ENTRY OF THE BACTERIUM DEAL SYMBIONT INTRACELLULARIS INTO CULTURED ENTEROCYTES AND ITS SUBSEQUENT RELEASE

Separate suspensions of two strains of ileaI symbiont (IS) intracellul aris, an obligate intracellular bacterium and the causative agent of p orcine proliferative enteropathy, were added to 40 or 80 per cent conf luent monolayers of established cultures of rat (IEC-18) or pig entero cytes (IPEC-J2). Peak numbers of intracellular organisms were detected within the enterocytes six days later, but no cytopathic effects were evident After an initial close association with the cell membrane of the enterocytes, single bacteria were internalised after three hours w ithin membranes-bound vacuoles. The formation of an electron-dense pro jection between cell membranes and external bacteria was only evident if the bacterial suspensions were centrifuged on to the monolayers. Th e release of internalised bacteria into the cytoplasm, with the breakd own and loss of membrane-bound vacuoles, was also evident three hours after infection. Internalised bacteria were associated with, but not o bserved within, coated membrane pits. Mitochondria were closely associ ated with internalised vacuoles and with released bacteria. Two to six days after infection, multiplication of the bacteria free in the cyto plasm was frequently observed. In infected cells six days after the in oculation of monolayers, groups of bacteria were found within large, b alloon-like, cytoplasmic protrusions, and the subsequent release of ba cteria from the monolayer provided a means of bacterial exit from the cells. Many events in the in vitro culture model closely resembled eve nts observed at the cellular level in animals infected with Is intrace llularis and the model provides a useful basis for investigating the p athogenetic mechanisms of this bacterium.