What characterizes gram-negative bacteria?

Gram-negative bacteria are characterized by their cell wall structure, which includes a thin layer of peptidoglycan surrounded by an outer membrane. This unique structure plays a critical role in their response to gram staining, a technique used to classify bacteria based on their cell wall composition.

In gram staining, the primary stain used is crystal violet, which penetrates all bacteria initially. After applying iodine, which helps fix the dye in the cells, a decolorizing agent, typically alcohol or acetone, is applied. Gram-negative bacteria have a thinner peptidoglycan layer that cannot retain the crystal violet after decolorization. As a result, when the counterstain (such as safranin) is applied, these bacteria take up the red/pink stain, leading to their characteristic appearance during microscopy. This distinctive color change is fundamental in differentiating gram-negative bacteria from gram-positive ones.

The other options do not accurately reflect gram-negative characteristics. The blue/green staining pertains to gram-positive bacteria due to the thicker peptidoglycan layer retaining the crystal violet. A thick peptidoglycan layer is a hallmark of gram-positive bacteria, not gram-negative. While some gram-negative bacteria may exhibit antibiotic resistance, it is not a defining