The transcriptome of Escherichia coli O157: H7 reveals a role for oxidative stress resistance in its survival from predation by Tetrahymena.

Pathogenic E. coli remains undigested upon phagocytosis by Tetrahymena and is egested from the ciliate as viable cells in its fecal pellets. Factors that are involved in the survival of Shiga toxin-producing E. coli serovar O157: H7 (EcO157) from digestion by Tetrahymena were identified by microarray analysis of its transcriptome in the protozoan phagosome. Numerous genes belonging to anaerobic metabolism and various stress responses were upregulated significantly ≥ 2-fold in EcO157 cells in the food vacuoles compared with in planktonic cells that remained uningested by the protist. Among these were the oxidative stress response genes, ahpF and katG. Fluorescence microscopy and staining with CellROX® Orange confirmed the presence of reactive oxygen species in food vacuoles containing EcO157 cells. Frequency distribution analysis of the percentage of EcO157 viable cells in Tetrahymena fecal pellets revealed that the ΔahpCF and ΔahpCFΔkatG mutants were less fit than the wild type strain and ΔkatG mutant after passage through the protist. Given the broad use of oxidants as sanitizers in the food industry, our observation of the oxidative stress response in EcO157 during its interaction with Tetrahymena emphasizes the importance of furthering our knowledge of the physiology of this human pathogen in environments relevant to its ecology and to food safety.

Salmonella transcriptional signature in Tetrahymena phagosomes and role of acid tolerance in passage through the protist.

Salmonella enterica Typhimurium remains undigested in the food vacuoles of the common protist, Tetrahymena. Contrary to its interaction with Acanthamoeba spp., S. Typhimurium is not cytotoxic to Tetrahymena and is egested as viable cells in its fecal pellets. Through microarray gene expression profiling we investigated the factors in S. Typhimurium that are involved in its resistance to digestion by Tetrahymena. The transcriptome of S. Typhimurium in Tetrahymena phagosomes showed that 989 and 1282 genes were altered in expression compared with that in water and in LB culture medium, respectively. A great proportion of the upregulated genes have a role in anaerobic metabolism and the use of alternate electron acceptors. Many genes required for survival and replication within macrophages and human epithelial cells also had increased expression in Tetrahymena, including mgtC, one of the most highly induced genes in all three cells types. A ΔmgtC mutant of S. Typhimurium did not show decreased viability in Tetrahymena, but paradoxically, was egested at a higher cell density than the wild type. The expression of adiA and adiY, which are involved in arginine-dependent acid resistance, also was increased in the protozoan phagosome. A ΔadiAY mutant had lower viability after passage through Tetrahymena, and a higher proportion of S. Typhimurium wild-type cells within pellets remained viable after exposure to pH 3.4 as compared with uningested cells. Our results provide evidence that acid resistance has a role in the resistance of Salmonella to digestion by Tetrahymena and that passage through the protist confers physiological advantages relevant to its contamination cycle.