**1. Introduction**

Sulfate-reducing bacteria (SRB) represent probably a trigger for the occurrence of inflammatory bowel diseases (IBD) since studies are connecting their presence with these diseases, especially their metabolic end product H2S in the gu<sup>t</sup> [1,2]. Other ailments (including rheumatic diseases and with ankylosing spondylitis) occur also in their presence [3]. SRB use sulfate as an electron acceptor in the process of dissimilatory sulfate reduction. The final product of this process is hydrogen sulfide [4]. Constant microorganism cultivation is happening in the large intestine since certain undigested food remains in it. [1,2]. Around 200 g of digestive material is found in the large intestine of an adult human [2,3,5,6]. These bacteria are in the fermentation process can cleave complex organic compounds

and form molecular hydrogen, different acids (acetic and lactic), same as other compounds. Lactic acid bacteria fermentative properties are directly responsible for the production of lactate [4]. Other groups of microorganisms can also use lactate and acetate, serving as electron donors and carbon sources [7–12]. The important role of human physiological processes is their capability to absorb sulfate and develop amino acids out of it (cysteine and methionine). The amount of the sulfate present in the intestine is related to human diet [13–16], meaning that it is highly influenced by individual's eating habits. The importance of daily sulfate intake can be overseen by the fact that staple food commodities represent high sulfate sources (>10 μmol/g) [13].

Although, sulfate amounts that are not used in amino acid synthesis represent good conditions for SRB [1,4,17–21]. SRB needs electron acceptor (sulfate serves this purpose) and they form hydrogen sulfide as their final product [22–27]. An exogenic electron donor, including lactate can be also used and oxidized to acetate [18,28]. The dominant SRB in the intestine of humans is *Desulfovibrio* genus [5,22,28]. The studies are emphasized connections between the presence of SRB in the intestines and the prevalence of ailments, such as cholecystitis, brain abscesses, and abdominal cavity ulcerative enterocolitis. Sulfate-reducing bacteria are not the only ones that produce H2S in the intestinal content. Numerous bacterial groups convert cysteine to H2S, pyruvate, and ammonia by cysteine desulfhydrase activity [2–4,12].

Though connections have been found, it is still not clear how these processes are affecting the prevalence of certain ailments. Meta-analysis is used widely in medical research, as in natural science. It is included in systematic reviews as a rigorous method for mapping the evidence gained by many authors. The meta-analysis should provide unbiased overviews of multiple results and should assess evidence quality and synthesize it. The first step of a systematic review is the research question that is deconstructed by sample consideration, the second step is intervention and then come outcome and comparator. The outcome of the meta-analysis depends on the study field, but in many cases, quantitative results are used [29].

The aim of the research was to compare a variation in the morphological distribution of sulfate-reducing microbial communities from healthy mice and mice with colitis, their production of hydrogen sulfide, and to study the occurrence of these bacterial populations during diseases of the small–large intestine axis.
