Characterization of the molecular mechanisms of calcium (Ca2+) regulation of bacterial physiology and virulence requires tools enabling measuring and monitoring the intracellular levels of free calcium (Ca2+in). Here, we describe a protocol optimized to use a recombinantly expressed Ca2+-binding protein, aequorin, for detecting Ca2+in in Pseudomonas aeruginosa. Upon binding to free Ca2+, aequorin undergoes chromophore oxidation and emits light, the log of which intensity linearly correlates with the amount of bound Ca2+, and therefore, can be used to measure the concentration of free Ca2+ available for binding. This protocol involves the introduction of the aequorin gene into P. aeruginosa, induction of apoaequorin production, reconstitution of the holoenzyme with its chromophore, and monitoring its luminescence. This protocol allows continuous measuring of Ca2+in concentration in vivo in response to various stimuli.
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