Factors influencing lysis time stochasticity in bacteriophage λ

John J Dennehy,Ing-Nang Wang

doi:10.1186/1471-2180-11-174

John J Dennehy, Ing-Nang Wang

Open Access

https://doi.org/10.1186/1471-2180-11-174

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Abstract

BackgroundDespite identical genotypes and seemingly uniform environments, stochastic gene expression and other dynamic intracellular processes can produce considerable phenotypic diversity within clonal microbes. One trait that provides a good model to explore the molecular basis of stochastic variation is the timing of host lysis by bacteriophage (phage).ResultsIndividual lysis events of thermally-inducible λ lysogens were observed using a temperature-controlled perfusion chamber mounted on an inverted microscope. Both mean lysis time (MLT) and its associated standard deviation (SD) were estimated. Using the SD as a measure of lysis time stochasticity, we showed that lysogenic cells in controlled environments varied widely in lysis times, and that the level of lysis time stochasticity depended on allelic variation in the holin sequence, late promoter (pR') activity, and host growth rate. In general, the MLT was positively correlated with the SD. Both lower pR' activities and lower host growth rates resulted in larger SDs. Results from premature lysis, induced by adding KCN at different time points after lysogen induction, showed a negative correlation between the timing of KCN addition and lysis time stochasticity.ConclusionsTaken together with results published by others, we conclude that a large fraction of λ lysis time stochasticity is the result of random events following the expression and diffusion of the holin protein. Consequently, factors influencing the timing of reaching critical holin concentrations in the cell membrane, such as holin production rate, strongly influence the mean lysis time and the lysis time stochasticity.

Highlights

Despite identical genotypes and seemingly uniform environments, stochastic gene expression and other dynamic intracellular processes can produce considerable phenotypic diversity within clonal microbes
Using a microscope-mounted, temperature-controlled perfusion chamber, we observed and recorded individual lysis events of thermally-induced Escherichia coli l lysogens (Figure 2A). These observations revealed a considerable amount of variation in lysis time for the wildtype (WT) l phage (Table 1; Figure 2B)
Given that phage progeny accumulate linearly at ~7.7 phage per minute beginning ~28 min after lysis induction [46], the ~30 min range of lysis times could result in a three-fold difference in burst size between phages that lyse early and those that lyse late

Summary

Introduction

Despite identical genotypes and seemingly uniform environments, stochastic gene expression and other dynamic intracellular processes can produce considerable phenotypic diversity within clonal microbes. Population heterogeneity, resulting from such molecular stochasticity, has been documented in many microbial organisms including bacteriophage (phage) l [10,11,12,13], Escherichia coli [14,15,16], Bacillus subtilis [17,18] and Saccharomyces cerevisiae [19,20,21,22,23,24] This within-population variation can have far reaching life history consequences. Mutants of S. cerevisiae showing greater heterogeneity in survival had higher rates of occasional-cell survival during high stress conditions than did wild-type cells [25] Because of their simplicity and ease of manipulation, phages are excellent models to explore the life history consequences of molecular stochasticity.

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