# The Cell Membrane of _Sulfolobus_ spp.--Homeoviscous Adaption and Biotechnological Applications
## 6 Conclusions  
Modifications in the lipid composition as a homeoviscous adaption strategy are done to alter the membrane packing and therefore maintain membrane fluidity. A lower DEL:TEL ratio increases the membrane rigidity and generates a less permeable membrane. Subsequently, the incorporation of cyclopentane rings enhances this process even further. The membrane lipid composition of _Sulfolobus_ spp. is influenced by growth conditions, such as pH, temperature and growth rate. The RI number is positively correlated with temperature, while decreasing when the growth rate increases. Both observations were reported in multiple publications. Generally, the DEL:TEL ratio was shown to change in response to temperature as well as growth rate. However, in two studies, no clear trend, and only in one case, a negative correlation in regards to varying growth rates, was observed. In terms of the presence of different polar head groups, again, the connection between temperature and growth rate remains obscure. Solely the presence of the calditol moiety was shown to increase acid tolerance. Altogether, highly cyclized GDGT could be a general response to stress in Sulfolobales. Generally, reports on the membrane lipid composition have to be critically reviewed according to the used extraction and detection methods, as these have an impact on the obtained results. A standard extraction method capable of extracting all lipid classes comprised in the archaeal membrane should be applied in all further studies. Chloroform, in combination with methanol, has been shown to achieve the best extraction results in terms of total lipid extraction efficiency. Conclusions in regard to the lipid composition upon results acquired by TLC are not adequate. Mass spectrometry, or mass spectrometry along with prior chromatography steps, are necessary for exact identification of the lipid class and its quantification. Apart from this, lipid standards for quantification have to be established, as currently only one is commercially available. While of fundamental interest for unraveling the interactions between microorganisms with their environment, the study of archaeal lipids is also of high biotechnological importance, since lipids of Sulfolobales can be used to generate archaeosomes, which have been shown to have superior properties when compared to the conventional liposomes. Studies with entrapped insulin revealed excellent stabilization of the archaeosome in the gastrointestinal tract. Further, skin permeation as a potential application has emerged, guiding the way for the establishment of new bioproducts. Conceptualization, K.R., D.J.W., O.S. and J.Q. ; investigation, K.R. ; writing--original draft preparation, K.R. ; writing--review and editing, J.Q., D.J.W. and O.S. ; visualization, J.Q. ; supervision, J.Q. ; project administration, D.J.W. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Open Access Funding by TU Wien. The authors acknowledge the TU Wien Bibliothek for financial support through its Open Access Funding Program. The authors declare no conflict of interest. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Field of Application** & **Specific Use** & **Properties Influenced by TELs** & **Reference** \\ \hline
**Oral Drug Delivery** & Delivery of insulin; antibiotic, cancer, hepatitis B and D, as well as osteoporosis treatment & Protection against drug degradation in the gastrointestinal tract & [124; 125; 126; 127; 128; 129] \\ \hline
**Dermal Delivery** & Delivery of a methylene blue as drug model through rat skin & Improved skin permeation & [130] \\ \hline
**Intravenous Drug** & Photodynamic therapy for anti-cancer treatment & Increased membrane rigidity for controlled release & [131] \\ \hline
**Gene Delivery** & Transfection of mammalian cells with TEL containing lipid nanoparticles & Increased stability and transfection efficiency & [132] \\ \hline \hline \end{tabular}
\end{table}