Within-host competition determines reproductive success of temperate bacteriophages

Abstract

Within-host competition between parasites is frequently invoked as a major force for parasite evolution, yet quantitative studies on its extent in an organismal group are lacking. Temperate bacteriophages are diverse and abundant parasites of bacteria, distinguished by their ability to enter a facultative dormant state in their host. Bacteria can accumulate multiple phages that may eventually abandon dormancy in response to host stress. Host resources are then converted into phage particles, whose release requires cell death. To study within-host competition between phages, I used the bacterium Escherichia coli and 11 lambdoid phages to construct single and double lysogens. Lysogenic bacterial cultures were then induced and time to host cell lysis and productivity of phages was measured. In double lysogens, this revealed strong competitive interactions as in all cases productivity of at least one phage declined. The outcome of within-host competition was often asymmetrical, and phages were found to vary hierarchically in within-host competitive ability. In double infections, the phage with the shorter lysis time determined the timing of cell lysis, which was associated with a competitive advantage when time differences were large. The results emphasize that within-host competition greatly affects phage fitness and that multiple infections should be considered an integral part of bacteriophage ecology.

Figure 1

Life cycle of temperate phages in the context of this study. Upon infection, phages establish lysogeny and reside dormant as prophages in the bacterial chromosome. When induced, they begin lytic development and lyse their host after some time to release virions. In this hypothetical example, the white phage has a shorter lysis time than the black phage and phages differ in productivity. If prophages share a cell and form a double lysogen, both lysis time and productivity may change. In this example, the white phage determines lysis time and competition lowers productivity of both phages.

Figure 2

The outcome of superinfection of 11 different single-lysogens with the same 11 phages.

Figure 3

Phage productivity (a) and lysis time (b) in single lysogens. Box plots summarizing baseline data of all 11 phages that were used in this study. Lysogenic cultures of E. coli MG1655 carrying a single prophage were induced with mitomycin C and turbidity was monitored to estimate lysis time. Productivity of different phages was measured as the average number of phages released per cell that was induced. Analysis of variance found significant variation among phages in both data sets. Letters indicate groups of phages that were found to differ significantly according to Tukey's range test.

Figure 4

Within-host competition between lambdoid phages. (a) Parameters describing the competitive ability of each phage. Competitive effect measures the effect of a phage on the productivity of its competitors; higher values indicate less effect. Competitive response measures the response of a phage to competition; lower values indicate a sensitive response. (b) Relative productivity of phages in double lysogens. Every data point corresponds to a unique pair of phages. To order data points and facilitate their comparison, phages with higher relative productivity in a pair were plotted on the y-axis. Error bars show the 95% confidence interval. The dotted line shows the function y=1−x. Points close to the line achieve a combined relative productivity of approximately one. Open symbols are lysogens where phages strongly interfere as defined by a combined relative productivity <0.1 (circles: λ/HK022, λ/mEp213, λ/mEpX1; squares: mEp234/Φ80, mEp506/Φ80), or where a phage in a double lysogen achieves a productivity that is significantly above its baseline productivity (diamonds: λ/mEpX2, mEp043/mEp234, mEp213/mEp234).

Figure 5

Shifts in lysis time and relation to productivity and competitive success. (a) Difference of lysis time of double lysogens from expectation under two hypotheses: (i) lysis time is determined by the faster phage in a double lysogen (gray bars), and (ii) lysis time is close to the average of the baseline lysis times of both phages in a double lysogen (open bars). (b) Phage relative productivity in relation to deviation from baseline lysis time. The solid curve is a spline that was fitted to the data. The dashed line indicates no deviation. (c) Competitive success of a phage in a double lysogen plotted against its deviation from baseline lysis time relative to that of its competitor (positive values indicate its baseline lysis time is closer than the competitor's). Closed circles are double lysogens with Φ80; open circles are all other double lysogens.