our growing familiarization with wild animals, pathogens can spread within Human kingdom. Research are constantly been done to contain the diseases and come over the pathogens. Most of them are based on drug discovery, one of the oldest fields of Human concern so far. Plants constitute the main source of drugs although interests have moved to bioactive microbial constituents in the last decades mainly against microbial infectious. Owing to the rich biodiversity in our planet, the search for bioactive compounds from untapped natural resources is among the important ongoing projects.

One of the main constituents of plants with pronounced therapeutic interests against infective diseases are volatile oil. They are found in almost every organ of a
