**4. Discussion**

The sulfate consumption and sulfide production, and the lactate consumption and acetate accumulation are important factors influencing the intestinal environment [7–10]. Intestinal sulfate-reducing bacteria, especially *Desulfovibrio* genus, are often found in the intestines and feces of people and animals with IBD. One of the main roles in the development of colitis, among other factors, can also be the species of this genus. These bacteria use sulfate as a terminal electron acceptor and organic compounds as electron donors in their metabolism [6,7]. This fact leads us to the conclusion that sulfate present in the daily diet plays an important role in the development of bowel disease. Sulfate is present mainly in the following food commodities: some breads, soya flour, dried fruits, brassicas, and sausages, as well as some beers, ciders, and wines. These data indicate that sulfate intake is highly dependent on diet and the small–large intestine axis [13].

In our previous research, principal component analysis indicated that the *Desulfovibrio* strains from individuals with colitis were grouped in one cluster by biomass accumulation and sulfide production, and the strains from healthy individuals formed another cluster by the same parameters. Sulfate and lactate consumption measured over time showed a negative correlation (Pearson correlations, *p* < 0.01). The linear regression ( *R*2) was lower in biomass accumulation and hydrogen sulfide production. Thus, biomass accumulation and sulfide production, together with measured kinetic parameters, play an important factor in bowel inflammation, including ulcerative colitis. Additionally, acetate produced by SRB can also be in synergic interaction with H2S, while sulfate consumption and lactate oxidation likely represent minor factors in bowel disease [16].

Our results provide an opportunity to find the optimum growing point of the bacteria. The study confirmed an intense growth of *D. piger* Vib-7 in the presence of higher concentrations of electron acceptor and donor, though the consequence is an intensive accumulation of sulfide and acetate. Data from the literature indicate that these conditions can be the cause of ulcerative colitis that can lead to cancer of the bowel. This statement is supported by the fact that hydrogen sulfide negatively affects intestinal mucosa and epithelial cells, inhibits the growth of colonocytes [4,14–18,34–37], causes phagocytosis, causes the death of intestinal bacteria [4,12,24], and induces hyperproliferation and metabolic abnormalities of epithelial cells [12]. The high level of metabolites and the presence of SRB are connected with the inflammation of the colon [4,6,36]. Therefore, the integrity of colonocytes is maintained by hydrogen sulfide concentration [35–37]. Sulfide production is higher among SRB isolated from individuals with ulcerative colitis [5,6].

Other research describing cross-correlation parameters of the SRB metabolic process indicated that the strains isolated from people with colitis shifted to the right side of the Y axis by biomass accumulation, sulfate consumption, lactate oxidation, as well as hydrogen sulfide and acetate production, compared with the strains isolated from healthy individuals. Di fferent percentages were observed in shifting to the right side of the Y axis: biomass accumulation 26%, sulfate consumption 1.5%, and sulfide production 5% [14]. It should be noted that the intestinal microbiota is a very complex system that may limit this study. There are a lot of interactions with clostridia, methanogens, lactic acid bacteria, etc. However, a central role in the development of IBD, especially ulcerative colitis, is SRB [1–3,11]. This bacterial group, producing hydrogen sulfide, can inhibit other microbiota, including lactic acid bacteria, methanogens, and many other intestinal microorganisms [2].

A diet high in sulfate ions (preservatives added to food often contain sulfur oxides) causes an increase in hydrogen sulfide concentration by SRB in rumens. The studies have revealed that the western diet contains over 16.6 mmol sulfate/day [13] and the feces of approximately 50% of healthy individuals contain SRB (up to 92% belong to the genus *Desulfovibrio*) [1,5]. Sulfate polysaccharides

such as mucin, chondroitin sulfate, and carrageenan are broadly consumed, and they represent good sources of sulfate for SRB [24]. It should also be noted that hydrogen sulfide can be toxic not only for intestinal cells, but also for its producers. The highest toxicity of H2S was measured in the presence of concentrations higher than 6 mM, where growth was stopped, though metabolic activities were not 100% inhibited. These findings are confirmed by cross correlation and principal component analysis that clearly support the above mentioned results. The presence of 5 mM H2S resulted in a two times longer lag phase and generation time was eight times longer. The results confirmed toxicity of H2S toward *Desulfovibrio* [18]. Beside sulfate and lactate, terminal oxidative processes in the human large intestine could be involved in the activities of SRB, and consequently the production of hydrogen sulfide in high concentrations that can cause inflammatory bowel disease development.
