Stochastic receptor expression allows sensitive bacteria to evade phage attack. Part I: experiments.

It has long been suspected that population heterogeneity, either at a genetic level or at a protein level, can improve the fitness of an organism under a variety of environmental stresses. However, quantitative measurements to substantiate such a hypothesis turn out to be rather difficult and have rarely been performed. Herein, we examine the effect of expression heterogeneity of lambda-phage receptors on the response of an Escherichia coli population to attack by a high concentration of lambda-phage. The distribution of the phage receptors in the population was characterized by flow cytometry, and the same bacterial population was then subjected to different phage pressures. We show that a minority population of bacteria that produces the receptor slowly and at low levels determines the long-term survivability of the bacterial population and that phage-resistant mutants can be efficiently isolated only when the persistent phage pressure >10(10) viruses/cm(3) is present. Below this phage pressure, persistors instead of mutants are dominant in the population.

Stochastic receptor expression allows sensitive bacteria to evade phage attack. Part II: theoretical analyses.

Stochastic gene expression in bacteria can create a diverse protein distribution. Most of the current studies have focused on fluctuations around the mean, which constitutes the majority of a bacterial population. However, when the bacterial population is subject to a severe selection pressure, it is the properties of the minority cells that determine the fate of the population. The central question is whether phenotype heterogeneity, such as a spread in the expression level of a critical protein, is sufficient to account for the persistence of the bacteria under the selection. A related question is how long such persistence can last before genetic mutation becomes significant. In this work, survival statistics of a bacterial population with a diverse phage-receptor number distribution is theoretically investigated when the cells are subject to phage pressures. The calculations are compared with our experimental observations presented in Part I in this issue. The fundamental basis of our analysis is the Berg-Purcell theoretical result for the reaction rate between a phage particle and a bacterium with a discrete number of receptors, and the observation that most phage-resistant mutants isolated in laboratory cultures are defective in phage binding. It is shown that a heterogeneous bacterial population is significantly more fit compared to a homogeneous population when confronting a phage attack.

On kinetics of phage adsorption.

Adsorption of lambda-phage on sensitive bacteria Escherichia coli is a classical problem but not all issues have been resolved. One of the outstanding problems is the rate of adsorption, which in some cases appears to exceed the theoretical limit imposed by the law of random diffusion. We revisit this problem by conducting experiments along with new theoretical analyses. Our measurements show that upon incubating lambda-phage with bacteria Ymel, the population of unbound phage in a salt buffer decreases with time and in general obeys a double-exponential function characterized by a fast (tau(1)) and a slow (tau(2)) decay time. We found that both the fast and the slow processes are specific to interactions between lambda-phage and its receptor LamB. Such specificity motivates a kinetic model that describes the interaction between the phage and the receptor as an on-and-off process followed by an irreversible binding. The latter may be a signature of the initiation of DNA translocation. The kinetic model successfully predicts the double exponential behavior seen in the experiment and allows the corresponding rate constants to be extracted from single measurements. The weak temperature dependence of the reversible and the irreversible binding rate suggests that phage retention by the receptor is entropic in nature and that a molecular key-lock interaction may be an appropriate description of the interaction between the phage tail and the receptor.