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Background  Some pathogenic bacteria have been developing as a part of terrestrial and aquatic microbial ecosystems. Bacteria are consumed by bacteriovorous protists which are readily consumed by larger organisms. Being natural predators, protozoa are also an instrument for selection of virulence traits in bacteria. Moreover, protozoa serve as a “Trojan horse” that deliver pathogens to the human body. Here, we suggested that carnivorous amoebas feeding on smaller bacteriovorous protists might serve as “Troy” themselves when pathogens are delivered to them with their preys. A dual role might be suggested for protozoa in the development of traits required for bacterial passage along the food chain.    Results  A model food chain was developed. Pathogenic bacteria  L. monocytogenes  or related saprophytic bacteria  L. innocua  constituted the base of the food chain, bacteriovorous ciliate  Tetrahymena pyriformis  was an intermediate consumer, and carnivorous amoeba  Amoeba proteus  was a consumer of the highest order. The population of  A. proteus  demonstrated variations in behaviour depending on whether saprophytic or virulent  Listeria  was used to feed the intermediate consumer,  T. pyriformis . Feeding of  A. proteus  with  T. pyriformis  that grazed on saprophytic bacteria caused prevalence of pseudopodia-possessing hungry amoebas. Statistically significant prevalence of amoebas with spherical morphology typical for fed amoebas was observed when pathogenic  L. monocytogenes  were included in the food chain. Moreover, consumption of tetrahymenas fed with saprophytic  L. innocua  improved growth of  A. proteus  population while  L. monocytogenes -filled tetrahymenas provided negative effect. Both pathogenic and saprophytic bacteria were delivered to  A. proteus  alive but only  L. monocytogenes  multiplied within amoebas. Observed differences in  A. proteus  population behaviour suggested that virulent  L. monocytogenes  might slow down restoration of  A. proteus  ability to hunt again and thus restrict the size of  A. proteus  population. Comparison of isogenic bacterial pairs that did or did not produce the haemolysin listeriolysin O (LLO) suggested a role for LLO in passing  L. monocytogenes  along the food chain.    Conclusions  Our results support the idea of protozoa as a means of pathogen delivery to consumers of a higher order and demonstrated a dual role of protozoa as both a “Trojan horse” and “Troy.”

Experimental Listeria–Tetrahymena–Amoeba food chain functioning depends on bacterial virulence traits