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.
Highlights
The appearance of plaques on a bacterial lawn is one of the enduring imageries in modern day biology
Effect of adsorption rate To assess the impact of adsorption rate on plaque size and plaque productivity, we constructed eight isogenic strains of phage l that only differed in their adsorption rate and virion size
Since all our side-tail fibers (Stf)+ and Stf- phages would have the same shape within the group and experience the same viscosity, we expect the ranking of the adsorption rates within each Stf group to remain the same
Summary
The appearance of plaques on a bacterial lawn is one of the enduring imageries in modern day biology. Until now, there has been no systematic experimental exploration on how different bacteriophage (phage) traits may influence the formation of a plaque. We constructed a series of isogenic l 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. Ever since the discovery of bacteriophages (phages), the prominent clearings that they produce on bacterial lawns (the lysis plaques) have fascinated countless microbiologists. The ability to form plaques is not restricted to phages only since. A change in plaque morphology during study is commonly used as an indication that the viral strain may have sustained a mutation(s) [10]. With proper calibration, the phage plaque size has been used as a surrogate for the fitness measurement [11] (see [12])
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