Interplay of flagellar motility and mucin degradation stimulates the association of Pseudomonas aeruginosa with human epithelial colorectal adenocarcinoma (Caco-2) cells

Pseudomonas aeruginosa can penetrate the extracellular mucin barrier formed by the intestinal epithelial cell layer and establish gut-derived sepsis in immunocompromised patients. We found that two efficient mechanisms, flagellar motility and mucin degradation, are needed for penetration of P. aeruginosa through the mucin barrier. Deletion of the flagellar motility-related gene, the filament protein gene fliC, the cap protein gene fliD, and the motor complex protein genes motABCD from P. aeruginosa PAO1 decreased association of P. aeruginosa with the apical surface of human epithelial colorectal adenocarcinoma (Caco-2) cells. A penetration experiment using an artificial mucin layer suggested that the decreased penetration is caused by attenuation of mucin penetration ability. Additionally, the presence of P. aeruginosa decreased the total mucin, including the secreted mucin protein MUC2, on the surface of the Caco-2 cell monolayer, regardless of flagellar motility. Construction of the PAO1 mutant series knocked out 12 putative serine protease genes and identified the mucD gene, which participated in degradation of total mucin, including MUC2. Furthermore, decreased association with the surface of the Caco-2 cell monolayer was observed in the mucD mutant, and the decrease was synergistically amplified by double knockout with fliC. We conclude that P. aeruginosa can penetrate the mucin layer using flagellar motility and mucin degradation, which is dependent on the MucD protease or the mucD gene-related protease.