Predation

The ability to detect luminescence activity of specific marked strains has been exploited to determine the effect of predation by protozoa on bacterial prey.48 Soil microcosms were inoculated with lux-marked bacterial cells (P. fluorescens) at specific matric potentials to ensure their location in small (neck size < 6 ^m) or intermediate size (neck size 6-30 ^m) pores, while Colpoda steinii was introduced to large pores (neck size 30-60 ^m) (Fig. 5.6). Viable cell concentrations of both populations were monitored and bacterial activity was assessed by luminometry. Bacteria in intermediate pores were predated at a greater rate than those introduced to small pores, and decreases in bacterial viable cell concentration were associated with increases in the ciliate population. Small pores therefore appear to provide protection from predation. Luminescence decreased, following inoculation, due to loss of bacterial activity through lack of nutrients and this effect was greater in the presence of the ciliate, due to predation. However, luminescence activity per cell was significantly greater for cells introduced into intermediate-sized pores, where predation was greatest. This suggests that predation, although reducing numbers of bacteria, results in the release and turnover of nutrients by the ciliate such that surviving bacterial cells have greater activity. Demonstration of this effect using traditional activity techniques was not possible because of problems in distinguishing between bacterial and ciliate activity.

Fig. 5.6. Changes in a) viable cell concentration, b) luminescence and c) luminescence viable cell-1 following introduction of P. fluorescens into small pores (neck diameter < 6 ^m) (o, •) or intermediate-sized pores (neck diameter 6-30 ^m) (□, ■ ) and introduction of sterile water (o, □) or C. steinii (•, ■) into large pores (neck diameter 30-60 ^m). Error bars are minimum significant difference values calculated from two way analysis of variance for p < 0.05. Bars are omitted where treatments were not significantly different (p > 0.05). Reprinted with permission from Wright D, Killham K, Glover LA et al. Appl Environ Microbiol 1995; 61:3537-3543.

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