**4. Energy flow theory**

The energy flow theory proposed by Harold Morowitz is useful for explaining the origin of life [8]. In the primitive Earth, millions of reduction-oxidation reactions took place, one of which occurred between molecular hydrogen (reductor) and carbon dioxide (oxidant). This redox reaction was not spontaneous. Therefore, primitive organisms, such as helminths, acquired the skills needed to manage this reaction via enzyme catalysis. The citric acid or Krebs cycle is one such example. In addition to the citric acid cycle, aerobic organisms, such as helminths, developed a group of metabolic cycles to obtain their capacity to manage oxygen because of their dual contrasting molecular characteristics **Figure 2**.

#### **Figure 2.**

*Redox throughout life's evolution. Two different groups of molecules that originate redox reactions. Principally, metals, in the early Earth contributed to its oxidation. Redox enzymes in organism, including helminths, contributed to their homeostasis.*

*Oxygen and Redox Reactions Contribute to the Protection of Free-Living and Parasite Helminths… DOI: http://dx.doi.org/10.5772/intechopen.102542*

### **5. The contrasting oxygen molecule**

As described previously, although molecular oxygen is vital for aerobic organisms, it is also a toxic mutagenic gas due to the production of intermediary oxygen molecules and reactive oxygen species (ROS) [9]. The toxicity of oxygen arises from its chemical electron acceptability by redox mechanisms, producing superoxide radicals (O•∙), hydrogen peroxide (H2O2), hydroxyl radicals (•OH), and singlet oxygen (O2), also known as reactive oxygen species (ROS). When concentration of ROS exceeds the capacity of the cells' defense systems, this results in the phenomenon of oxidative stress, which is characterized by an increase in the reduction potential or a large decrease in the reducing capacity of the cellular redox couples.

Oxidative stress is associated with damage to biological molecules. ROS can oxidize amino acid chains and cross-link proteins, as well as oxidize protein backbones. The highly reactive hydroxyl radical (•OH) reacts with DNA via the addition of double bonds of DNA bases and by the abstraction of a hydrogen atom from the methyl group of thymine and each of the C∙H bonds of 2-deoxyribose. Furthermore, ROS also induces the process of lipid peroxidation in lipoprotein particles or membranes, giving rise to a variety of products, including short chain aldehydes, such as malondialdehyde or 4-hydroxynonenal, alkanes, alkenes, conjugated dienes, and a variety of hydroxides and hydroperoxides.

One way to understand how oxidative stress works in free-living helminths is to appreciate the process by which these organisms can be affected by bacterial virulence. This observation is clear from the studies developed in *C. elegans*, which produced hundreds of mutant worms with enough different genes [10] and mutants for the study of all aspects of this organism. Therefore, this worm is an excellent host model for the study of the evolutionarily conserved mechanisms of microbial pathogens.

Based on this, microbes that cause diseases in mammalian hosts have also been shown to be important for diseases in *C. elegans*, and as a terrestrial microbiome, *C. elegans* can be fed not only with the auxotrophic *Escherichia coli* strain OP50, which is harmless to the worm, but also with a variety of pathogenic bacteria. This means that a way to understand how a worm can get a disease is just to see them without movement in culture in the presence of the pathogen bacteria. Specifically, it was observed that C. elegans was killed when the *Pseudomonas aeruginosa* PA14 strain or another pathogenic microbe was provided as a food source [11].

A historical summary of the major results obtained in the study of the *C. elegans* model is as follows: (a) Sifri et al. [12] identified *C. elegans* as a new, simple, and cheap model organism for the study the pathogenesis of the Gram-negative bacteria *P. aeruginosa* and *Salmonella enterica* serovar *Typhimurium*. (b) Garsin et al. [13] showed that the Gram-positive bacteria *Enterococcus faecalis* and *Streptococcus pneumoniae* kill *C. elegans*. (c) Hodgkin et al. [14] demonstrated that the genetically amenable nematode *C. elegans* is ideally suited to identify host factors. Therefore, the biological complementarity between *C. elegans* and pathogenic microorganisms is well suited for the study of bacterial virulence, not least because of the vast bacterial strains available for this aim.
