During a Guthrie newborn screening for phenylketonuria, growth of Bacillus subtilis around the disk indicates which condition?

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Multiple Choice

During a Guthrie newborn screening for phenylketonuria, growth of Bacillus subtilis around the disk indicates which condition?

The test works by using Bacillus subtilis grown on agar with an inhibitor in the medium. The disk carries blood from the newborn, and phenylalanine from the blood diffuses into the agar. If phenylalanine levels are high, as in phenylketonuria, it blocks the inhibitor (beta-2-thienylalanine), allowing the bacteria to grow around the disk and form a visible growth halo. So observing growth around the disk is a positive sign of elevated phenylalanine in the blood. If there were no phenylalanine to counteract the inhibitor, you’d expect no growth near the disk (no halo). A clear zone around the disk would indicate inhibition without counteraction, and growth only when phenylalanine is absent would be the opposite of what PKU screening looks for. Thus, the growth around the disk due to phenylalanine inhibiting the growth-inhibitor best explains a positive PKU screen.

The test works by using Bacillus subtilis grown on agar with an inhibitor in the medium. The disk carries blood from the newborn, and phenylalanine from the blood diffuses into the agar. If phenylalanine levels are high, as in phenylketonuria, it blocks the inhibitor (beta-2-thienylalanine), allowing the bacteria to grow around the disk and form a visible growth halo. So observing growth around the disk is a positive sign of elevated phenylalanine in the blood.

If there were no phenylalanine to counteract the inhibitor, you’d expect no growth near the disk (no halo). A clear zone around the disk would indicate inhibition without counteraction, and growth only when phenylalanine is absent would be the opposite of what PKU screening looks for. Thus, the growth around the disk due to phenylalanine inhibiting the growth-inhibitor best explains a positive PKU screen.

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