In an introductory light microscopy lab, which microscope setting is best for viewing living, moving bacteria with improved contrast without staining?

• A. Dark-field microscopy with the light intensity reduced to minimum
• B. Phase-contrast microscopy
• C. Oil-immersion bright-field microscopy with a stained smear
• D. Bright-field microscopy with the condenser iris wide open

Answer: (B)

Verified step by step guidance

1. Understand the goal: We want to observe living, moving bacteria with improved contrast without staining. This means the technique should enhance visibility of transparent specimens without killing or altering them.
2. Recall that bright-field microscopy typically requires staining to improve contrast, which can kill or fix bacteria, so it is not ideal for viewing live, unstained bacteria.
3. Consider dark-field microscopy: it enhances contrast by only collecting scattered light, making specimens appear bright against a dark background. However, reducing light intensity to minimum may make the image too dim and is not the best standard setting for live bacteria.
4. Phase-contrast microscopy converts phase shifts in light passing through transparent specimens into differences in brightness, allowing visualization of live, unstained cells with good contrast and detail.
5. Therefore, the best microscope setting for viewing living, moving bacteria with improved contrast without staining is phase-contrast microscopy.