Effects of bacteriophage traits on plaque formation

Abstract

BackgroundThe appearance of plaques on a bacterial lawn is one of the enduring imageries in modern day biology. The seeming simplicity of a plaque has invited many hypotheses and models in trying to describe and explain the details of its formation. However, until now, there has been no systematic experimental exploration on how different bacteriophage (phage) traits may influence the formation of a plaque. In this study, we constructed a series of isogenic λ phages that differ in their adsorption rate, lysis timing, or morphology so that we can determine the effects if these changes on three plaque properties: size, progeny productivity, and phage concentration within plaques.ResultsWe found that the adsorption rate has a diminishing, but negative impact on all three plaque measurements. Interestingly, there exists a concave relationship between the lysis time and plaque size, resulting in an apparent optimal lysis time that maximizes the plaque size. Although suggestive in appearance, we did not detect a significant effect of lysis time on plaque productivity. Nonetheless, the combined effects of plaque size and productivity resulted in an apparent convex relationship between the lysis time and phage concentration within plaques. Lastly, we found that virion morphology also affected plaque size. We compared our results to the available models on plaque size and productivity. For the models in their current forms, a few of them can capture the qualitative aspects of our results, but not consistently in both plaque properties.ConclusionsBy using a collection of isogenic phage strains, we were able to investigate the effects of individual phage traits on plaque size, plaque productivity, and average phage concentration in a plaque while holding all other traits constant. The controlled nature of our study allowed us to test several model predictions on plaque size and plaque productivity. It seems that a more realistic theoretical approach to plaque formation is needed in order to capture the complex interaction between phage and its bacterium host in a spatially restricted environment.

Population Dynamics of Phage and Bacteria in Spatially Structured Habitats Using Phage λ and Escherichia coli

“…The dependence of plaque size on the adsorption rate is rather weak (Fig. 2e), consistent with previous experimental observations (38). The initial bacterial density, B 0 , influences both the expansion and the final size because the bacteria absorb large amounts of phages and because a large initial density limits the time before nutrient depletion influences the growth.…”

“…It should be noted that our diffusion constant D p may well be smaller than that in pure water since the phage will attach and detach with bacteria, making D p decrease with the adsorption rate. Experimentally, it has been known that the phage with side tail fibers makes plaques with about half the area of those without (38). Existence of side tail fibers affects the adsorption rate by factor 5-to 100-fold (37,38) and thereby also the effective diffusion constant.…”

“…Experimentally, it has been known that the phage with side tail fibers makes plaques with about half the area of those without (38). Existence of side tail fibers affects the adsorption rate by factor 5-to 100-fold (37,38) and thereby also the effective diffusion constant. It should also be noted that overall results depend on the time scale relative to the maximum growth rate, which we kept constant in the present simulation; changing the growth condition would effectively rescale the relevant rates.…”

“…Most of them focus on virulent phages and formation of clear plaques. The experimental work by Hendrix and Duda (37) and more recently by Gallet et al (38) used various phages mutant in their tail fibers to address the effects on the plaque expansion speed and plaque size. Most modeling studies examine the plaque enlargement at a constant speed in a lawn of constant bacterial density, as seen with virulent phage T7 (39)(40)(41)(42)(43)(44).…”

Fitness benefits of low infectivity in a spatially structured population of bacteriophages

“…We use the term plaque density to refer to the concentration of phage within a small circular region within a single plaque (similar to plaque productivity in [30]). Phage were plated using dilutions that produced between 10 and 50 plaques per plate.…”