| Summary: | <p>Abstract</p> <p>Background</p> <p>The gram-positive pathogenic bacterium <it>Listeria monocytogenes </it>is widely spread in the nature. <it>L. monocytogenes </it>was reported to be isolated from soil, water, sewage and sludge. Listeriolysin O (LLO) is a <it>L. monocytogenes </it>major virulence factor. In the course of infection in mammals, LLO is required for intracellular survival and apoptosis induction in lymphocytes. In this study, we explored the potential of LLO to promote interactions between <it>L. monocytogenes </it>and the ubiquitous inhabitant of natural ecosystems bacteriovorous free-living ciliate <it>Tetrahymena pyriformis</it>.</p> <p>Results</p> <p>Wild type <it>L. monocytogenes </it>reduced <it>T. pyriformis </it>trophozoite counts and stimulated encystment. The effects were observed starting from 48 h of co-incubation. On the day 14, trophozoites were eliminated from the co-culture while about 5 × 10<sup>4 </sup>cells/ml remained in the axenic <it>T. pyriformis </it>culture. The deficient in the LLO-encoding <it>hly </it>gene <it>L. monocytogenes </it>strain failed to cause mortality among protozoa and to trigger protozoan encystment. Replenishment of the <it>hly </it>gene in the mutant strain restored toxicity towards protozoa and induction of protozoan encystment. The saprophytic non-haemolytic species <it>L. innocua </it>transformed with the LLO-expressing plasmid caused extensive mortality and encystment in ciliates. During the first week of co-incubation, LLO-producing <it>L. monocytogenes </it>demonstrated higher growth rates in association with <it>T. pyriformis </it>than the LLO-deficient isogenic strain. At latter stages of co-incubation bacterial counts were similar for both strains. <it>T. pyriformis </it>cysts infected with wild type <it>L. monocytogenes </it>caused listerial infection in guinea pigs upon ocular and oral inoculation. The infection was proved by bacterial plating from the internal organs.</p> <p>Conclusions</p> <p>The <it>L. monocytogenes </it>virulence factor LLO promotes bacterial survival and growth in the presence of bacteriovorous ciliate <it>T. pyriformis</it>. LLO is responsible for <it>L. monocytogenes </it>toxicity for protozoa and induction of protozoan encystment. <it>L. monocytogenes </it>entrapped in cysts remained viable and virulent. In whole, LLO activity seems to support bacterial survival in the natural habitat outside of a host.</p>
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