, 2007). However, Tetrahymena had been reported to be more suitable than the amoeba model in high-throughput screening to identify inhibitors of Klebsiella pneumoniae virulence (Benghezal et al., 2007). The ciliate Tetrahymena is a eukaryotic unicellular microorganism with a defined genetic BTK inhibitor background that provides an ideal interface between pathogen and host, allowing for the

elucidation of molecular interactions between host and pathogen (Orias, 1998). Recently, several Tetrahymena–bacteria infection models have been described. For example, Kikuhara et al. (1994) and Steele & McLennan (1996) reported on the interaction between Legionella pneumophila and Tetrahymena thermophila and Benghezal et al. (2007) designed a simple surrogate host model system using Tetrahymena pyriformis and K. pneumoniae cells to assess bacterial virulence while also identifying antivirulence this website molecules. In addition, interactions between Tetrahymena and other bacteria, such as Yersinia pestis (Pushkareva, 2003; Breneva & Maramovich, 2008), Escherichia coli (Bukharin et al., 2008), Vibrio fischeri (Bonnet et al., 2008) and so on, have also been reported. Tetrahymena is common in the freshwater environment, where A. hydrophila is naturally confronted with it. However, the interaction mode between the two organisms

is not clear. In this study, we co-cultured both virulent and avirulent A. hydrophila strains with T. thermophila and recorded changes in the biomass of both A. hydrophila and T. thermophila. In addition, we analyzed infection mechanisms to evaluate the potential use of T. thermophila as an A. hydrophila infection model. The A. hydrophila J-1 strain, used as a vaccine strain in China, was a clinical isolate associated with a natural outbreak linked to cyprinoid fish in Nanjing, China (Chen & Lu, 1991). The A. hydrophila strain ID-8 NJ-4 was isolated from healthy cultured cyprinoid fish in Nanjing, China, and its very low virulence was confirmed by the results of three consecutive infections of cyprinoid fish carried out before this study (unpublished data). In this study,

the two bacterial strains were used as virulent and avirulent strains, respectively. Tetrahymena thermophila BF1 was obtained from Dr Miao Wei, Institute of Hydrobiology, Chinese Academy of Sciences. Tetrahymena thermophila BF1 was cultured at 30 °C and stock cultures were maintained axenically in PYG medium containing 1% proteose peptone, 0.1% yeast extract and 0.1% glucose. Aeromonas hydrophila strains J-1 and NJ-4 were routinely cultured in Luria–Bertani (LB) broth or on Luria agar plates at 28 °C. Tetrahymena thermophila cells were cultured for 48 h and the cultures were concentrated from 6 to 1 mL by centrifugation at 2000 g for 10 min at 15 °C, thus resulting in a concentration of approximately 108 cells mL−1. Five hundred microliters of suspensions of late-log-phase A. hydrophila J-1 and NJ-4 cultures were mixed with the same volume of T. thermophila cells, respectively.