**2. The hostile gastric environment**

Compared to other gastrointestinal (GI) segments, the stomach has a physiological environment that is significantly more hostile to bacterial colonization and is a crucial part of the dynamics of the gastric microbiota. Primary reason for this is the gastric juice, which is composed of two main components—proteolytic enzymes and hydrochloric acid (HCl). The hydrochloric acid creates a strong acidic environment by maintaining a pH of 1–2 in the gastric lumen, which together with the proteolytic features of the gastric enzymes creates an intragastric environment that serves both digestive and protective roles. This environment facilitates the denaturation of proteins and nutrient absorption but also severely limits bacterial colonization and survival, preventing infection by pathogens [4]. The low pH value is the main restrictive component of the gastric juice [5]. To prevent damage to the mucosa from the acid and enzymes, neck cells of the gastric glands secrete mucus on the surface of the gastric epithelium. This mucus layer establishes a pH gradient that increases the pH up to 6–7 at the surface of the mucosa [6]. This is due to the unique properties of the mucus which permit acid to flow from parietal cells into crypts which communicate with the lumen, but do not allow acid at pH <4 from penetrating the mucus layer [6]. The mucus layer consists of several different mucin molecules, including MUC1, MUC5AC, MUC5AB, and MUC6, and forms two sublayers, an inner mucus layer that is firmly attached to the epithelia and a loose mucus layer, which is in direct contact with the lumen [7, 8]. Additional factors that contribute to the strong antimicrobial environment of the stomach are the accidental bile reflux and the gastric peristalsis.
