The adsorption of bacteriophages (phages) onto host cells is, in all but a few rare cases, a sine qua non condition for the onset of the infection process. Understanding the mechanisms involved and the factors affecting it is, thus, crucial for the investigation of host-phage interactions. This review provides a survey of the phage host receptors involved in recognition and adsorption and their interactions during attachment. Comprehension of the whole infection process, starting with the adsorption step, can enable and accelerate our understanding of phage ecology and the development of phage-based technologies. To assist in this effort, we have established an open-access resource--the Phage Receptor Database (PhReD)--to serve as a repository for information on known and newly identified phage receptors.

Background: One of the main challenges in developing phage therapy and manufacturing phage products is the reliable evaluation of their efficacy, performance, and quality. Since phage virulence is intrinsically difficult to fully capture, researchers have turned to rapid but partially inadequate methods for its evaluation. Materials and Methods: This study demonstrates a standardized quantitative method to assess phage virulence based on three parameters: the virulence index (V P )-quantifying the virulence of a phage against a host, the local virulence (v i )-assessing killing potential at given multiplicities of infection (MOIs), and MV 50 -the MOI at which the phage achieves 50% of its maximum theoretical virulence. This was shown through comparative analysis of the virulence of phages T4, T5, and T7. Results: Under the conditions tested, phage T7 displayed the highest virulence, followed by phage T4 and, finally, by phage T5. The impact of parameters such as temperature and medium composition on virulence was shown for each phage. The use of the method to evaluate the virulence of combinations of phages-for example, for cocktail formulation-is also shown with phages T5 and T7. Conclusions: The method presented provides a platform for high-throughput quantitative assessment of phage virulence and quality control of phage products. It can also be applied to phage screening, evaluation of phage strains, phage mutants, infection conditions and/or the susceptibility of host strains, and the formulation of phage cocktails.

The process of a bacteriophage attaching to its host cell is a combination of physical diffusion, biochemical surface interactions, and reaction-induced conformational changes in receptor proteins. Local variations in the physico-chemical properties of the medium, the phage׳s mode of action, and the physiology of the host cell also all influence adsorption kinetics. These characteristics can affect a specific phage׳s binding capabilities and the susceptibility of the host cell to phage attack. Despite the complexity of this process, describing adsorption kinetics of a population of bacteriophages binding to a culture of cells has been accomplished with relatively simple equations governed by the laws of mass-action. Many permutations and modifications to the basic set of reactions have been suggested through the years. While no single solution emerges as a universal answer, this review provides the fundamentals of current phage adsorption modeling and will guide researchers in the selection of valid, appropriate models.

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Synchronized Escherichia coli cultures were infected with bacteriophage T4 at discrete points in the cell growth cycle. The cell cycle had a significant impact on the outcome of infection. Cell burst size was smallest for newly formed cells and increased dramatically as these progressed in the cell cycle. The largest burst sizes were achieved when infecting cells immediately prior to cell division. When cells were infected during cell division, the burst size was reduced back to its initial value. Interestingly, lysis time was longest for young cells, reached a minimum at the same point that burst size reached its maximum value, and then increased at the commencement of cell division. Consequently, phage productivity in cells about to undergo cell division was almost three times greater than the productivity of young, newly formed cells. The availability of intracellular resources is believed to be the major driving force behind phage productivity during infection. Indeed, intracellular RNA contents at the time of infection were found to correlate strongly with phage productivity. There was no significant relationship between cell DNA levels and phage productivity. Finally, burst size experiments suggested that the cell cycle also influenced the likelihood of a phage to undergo productive infection.