3.3.2. Parasite-Host Relationships

Parasitic platyhelminths establish a chemical dialogue with the host by taking elements from it and by sending molecules from the parasite, having a different impact in their relationship with the host. In addition to oxygen diffusion, this class of parasites must ensure access to the nutrients they need for their development, so the presence of a sophisticated recognition and acquisition system through the use of specific transporters (similar to those of the host) is not surprising [30,188]. However, the parasite-host relationship is not limited to that, as the parasite can use the metabolism of the host cells to its advantage [93],

Through proteomic analysis, the presence of intact and functional host proteins has been confirmed in the hydatid fluid of *E. granulosus* and in the vesicular fluid of various species of the genus *Taenia* [50]. Although the ratio of parasite/host proteins is specific to each organism, the composition of these fluids against the composition of the serum of the respective host has been analyzed [50,189–192]. Some of the most abundant host proteins reported are serum albumin and immunoglobulins [87,190]. In the case of the former, the parasite can use it to maintain internal osmotic pressure, whereas the latter could help to prevent antigen exposure of the immune system [192,193]. Surprisingly, it has been reported that these organisms can incorporate various host antioxidant proteins to their antioxidant repertoire, such as the SOD, Prxs, and CAT isoforms [81,87].

In parasitic flatworms, the presence of various families of transporters specialized in the removal of metabolites and drugs has been reported. Although this representation is not homogeneous in flatworms, its participation in detoxification processes has been demonstrated [194–201]. In a previous experiment, we inhibited the TGR enzyme activity of *T. crassiceps* cysticerci under in vitro conditions and observed the appearance of GSSG in the culture medium, which led to the proposition that the cestode expels GSSG excess as a mechanism to avoid the change of the redox environment inside [165]. By searching for a transporter capable of carrying out the translocation of this oxidized species, we were able to identify some multidrug resistance (MDR) transporters in the genome of *T. solium*, which may potentially be responsible for carrying out this function [165].

### **4. Conclusions**

Oxygen has a dual function in organisms. In aerobic organisms it works mainly as a final electron acceptor during respiration, which results in greater energy production through the catabolic pathways, and its presence is related to the generation of ROS resulting in the expression of antioxidant systems involved in redox homeostasis maintenance.

Among flatworms, trematodes and cestodes have life cycles that develop in environments with different oxygen tension, which determine the development of special characteristics that have allowed them to adapt to varied conditions, such as:


• Finally, the exposure of the tegument to a higher concentration of O2 implies a greater production of ROS in it, as indirectly demonstrated by a significant presence of antioxidant enzymes in this region (SOD, GPx, Prx).

These overall data provide more information on the type of metabolism that is performed in the parasite in relation to pO2. However, as Boyunaga comments [82], "one must be cautious when trying to relate this O2 tension where these parasites develop" with the type of energy metabolism they carry out, since the reports in the literature can be controversial. Thus, an important aspect that must be considered is the presence of both types of metabolism, aerobic and anaerobic, in the same organism and its relation to the size of the parasite, the stage of the life cycle, and the degree of purity of the mitochondrion (at least two mitochondrial types in these organisms). Having pure populations of mitochondria would make it possible to determine with greater certainty what type of energy metabolism occurs at what time in the life cycle and in what region of the parasite.

**Funding:** This work was supported by the research grant IN217920 from Dirección General de Asuntos del Personal Académico (DGAPA), at Universidad Nacional Autónoma de México (UNAM).

**Acknowledgments:** Authors want to thank Ingrid Helena Mascher Gramlich, for proofreading the English-language version of this manuscript. Authors want to thank Fashion Designer Juan Antonio Rocha-Santiago for redesign and elaboration of the figures used in this paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
