Experiments on white mice, made with the use of genetically linked pair of P. mirabilis differing in the presence of protease activity, have demonstrated the role of this activity in the aggravation of the infectious process, observed only in cases of the parenteral introduction of microorganisms. The damaging action of proteases manifests by decreased antiprotease capacity of the blood and the dissemination of bacteria into organs and tissues with the predominant colonization of the urinary tract. The damage of the epithelial barrier of the gastrointestinal mucosa after the intragastric administration of bacteria can be observed after preliminary destruction of the protective layer consisting of mucin and containing sIgA.