| This tube contains enough Barium so that when it is shook up and the Barium is completely suspended, the optical density of the suspension is 0.5. Antibiotic suseptibility testing by the disk-diffusion (Kirby-Bauer) method is standardized for bacterial inoculums with this optical density. Bacteria are diluted in sterile saline until the optical density matches the McFarland standard, then used to streak a lawn of bacteria to be used for Kirby-Bauer.
The Barium settles out during storage. By holding the tubes up to paper with printing on it, one can easily see the difference between shook (left) and unshook (right) McFarland standards.
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