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Legionella pneumophila is ingested by both human macrophages and amoebae, and it multiplies within similar endocytic compartments in both eukaryotic species. Inhibitors of eukaryotic protein synthesis, such as cycloheximide and emetine, had no effect on the uptake of L. pneumophila by macrophages but completely abolished ingestion by the amoeba Hartmannella vermiformis. Therefore, host cell protein synthesis is required for the bacterium to infect the amoeba but not human macrophages. To identify proteins expressed by H. vermiformis upon contact with L. pneumophila, we radiolabeled amoebal proteins after contact with bacteria in bacteriostatic concentrations of tetracycline to inhibit bacterial protein synthesis. We analyzed protein expression by two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis and found that 33 amoebal proteins were induced; 12 of these were not detected in resting amoebae. Eleven other amoebal proteins were repressed; four of them became undetectable. In contrast, no phenotypic changes were observed in H. vermiformis upon contact with Escherichia coli or heat-killed L. pneumophila. An isogenic, avirulent variant of L. pneumophila, incapable of infecting either macrophages or amoebae, induced a different pattern of protein expression upon contact with H. vermiformis. Our data showed that amoebae manifested a specific phenotypic response upon contact with virulent L. pneumophila. This phenotypic modulation may be necessary for uptake of the bacteria into an endocytic compartment that permits bacterial survival and multiplication.

Protein expression by the protozoan Hartmannella vermiformis upon contact with its bacterial parasite Legionella pneumophila