CHAPTER ONE

INTRODUCTION

1. BACKGROUND OF THE STUDY

Bacteria and fungi play a fundamental role in the biogeochemical cycles in nature. These micro organisms remineralize organic matter to carbon dioxide, water, and various inorganic salts. Because bacteria are ubiquitous, and capable of rapid growth when provided with nutrients and conditions favourable for metabolism and cell division, they are involved in catalysis and synthesis of organic matter in the aquatic and terrestrial environments. Many substances, such as lignin, cellulose, chitin, pectin, agar, hydrocarbons, phenols, and other organic chemicals, are degraded by microbial action. The rate of decomposition of organic compounds depends upon their chemical structure and complexity and upon environmental conditions. The nitrogen cycle, including fIxation of molecular nitrogen and denitrifIcation, is mediated by microorganisms in the natural environment. Other biogeochemical cycles, including the sulphur, phosphorus, iron, and manganese cycles also depend primarily upon microbial activity. Transformation and mobilization of heavy metals, degradation of pesticides, herbicides, and other man-made, allochthonous materials are left, ultimately to the microorganisms, for recycling. The toxic effects of pollutants on the autochthonous microbial populations, therefore, become of major significance in ecotoxicology (Popoff, 2015).

Recently, novel advances have been performed in bacterial toxins which have been mainly facilitated by the development/improvement of molecular technologies. Indeed, whole genome sequencing of saprophytic and pathogenic bacteria with subsequent genomics analysis allowed the identification of novel toxins and the exploration of their spreading and evolution in the bacterial world. For example, typhoid toxin was initially suspected in cells invaded by Salmonella typhi and was subsequently characterized by genomic analysis and crystal structure, which revealed a novel toxin organization consisting of two distinct enzymatic subunits and five binding subunits. Typhoid toxin seems to have evolved from assembly of several ancestor toxin genes spread in other bacteria, such as cytolethal distending toxins and pertussis toxin (Alouf, 2015). The rapidly increasing progress in the whole genome sequencing of microorganisms would probably result in identification of novel toxins and in-depth understanding of their evolution. Moreover, refinement of the crystal structure investigations allowed unravelling the 3D structure of large or complex toxins, such as the whole structure of the large clostridial glucosylating toxin A from Clostridium difficile (Collier, 1975) or botulinum progenitor toxin complex. However, many aspects of these multifunctional toxin proteins remain to be discovered. Albeit cellular receptors have been identified for some toxins, they are largely unknown for many others. Strategic features, such as toxin dissemination into the host to target cells, interaction with the cell membrane, crossing the phospholipid bilayer, intracellular trafficking, and precise mechanisms of cell alteration are still under investigation for many toxins.

In addition to their interest in fundamental science, bacterial toxins are key players in various applied developments, including tools for diagnosis, prevention, and therapy of diseases, due to toxigenic bacteria. Indeed, the diagnosis of several diseases is based on the detection and identification of toxins in biological and/or environmental samples, such as foods. The development of sensitive and rapid in vitro methods of toxin characterization are still in progress using new or improved technologies, such as mass spectrometry, refined ELISA, or fluorescent techniques. Since the pioneering historical works, detoxified toxins (anatoxins) were shown to elicit a solid preventive response against diseases due to toxigenic bacteria. Anatoxins are among the most efficient vaccines against bacterial diseases. Recombinant toxin subunits, which are biologically inactive, but retain the immunogenicity, offer the advantage to be safer than the classical detoxified toxins. Development of efficient toxin inhibitors are of major importance (see the special issue of Toxins edited by H. Barth). Although bacterial toxins are very poisonous compounds, some of their properties have powerful therapeutic applications. The most representative example is that of botulinum toxins which are used for their anti-cholinergic effects. Thereby, the most potent toxins are the drugs which have the most numerous medical indications from the treatment of dystonia, strabismus, hypersecretory activity of cholinergic glands, urinary bladder dysfunction, pain, cosmetology, etc. Botulinum toxins are mainly used as wild-type purified proteins, but engineering of these molecules is in progress to exploit specific properties of these toxins, such as therapeutic effects on sensitive neurons and the treatment of pain (Rubin, 1988). Recombinant toxins have also been engineered to specifically kill malignant cells (immunotoxins), or to transport therapeutic compounds into specific host compartments hardly accessible by routine administration pathways, such as the central nervous system. In addition, toxin development concerns not only medical applications, but also broad technical innovations. Thereby, the specific pore-forming activity of Staphylococcus aureus alpha toxin is used in novel processes of DNA sequencing.

1. STATEMENT OF THE PROBLEM

Toxins are powerful pathogenicity factors produced by certain bacteria, fungi, animals, and plants which mediate drastic interactions of these pathogens on the organism host. Notably, bacterial toxins were the first compounds which were identified as responsible for severe bacterial diseases in man and animals. Numerous studies and publications were dedicated to bacterial toxin characterization and deciphering of their mechanisms of action (Chumbler, 2016).

In the last decades, a great effort has been done to unravel the mechanisms of action of these very active proteins able to induce so severe symptoms in higher organisms. Clostridium perfringens alpha toxin was the first bacterial toxin identified as developing an enzymatic activity consisting of phospholipase C activity at the cell membrane surface (Galan, 2016). Then, diphtheria toxin (DT) was the first toxin that was characterized to exert a novel enzymatic activity intracellularly (Chumbler, 2016). Thereby, DT modifies the elongation factor-2 (EF-2) by ADP-ribosylation, leading to inhibition of cell protein synthesis and cell death. ADP-ribosylation was then recognized to be shared by numerous other intracellularly-active toxins. Other bacterial toxins develop an enzymatic activity commonly used by many cellular or bacterial enzymes, such as protease, glucosylase, DNase, or RNase, but towards a specific intracellular target promoting specific effects.

1. AIM AND OBJECTIVES

The aim of the study is to investigate microbial toxic and their mode of action

Specifically, the study seeks to:

1. Examine the effects of microbial toxic on living organism
2. Ascertain some of the factors produced by certain bacteria, fungi, animals, and plants.
3. To determine the types of bacteria toxins and their mode of transmittion
4. Assess the mode of action of microbial toxin
   1. RESEARCH QUESTIONS

The following research questions will be addressed in the study as presented below:

1. What are the effects of microbial toxic on living organism?
2. What are the factors produced by certain bacteria, fungi, animals, and plants?
3. What are the types of bacteria toxins and their mode of transmittion?
4. What is the mode of action of microbial toxin?
   1. SIGNIFICANCE OF THE STUDY

The study is on microbial toxic and mode of action. This is very important because bacteria are ubiquitous, and capable of rapid growth when provided with nutrients and conditions favourable for metabolism and cell division, they are involved in catalysis and synthesis of organic matter in the aquatic and terrestrial environments.

Therefore, the study will unravel the mechanisms of action of microbial toxic, as well as the effects and some of the factors produced by certain bacteria, fungi, animals, and plants.

The findings of the study will be helpful and useful to the environmental biologist and chemical sciences in combating microbial toxic.

1. SCOPE OF THE STUDY

This study is focuses mainly on investigation of microbial toxic and their mode of action, specifically; the interest of the study will be restricted to the effects of microbial toxic on living organism; factors produced by certain bacteria, fungi, animals, and plants; examine whether bacteria toxin is responsible for severe bacterial diseases in man and animals.

1.7 DEFINITION OF TERMS

Bacterial toxins

Bacterial toxins are toxic substances that are produced and released by bacteria to target other bacterial or host cells. Bacteria often have antitoxins to avoid the deleterious effects of toxins.

Toxic

The definition of toxic is something poisonous, or something very harmful or bad. A poison that will cause you to become very ill is an example of something that would be described as toxic. A relationship that is very bad for your mental health is an example of something that would be described as toxic.

Bacteria

Member of a large group of unicellular microorganisms which have cell walls but lack organelles and an organized nucleus, including some that can cause disease