RESEARCH & SUPPORT
Bacterial threat assessment of phage infection
When cells lyse, danger signals are released into the environment that warn adjacent cells of nearby danger. However, the molecules that serve as bacterial danger signals are poorly defined, and how danger signals affect bacterial responses to threats such as bacteriophage infection are unknown. Here, we demonstrate that polyamines released by lysed bacteria are internalized by adjacent cells. In the absence of bacteriophage infection, intracellular polyamine levels quickly return to baseline. When bacteriophage infect a cell, they inject linear DNA that causes intracellular polyamine levels to remain high, interfering with bacteriophage DNA replication. Our results indicate that polyamines released by lysed cells and linear DNA associated with bacteriophage infection are danger signals that guide bacterial threat assessment of bacteriophage infection.
When the Pf4 prophage is deleted from P. aeruginosa PAO1 (∆Pf4), bacteria produce more of the green pigment virulence factor pyocyanin. However, ∆Pf4 bacteria are less virulent in animal models of infection. The reduced virulence of ∆Pf4 despite high levels of pyocyanin production may be explained by our finding that C. elegans mutants unable to sense bacterial pigments through the aryl hydrocarbon receptor are more susceptible to ∆Pf4 infection compared to wild-type C. elegans. Collectively, our data support a model where suppression of quorum-regulated virulence factors by Pf4 allows P. aeruginosa to evade detection by innate host immune responses.
Prophage-host-microbe interactions
Bacteriophage in Lyme disease
Most Lyme disease spirochetes are infected by bacteriophage. We are exploring the role these phages play in the vector-vertebrate lifecycle of Borrelia burgdorferi and in the pathogenesis of Lyme disease.
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Filamentous bacteriophage in the biofilm matrix
When the bacterial pathogen Pseudomonas aeruginosa forms a biofilm—bacteria encased in a polymer-rich matrix—some of the most highly expressed genes belong to filamentous Pf bacteriophage. Utilizing biophysical and genetic approaches, we found that a crowding mechanism called depletion attraction forced the bacteriophage within the biofilm matrix to assemble into a highly ordered liquid crystalline structure. We showed that relative to a non-crystalline matrix, bacteria within a liquid-crystalline matrix were better able to survive desiccation and antibiotic treatment (Cell Host & Microbe, Nov. 2015).
We also discovered that crowding induced by a wide range of host polymers caused filamentous bacteriophage to assemble liquid crystalline networks, increasing viscosity. Moving this observation into a murine model of pneumonia, we found that bacteria producing filamentous bacteriophage modulated the immune response and became physically trapped within the lung, establishing a non-invasive infection phenotype (Infection and Immunity, Jan. 2017, featured on journal cover).
The birefringence of the liquid crystalline biofilm matrix can be visualized by placing colonies between crossed polarizing lenses. The liquid crystalline matrix can change the polarization of light allowing it to pass through both polarizing lenses. Thus, non-liquid crystalline colonies appear opaque (red arrow) while liquid crystalline colonies appear bright (blue arrow). The organization of the biofilm matrix into a liquid crystal enhances adhesion, antibiotic tolerance, and desiccation survival. Further, Pf phage can increase the viscosity of host polymers such as mucin and DNA. Secor et al., 2016, Microbial Cell, 3:1 pp. 49-52.
