Cytokine secretion by stimulated monocytes depends on the growth phase andheat treatment of bacteria: A comparative study between lactic acid bacteria and invasive pathogens

The consumption of food containing lactic acid bacteria (LAB) has been show n to exert immunomodulatory effects in humans. The specific cellular intera ction of these bacteria with immune-competent cells has not yet been fully understood, Since the TNF-alpha secretion of stimulated monocytes is an imp ortant initial response to a bacterial challenge, we investigated the poten tial of LAB originating from the human intestine or fermented food in compa rison to the effect of invasive pathogens, The challenge of monocytes with three LAB strains, Listeria monocytogenes or enterohaemorrhagic Escherichia coli (EHEC) elicited a strain specific, dose-dependent biphasic TNF-alpha secretion. The concentration (EDmax) of bacteria or bacterial cell wall com ponents necessary to induce maximal TNF-alpha secretion (TNFmax) by monocyt es was mathematically approximated. It was shown for exponentially growing LAB strains that the maximal TNF-alpha secretion (TNFmax) was stronger (57 to 78%) upon stimulation with living bacteria than with heat killed cells. In contrast to log-phase bacteria, the maximal TNF-alpha secretion of monoc ytes (TNFmax) was higher (15 to 55%) after the stimulation with heat killed , stationary-phase bacteria when compared to that of live LAB. Thus, monocy te stimulation was clearly affected by the growth phase of bacteria. Purifi ed cell walls of LAB straines revealed only a limited potential for monocyt e stimulation, LPS exhibited a higher capacity to stimulate monocytes than purified Gram positive cell wads or muramyldipeptide, In comparison to path ogenic bacteria, the maximal secretory TNF-alpha response (TNFmax) was up t o 2 fold higher with LAB strains. In general, the amount of bacteria (EDmax ) necessary to induce maximal TNF-alpha secretion (TNFmax) was approximatly 1 to 3 log higher for heat killed bacteria when compared to live bacterial cells illustrating the significant lower potential of heat killed bacteria to activate monocytes.