Possible parallels between LLO-mediated mechanisms

causing apoptosis in immune cells and encystment in protozoa require a special investigation. Despite the growing number of evidences that a prey-predator model describing interactions between protists and saprophytic bacteria, is not appropriate to explain the interactions of bacteriovorous protozoa and pathogenic bacteria, the mechanisms that permit pathogenic find more bacteria to avoid protozoan grazing are not clear. It was suggested that these mechanisms may involve at least in part the means that pathogens utilize to survive in higher eukaryotes [28–30, 35]. Moreover, it was suggested that the resistance to digestion by bacteriovorous protozoa might be an evolutionary precursor of bacterial adaptation to intracellular survival in LY2874455 clinical trial mammalian professional phagocytes such as macrophages. Our results support this hypothesis by demonstration of the role that the major virulence factor listeriolysin O (LLO) plays in interpopulation relationships of the pathogenic bacterium selleck L. monocytogenes and the bacteriovorous ciliate T. pyriformis.

Discussing the input of LLO in interactions of L. monocytogenes with mammals and protozoa, it is necessary to take notice of LLO expression under different conditions. Expression of the PrfA protein, which is a master-regulator of virulence genes in L. monocytogenes [2], Inositol oxygenase changes in a temperature-sensitive manner that results in very low expression of PrfA-controlled genes under environmental temperatures while their expression increases at the temperatures of mammalian body [36]. In contrast to other virulence factors, the LLO-encoding hly gene expression is regulated by both PrfA-dependent and PrfA-independent promoters [37]. Low LLO expression at environmental conditions driven by the PrfA-independent

promoter and the low-active PrfA-dependent promoter is sufficient to provide L. monocytogenes with benefits in its interactions with other members of the natural ecosystems. Increasing LLO expression, e.g. via introduction of the PrfA* protein, which stimulates higher expression from the PrfA-dependent promoter, distorts the balance causing mortality not only among trophozoites but as well among cysts as we observed for L. innocua carrying pHly/PrfA* plasmid. Therefore, mutations resulting in increased LLO production might be detrimental for survival in the nature. It is interesting, that another Listeria virulent species, L. ivanovii, which is highly haemolytic and is not able to repress virulence factor production via a described PrfA-dependent mechanism [38], is much more rear isolated from environment than L. monocytogenes [39, 40]. Thus, LLO expression might be beneficial under different conditions but it is required a tight regulation in dependence on external conditions.