Background: Peroxide-producing lactobacilli provide protection from infection for the female reproductive tract. However, in vitro studies demonstrated that H₂O₂-produced by Lactobacillus is not the cause of inhibition of pathogens. It is not exactly known how H₂O₂-producing lactobacilli are involved in the protection of the vaginal environment. Objectives: This study aimed to evaluate the importance of the interaction between H₂O₂-producing lactobacilli and their host for the resistance of the vaginal biotope. Materials and Methods: In this study, we used vaginal lactobacilli (11 H₂O₂-roducing strains and 11 non-H₂O₂-producing strains). The influence of epithelial cells on the growth and antibacterial activity of lactobacilli were evaluated. The effects of lactobacilli on the antibacterial activity of the epithelial cells, muramidase and lactoferrin were also determined. Results: Vaginal epithelial cells stimulated the growth and antibacterial activity of H₂O₂-producing lactobacilli in a greater extent than that of the non-H₂O₂-producing lactobacilli. Mainly, the H₂O₂-producing lactobacilli were capable of increasing the activity of the host antimicrobial peptides (muramidase and lactoferrin) as well as the antibacterial activity of the epithelial cells. Conclusions: The involvement of the peroxide-producing lactobacilli in the protection of vagina was due to their ability to effectively interact with the host. This is expressed on one side to stimulate the growth and antagonistic activity of lactobacilli and on the other side to increase the antibacterial activity of the host defense factors (muramidase, lactoferrin and metabolites of epithelial cells). Keywords: Epithelial Cells; Host-Pathogen Interactions; Hydrogen Peroxide; Immunity; Innate; Lactoferrin; Muramidase; Vagina; Lactobacillus

The development of polymeric carriers based on partially deacetylated chitin nanowhiskers (CNWs) and anionic sulfated polysaccharides is an attractive strategy for improved vaginal delivery with modified drug release profiles. This study focuses on the development of metronidazole (MET)-containing cryogels based on carrageenan (CRG) and CNWs. The desired cryogels were obtained by electrostatic interactions between the amino groups of CNWs and the sulfate groups of CRG and by the formation of additional hydrogen bonds, as well as by entanglement of carrageenan macrochains. It was shown that the introduction of 5% CNWs significantly increased the strength of the initial hydrogel and ensured the formation of a homogeneous cryogel structure, resulting in sustained MET release within 24 h. At the same time, when the CNW content was increased to 10%, the system collapsed with the formation of discrete cryogels, demonstrating MET release within 12 h. The mechanism of prolonged drug release was mediated by polymer swelling and chain relaxation in the polymer matrix and correlated well with the Korsmeyer–Peppas and Peppas–Sahlin models. In vitro tests showed that the developed cryogels had a prolonged (24 h) antiprotozoal effect against Trichomonas, including MET-resistant strains. Thus, the new cryogels with MET may be promising dosage forms for the treatment of vaginal infections.