Invasion of the Host Epithelium by the Microorganisms: Good or Bad News for the Host?

The host epithelium has permanent contact with the environment and a multitude of diverse microorganisms, resulting in a network of the host's defense mechanisms. Pathogens use various strategies to invade epithelial barriers, to hijack eukaryotic host function to their own benefit and use the epithelium as a reservoir for dissemination throughout the host. Alteration of the host cell apoptosis, promotion of cell proliferation or conversely, inhibition of cell growth and modulation of the cell differentiation by blocking of cell cycle progression are some of them. The mechanisms all of the stratagems employed by the pathogens are not fully elucidated, but they can contribute to the virulence of those microorganisms. However, the latest investigation of the interaction between host epithelium and microorganisms suggest that the epithelium is not a simple mechanical barrier: epithelial cells recognize microorganisms and initiate appropriate signaling which contribute to the endocytosis of microorganisms. It appears that capture of microorganisms by the epithelial cells is selective and that the different endocytic mechanisms may be enhanced by proinflammatory cytokines. The specificity of the recognition is illustrated by the various studies, showing that the epithelial cells distinguish the different morphotypes of the microorganisms. Using the model of the infection of respiratory epithelium by opportunistic pathogen Aspergillus fumigatus, we have shown that the airway epithelial cells identify the most invasive fungal form that may be beneficial for the host defense. Moreover, host epithelium exposed to the microorganisms, express various cytokines and different protective substances, such as antimicrobial peptides, with direct microbicidal or chemotactic activities, which might contribute to the regulation of host adaptive immunity against microbial invasion. Autocrine mechanismes of antimicrobial peptides expression was shown with the epithelial cells exposed to Aspergillus fumigatus. Further study of the regulation of antimicrobial peptides expression might provide the new approaches that may enhance its expression for potential therapeutic use.Nevertheless, despite permanent exposure to a considerable amount of the microorganisms present in the environment, epithelium possesses the enormous capacity to keep its integrity, suggesting that some microbial strategies link to the mechanisms, which control the structural integrity of the tissue. Recent evidence supports the role of microbial factors in the maintenance of the integrity of the epithelial tissue: it was shown that Staphylococcus aureus as well as other microbial products induce epithelial repair, survival and growth and that such compensatory epithelial responses are mediated by autonomous non-inflammatory pathway. Therefore the outcome of the interaction between the host epithelium and microorganisms depends on multiple features.