Cell interactions of Listeria monocytogenes L forms and peritoneal exudative cells in rats

Listeria monocytogenes 4b and its forms without cell walls (L forms of a protoplastic type) were used to study in vivo interactions with host cells. Samples of peritoneal lavage fluid were obtained from rats intraperitoneally inoculated at intervals between 1 and 15 days after challenge, for scanning electron microscopic, bacteriological, biochemical, and cytometrical investigations. Scanning electron microscopic examination revealed continuous adhesion of L forms on the macrophage surface up to 15 days after inoculation. The persistence of the L forms within the peritoneal cavity was also shown bacteriologically at all sample times, while the parental bacterial forms were isolated from the peritoneal cavity up to 7 days after challenge. The total count of peritoneal exudative cells determined by automated flow peroxidase cytometry peaked on the 15th day in animals infected with parental forms, while in animals infected with L forms the peak was lower and the macrophage population was predominant. The glycolytic and acid phosphatase activity of peritoneal exudative cells was two times higher in rats infected with L forms as compared with rats infected with the L. monocytogenes parental forms on the 3rd day after challenge. An understanding of the nature of the interactions between L forms of L. monocytogenes and peritoneal exudative cells found in vivo could be used to establish the influence of L forms on host cellular defense mechanisms.Key words: Listeria monocytogenes, L forms, peritoneal exudative cells, electron microscopy.