Like us, bacteria have their own microbial attackers, in the form of bacteria-specific viruses called bacteriophage, or phage. These phage come in a variety of flavors but can be broadly categorized into virulent, which immediately begin to replicate and lyse (burst) the infected cell quickly, and temperate, which incorporate the phage DNA into the bacterial genome. This non-replicative state, called lysogeny, allows the phage to be passed to daughter cells when the bacterium divides, but does not generate multiple progeny phage until activated. When activated, the lysogenic phage enter the lytic cycle, killing E. coli host cells as the phage replicate; lysis of E. coli via any lytic phage can be visualized in the resulting plaque (or clearance) of the surrounding killed bacterial cells (see figure, left).
The wild-type λ phage can act either as a virulent or a temperate phage, while the cI- mutant λ phage is a virulent phage that kills all of its host cells. These different lifestyles are reflected in close-ups of the plaques caused by these respective types: a wild-type plaque has some surviving bacteria, some of which carry lysogenic phage, while the mutant phage leaves almost no survivors (see right).
These phage mutants also have differing abilities to diffuse through a population – both in size and in speed of plaque spread. Lysogenic infections happen more frequently with slower bacterial growth and higher phage concentrations. Combining computer simulations with growth experiments, first author Namiko Mitarai and senior author Kim Sneppen studied the difference in λ phage diffusion in an E. coli bacterial lawn. This allowed the authors to test how phage diffusion can be affected by bacterial growth, phage density, and phage adsorption to bacterial cells – and how these, in turn, affect localized infections within a phage plaque. Their results are published in the Journal of Bacteriology.
From their experiments, the authors concluded that a temperate phage has fewer surviving lysogens (quiescently infected bacteria) in the center of the plaque than the periphery, in part due to phage density: the higher density leads to more lysogenic infections; as the phage diffuse, the concentration drops, leading to more lytic infections. These simple models and experiments were conducted in a short time frame, with the next step to be future investigation of infection time courses.
Bacteriophage were discovered before antibiotics, but research into phage applicability for fighting bacterial infections was put by the wayside once mass-producing antibiotics became feasible. As drug-resistant infections increase in number and severity, some researchers have returned to this natural bacterial enemy that has no known effects on people. Understanding the dynamic environment in one small plaque – where and when phage are most active – will be important for proper deployment of phage against open lesions or biofilms. If these experiments show survival differences over a few millimeters, imagine the variation in an even larger wound! These studies will help guide phage selection and usage in future applications of phage therapy.
-- Julie Wolf