Intracellular survival of enteric bacteria in cultured human enterocytes

Translocating enteric bacteria have been visualized within intact intestinal epithelial cells in animal models of bacterial translocation. Although the ability of the enterocyte to engulf bacteria has been well documented in both in vivo and in vitro experimental models, relatively little is known about the enterocyte's ability to degrade internalized bacteria. Intracellular survival of eight strains of enteric bacteria (two strains of Listeria monocytogenes, Salmonella typhimurium, Proteus mirabilis, two strains of Escherichia coli, and two strains of Enterococcus faecalis) was quantified over a 20 h period using two different types of terminally differentiated polarized enterocytes considered relevant in vitro models of human small intestinal epithelium, namely Caco-2 and HT-29 cells. Caco-2 enterocytes were generally more permissive for bacterial uptake when compared with HT-29 enterocytes. However, bacterial survival was similar within each type of enterocyte, and most strains of enteric bacteria remained viable within enterocytes for the 20 h duration of the assay. In addition, with the exception of one strain of L. monocytogenes in Caco-2 cells, intracellular enteric bacteria had no noticeable effect on host enterocyte viability for this 20 h duration. Transmission electron microscopy was used to visualize both intact and degraded bacteria within individual enterocytes, suggesting that prolonged bacterial survival might have resulted from simultaneous bacterial degradation and replication. Thus, although enterocytes internalize enteric bacteria, enterocytes might not kill internalized bacteria as efficiently as leukocytes. Observations of bacterial intracellular survival supported the hypothesis that the enterocyte might be a portal of entry for translocating microbes, and observations of intracellular bacterial degradation might have implications for the role of the enterocyte as an antigen-presenting cell.