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.

Effects of bacterial prey species and their concentration on growth of the amoebae Acanthamoeba castellanii and Hartmannella vermiformis.

Two amoebae were presented with six bacterial prey at a range of concentrations, and the growth parameters of the amoebae were deduced. All but one bacterium (Synechococcus) resulted in a positive growth response, but the gram-positive bacterium Staphylococcus aureus proved to be difficult to digest and the heavily pigmented bacterium Klebsiella ozaenae induced unusual amoebic behavior prior to ingestion.

A comparison of the growth and starvation responses of Acanthamoeba castellanii and Hartmannella vermiformis in the presence of suspended and attached Escherichia coli K12.

The growth and starvation responses of Acanthamoeba castellanii and Hartmannella vermiformis were investigated in the presence and absence of Escherichia coli on an agar surface or within shaken suspensions. The amoebae perceived all the suspended systems to be unfavourable for growth, despite being challenged with high levels of prey, and as a consequence they exhibited a starvation response. However, the response differed between species, with A. castellanii producing characteristic cysts and H. vermiformis producing round bodies. These amoebic forms were reactivated into feeding trophozoites in the presence of bacterial aggregates, which formed in the suspended systems after 68 h of incubation. In contrast, both species of amoebae grew well in the presence of attached E. coli at a concentration of 1 x 10(6) cells cm(-2) of agar and yielded specific growth rates of c. 0.04 h(-1). Starvation responses were induced at the end of the growth phase, and these were equivalent to those recorded in the suspended systems. We conclude that, when suspended, amoebae in the 'floating form' cannot feed effectively on suspended prey, and hence the starvation response is initiated. Thus the majority of amoebic feeding is via trophozoite grazing of attached bacterial prey.