Growth of Acanthamoeba castellanii and Hartmannella vermiformis on live, heat-killed and DTAF-stained bacterial prey

The growth responses of two species of amoeba were evaluated in the presence of live, heat-killed and heat-killed/5-(4,6-dichlorotriazin-2-yl) aminofluorescein (DTAF)-stained cells of Escherichia coli, Pseudomonas aeruginosa, Klebsiella aerogenes, Klebsiella ozaenae and Staphylococcus aureus. The specific growth rates of both species were significantly higher with live bacterial prey, the only exception being Hartmannella vermiformis feeding on S. aureus, for which growth rates were equivalent on all prey states. There was no significant difference between growth rates, yield or ingestion rates of amoebae feeding on heat-killed or heat-killed/stained bacterial cells, suggesting that it was the heat-killing process that influenced the amoeba−bacteria interaction. Pretreatment of prey cells had a greater influence on amoebic processing of Gram-negative bacteria compared with the Gram-positive bacterium, which appeared to be as a result of the former cells being more difficult to digest and/or losing their ability to deter amoebic ingestion. These antipredatory mechanisms included microcolony formation in P. aeruginosa, toxin production in K. ozaenae, and the presence of an intact capsule in K. aerogenes. E. coli and S. aureus did not appear to possess an antipredator mechanism, although intact cells of the S. aureus were observed in faecal pellets, suggesting that any antipredatory mechanism was occurring at the digestion stage.