## 2 SRB Categorization and Characteristics
### Class Delataproteabacteria  
SRB include microorganisms displaying different shapes, sizes, metabolic pathways, cell wall types, and more [44; 46; 47; 48]. However, a dominating component of SRB taxonomy consists of the members of the Gram-negative bacteria, class _Deltaproteobacteria_ of phylum _Proteobacteria_, including the SRB-containing orders _Desulfovibionales_ and _Desulfoucterales_. The division is largely based on 16S rRNA gene sequencing [44]. Members of order _Desulfovibionales_ are motile, anaerobic rods that are mostly mesophilic or thermophilic, and rarely psychrophilic. The order _Desulfoucterales_ is more morphologically diverse than the order _Desulfovibionaceae_, including not only rods but cocci and filamentous bacteria as well. They are mobile, thermally mesophilic, and sometimes psychrophilic. Slightly curved rods of _Desulfoucterales_ mainly use lactate as an electron donor that is further oxidized to acetate, and they also require vitamins to grow. This order occurs in anoxygenic aquatic habitats. The _Desulfoucterales_ family _Desulfoublaceae_ includes the genus _Desulfoublus_. These lemon-shaped rods populate water sediments (both freshwater and marine water). They use mainly propionate as an electron donor that oxidizes incompletely to acetate [44; 45].  
The _Desulfovibionaceae_ family consists of the following genus: _Bilophila_, _Desulfouculum_, _Desulfoucrubacter_, _Desulfoucurus_, _Desulfohalovirio_, _Desulfoulivirio_, and _Desulfovirio_. _Desulfohalobiaceae_, a family within the order _Desulfovibionales_, embraces the genera _Desulfohalobium_, _Desulfoontranrospipra_, _Desulfoontranovibrio_, _Desulfoothermus_, _Desulfoourcirculus_, and _Desulfohalophilus_[44; 48]. The Desulfovibionaceae family consists of mesophilic organisms that oxidize organic substances to acetate, most of which have a Vibrio shape and one polar flagellar. An important representative is the genus _Desulfoviibrio_, since this genus is the most often found in the environment (it encompasses the majority of species, and they are the most often found within the human and animal intestinal microbiota, especially among patients with developed intestinal bowel diseases) [44; 49]. _Desulfovirio__piger_, which in contrast to _D. desulfuricans_ consists of non-motile straight rods, represents the most abundant SRB in the human gastrointestinal tract [34]. This species in particular has been found to be present at significantly higher levels in patients suffering from chronic bowel inflammation [50; 51]. By contrast, _Desulfovirio__ferophilus_ occurs in the marine environments that are oxygen free, where it forms crusts on the iron surfaces, resulting in corrosion. Both species are using iron as an electron donor [52]. _Desulfoplia_ corrodens requires an organic substrate to grow, such as acetate [53]. The family _Desul  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Species** & **Phylum** & **Order** & **Shape** & **Gram Staining** & **Mobility** \\ \hline _Desulfovirio tiger_ & _Proteobacteria_ & _Desulfovibionales_ & rods & negative & no \\ \hline _Desulfovirio desulfuricans_ & _Proteobacteria_ & _Desulfovibionales_ & vibrios & negative & yes \\ \hline _Desulfovirio vulgaris_ & _Proteobacteria_ & _Desulfovibionales_ & vibrios & negative & yes \\ \hline _Desulfoucter hydrogenophilus_ & _Proteobacteria_ & _Desulfovibionales_ & oval rods & negative & yes \\ \hline _Desulfobulus propionate_ & _Proteobacteria_ & _Desulfovibionales_ & elliptical rods & negative & yes \\ \hline _Desulfoumaculum acetoxidants_ & _Firmicutes_ & _Clostridiales_ & rods & positive & yes \\ \hline _Caldivirga mauilingensis_ & _Crenarchaeota_ & _Thermoproteales_ & rods & – & yes \\ \hline \hline \end{tabular}
\end{table}
Table 1: Some types of sulfate-reducing bacteria (SRB) [44].  
fohalobiaceae_ also includes the genus _Desulfomonas_, an irregular, immobile rod found in human stool [32; 54].