INTRACELLULAR SURVIVAL OF ENTERIC BACTERIA IN CULTURED HUMAN ENTEROCYTES

Translocating enteric bacteria have been visualized within intact inte stinal epithelial cells in animal models of bacterial translocation. A lthough the ability of the enterocyte to engulf bacteria has been well documented in both in vivo and in vitro experimental models, relative ly little is known about the enterocyte's ability to degrade internali zed bacteria. Intracellular survival of eight strains of enteric bacte ria (two strains of Listeria monocytogenes, Salmonella typhimurium, Pr oteus mirabilis, two strains of Escherichia coli, and two strains of E nterococcus faecalis) was quantified over a 20 h period using two diff erent types of terminally differentiated polarized enterocytes conside red relevant in vitro models of human small intestinal epithelium, nam ely Caco-2 and HT-29 cells. Caco-2 enterocytes were generally more per missive for bacterial uptake when compared with HT-29 enterocytes. How ever, bacterial survival was similar within each type of enterocyte, a nd most strains of enteric bacteria remained viable within enterocytes for the 20 h duration of the assay. In addition, with the exception o f one strain of L. monocytogenes in Caco-2 cells, intracellular enteri c bacteria had no noticeable effect on host enterocyte viability for t his 20 h duration. Transmission electron microscopy was used to visual ize both intact and degraded bacteria within individual enterocytes, s uggesting that prolonged bacterial survival might have resulted from s imultaneous bacterial degradation and replication. Thus, although ente rocytes internalize enteric bacteria, enterocytes might not kill inter nalized bacteria as efficiently as leukocytes. Observations of bacteri al intracellular survival supported the hypothesis that the enterocyte might be a portal of entry for translocating microbes, and observatio ns of intracellular bacterial degradation might have implications for the role of the enterocyte as an antigen-presenting cell.