Viscosity has dichotomous effects on Bdellovibrio bacteriovorus HD100 predation

Summary Bdellovibrio bacteriovorus HD100 is a highly motile predatory bacterium that consumes other Gram‐negative bacteria for its sustenance. Here, we describe the impacts the media viscosity has both on the motility of predator and its attack rates. Experiments performed in polyethylene glycol (PEG) solutions, a linear polymer, found a viscosity of 10 mPa s (5% PEG) negatively impacted predation over a 24‐h period. When the viscosity was increased to 27 mPa s (10% PEG), predation was nearly abolished. Tests with three other B. bacteriovorus strains, i.e., 109J and two natural isolates, found identical results. Short‐term (2‐h) experiments, however, found attack rates were improved in 1% PEG, which had a viscosity of 5.4 mPa s, using bioluminescent prey and their viabilities. In contrast, when experiments were performed in dextran, a branched polymer, no increase in predation was seen even though the viscosity was a comparable 5.1 mPa s. The enhanced attack rates in this solution coincided with a 31% increase in B. bacteriovorus HD100 swimming speeds (62 μm s−1 in 1% PEG vs. 47.5 μm s−1 in HEPES‐salt).