Which color indicates gram-negative bacteria on a microscopic slide?

The correct choice of red indicates gram-negative bacteria on a microscopic slide due to the specific staining properties of the Gram stain procedure. In this process, bacteria are classified into two groups based on their cell wall structure.

Gram-negative bacteria have a thinner peptidoglycan layer surrounded by an outer membrane containing lipopolysaccharides. When subjected to the Gram stain, these bacteria do not retain the primary stain (crystal violet) and are washed out when treated with alcohol or acetone. They are then counterstained with safranin, which imparts a red color. This distinguishing characteristic is fundamental in microbiology, as it not only helps in identifying and classifying bacteria but also has implications for treatment, as gram-negative bacteria often display resistance to certain antibiotics.

In contrast, gram-positive bacteria, which retain the crystal violet dye and appear blue or purple, have a thicker peptidoglycan layer that traps the dye after the decolorization step. Understanding this fundamental difference is essential for proper identification and treatment of bacterial infections in veterinary medicine.