Submitted to: The ISME Journal: Multidisciplinary Journal of Microbial Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/18/2010
Publication Date: 8/10/2010
Citation: Rehfuss, M.Y., Parker, C., Brandl, M. 2011. Salmonella transcriptional signature in Tetrahymena phagosomes and role of acid tolerance in passage through the protist. The ISME Journal:5:262-273.
Interpretive Summary: Salmonella enterica Typhimurium remains undigested in the food vacuoles of the common protist, Tetrahymena, and is egested as viable cells in its fecal pellets. Grazing by protozoa is a major factor in shaping bacterial populations in aquatic, soil and anthropogenic ecosystems. Because protozoan predation significantly reduces overall numbers of bacterial prey, foodborne pathogens that resist grazing by protozoa may display increased environmental fitness and have increased persistence throughout the food chain. We have identified by microarray analysis of global gene expression, numerous genes that have a role in the resistance of Salmonella to digestion by Tetrahymena, including one involved in acid tolerance and another in maintenance of Salmonella in human macrophages. Additionally, we have observed that Salmonella cells are more resistant to acid conditions after passage through the protist than cells that remain uningested and free in suspension. Along with its upregulation of several key virulence genes in the protozoan vacuoles, this enhanced acid resistance of Salmonella may indicate that the interaction of the pathogen with Tetrahymena in the agricultural environment may contribute to its contamination cycle.
Technical Abstract: Salmonella enterica Typhimurium remains undigested in the food vacuoles of the common protist, Tetrahymena, and is egested as viable cells in its fecal pellets. We investigated the interaction of S. Typhimurium with Tetrahymena by microarray gene expression profiling. The transcriptome of S. Typhimurium in Tetrahymena phagosomes revealed that 16% of its genes were altered in expression, many of which reflect a physiological response in the pathogen analogous to that in the vacuole of human macrophages. A great proportion of the upregulated genes have a role in anaerobic metabolism and the use of alternate electron acceptors. Several virulence genes required for survival and replication within human macrophages also had increased expression, including type III secretion system genes and PhoP-regulated genes, such as pagP/C/K, envE, mig-14, virK and mgtB/C. A 'mgtC mutant of S. Typhimurium did not display decreased viability in Tetrahymena, but paradoxically, was maintained at a higher cell density in pellets than the wild-type strain. The expression of adiA and adiY, which are involved in arginine-dependent acid resistance, also was increased in the protozoan phagosome. A 'adiA-adiY mutant had lower viability after passage through Tetrahymena, providing evidence that acid resistance has a role in the survival of the pathogen to digestion by this protist. Furthermore, a higher proportion of S. Typhimurium cells within pellets remained viable after exposure to pH 3.4 compared to uningested cells in the same suspension. Our results suggest that S. Typhimurium experiences conditions in Tetrahymena vacuoles that are similar to those in human macrophages and may have implications for its contamination cycle.