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Impedance Measurements Could Accelerate Phage-Based Identification of Bacillus anthracis and Other Bacteria

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Technical Report

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USAMRIID Frederick United States

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The high degree of specificity that phage exhibit for their hosts has made them valuable tools for identifying bacterial species and subspecies. This is particularly true in the field of biodefense where phage have a long history of being used to identify Bacillus anthracis and Yersinia pestis isolates. However, phage assays generally require that suspect colonies be sub-cultured onto a fresh agar plate to generate a dense lawn against which the phage-induced plaques can be observed. Following a potential exposure to bacterial threat agents, this additional incubation step can consume valuable time. Researchers have shown that temporal changes in the dielectric permittivity of bacterial micro-cultures differ for chemically or physically stressed and unstressed cells. In fact, a distinctive shift in the relative responses caused by heat shock, antibiotics, or phage infection can be detected as early as one hour after exposing as few as 105 CFU bacteria to the stressor. We predicted that similar responses could be used to detect phage-induced stress in susceptible B. anthracis micro-cultures and thereby reduce both the time and biomass required to perform phage-based diagnostic assays for this pathogen. After exposing small quantities of Bacillus cultures to phage, we tracked the cultures for up to 90 minutes using microscopy, impedance measurements, and plated serial dilutions. Our results show that only the phage-sensitive B. anthracis cultures exhibit rapid and distinctive drops in dielectric permittivity when exposed to the phage. This data suggests that impedance measurements might be used to improve traditional phage-based identification assays, including those used in the field of biodefense.

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