"Within-host competition determines reproductive success of temperate bacteriophages".
Reviewed: 11/04/11
Parasites compete with one another just like species in any other trophic level in the ecosystem. This study approached parasitism from this perspective, in a competitor-on-competitor and resource-limited fashion. The results are important ecologically as they suggest that the conceptual tools already present in the field of ecology to think about traditional competition may apply to coinfected parasites within a common host as well.
The study organisms were the ubiquitous E. coli bacteria and 11 bacteriophages. Bacteria are easily cultured and many different molecular and genetic techniques have been previously developed to control their growth in a laboratory setting. Bacteriophages infect by two related but slightly different mechanisms. One involves the lytic cycle, where the parasite hijacks the host machinery to reproduce its own genetic material and then initiates cell rupture and death. The second is the lysogenic cycle, where the phage inserts its DNA into the host DNA, becoming a prophage. The prophage stays internal, and gets replicated with each cell division, until external pressures induce the lytic cycle and cell death. A secondary infection of a prophage into a bacteria already infected with a prophage must be a constant threat, as premature stimulus of the lytic cycle could lower reproductive output for either or both bacteriophages.
This concept shapes the basis of this experiment. Baseline performance of uni-infected prophages was analyzed first in order to note deviations from the normal pattern of growth when coinfected with a secondary prophage. Productivity, phages released per lysed cell, and lysis time, length of the rupture process from moment of externally applied signal, were the primary response variables analyzed.
In the majority of the coinfections, baseline productivity was not maintained by both phages at the same time, indicating a potential hierarchy of competitive ability. Total combined productivity of the two phages was close to 100% in most cases. This lends support to the idea that a high degree of resource/exploitative competition was taking place. However, there was a significant loss of total productivity that could not be explained by the sharing of resources alone, indicating some direct/interference competition between the two coinfected prophages, though the mechanism is not known.
Crucially this study found that coinfection alters productivity of the parasites and that this alteration is differential based on competitive ability. However, coexistence of phages still occurs in nature; one super-phage has not outcompeted all other phages in all settings. This may be because a small proportion of the variance in productivity was explained by the specific combination of the two coinfected phages, and not just a sum of their main effects. The results of this study would also benefit by testing a similar experiment across an array of nutrient conditions and/or host genotype or species. The phages in this experiment resided within a host that was largely not stressed by external environmental factors up until the lytic cycle was induced. An experiment involving staggered pH level differences, or deviations in nutrient content from the idealized petri plate, would be highly applicable to natural world microbial communities, as well as to larger questions about coinfection as drivers in ecological processes.
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