VIRULENCE OF CULTURE FILTRATE FROM HEAT-INJURED AND REPAIRED LISTERIASTRAINS - ASSAY ON BOVINE MAMMARY EPITHELIAL (MAC-T) CELLS

The cytotoxic effect of culture filtrates from healthy, heat-stressed, and repaired Listeria monocytogenes and L, innocua on the bovine mamm ary epithelial cell line MAC-T was examined. Culture filtrates were co llected from Listeria spp, following treatments which included: (i) 18 h of growth of Listeria at 37 degrees C; (ii) sublethal heat treatmen t at 56 degrees C for 50 minutes; (iii) repair of the injured cells at 37 degrees C for 7 h; (iv) growth of repaired bacterial cells at 37 d egrees C for 36 h; and (v) heat injury at 56 degrees C for 50 min of t he cell population obtained after the initial repair and growth. Strai ns chosen for study included two genetic mutants of L. monocytogenes: a hemolysin-negative mutant, CNL 85/162 (Hly(+)) and a hemolysin-posit ive revertant, CNL 85/163 (Hly(+)). Culture filtrates obtained from Hl y(-) bacteria did not prevent adhesion of the mammary epithelial cells and slightly stimulated their growth. In contrast, culture filtrates from Hly(+) bacteria grown for 18 h significantly reduced the ability of MAC-T cells to adhere to the cell culture dishes, prevented the gro wth of those cells that were attached to the dishes, and caused cell d eath. Supernatants from Hly(+) and Hly(-) following injury and during repair had no lethal effect on MAC-T cells. The effects of culture med ium obtained after growth of the repaired Listeria cells on MAC-T cell s were similar to those recorded for medium from the first 18 h growth for both strains, indicating that cells regain virulence potential on ce they have repaired and reinitiated growth. Culture filtrates obtain ed from L. monocytogenes Scott A showed results similar to those of Hl y(+), decreasing adherence and growth of MAC-T cells, while L, innocua culture filtrate had no adverse effect. The results of these experime nts suggest that when injured, L. monocytogenes does nor demonstrate a dverse effects towards MAC-T cells. Once repair is completed and the L isteria are growing, activity towards MAC-T cells is restored.