What characteristic defines Gram-negative bacteria?

The defining characteristic of Gram-negative bacteria is that they stain pinkish-red during the Gram staining procedure. This staining process differentiates bacterial species based on the structure of their cell walls. Gram-negative bacteria have a thinner peptidoglycan layer in their cell wall and possess an outer membrane that contains lipopolysaccharides. When subjected to Gram staining, they do not retain the crystal violet dye used in the initial step and instead take up the counterstain, which is usually safranin, leading to the pinkish-red coloration observed under the microscope.

This staining property is crucial for microbiologists as it not only helps in identifying the type of bacteria but can also indicate their potential antibiotic resistance, since Gram-negative bacteria often exhibit higher resistance to certain antibiotics due to their outer membrane. Understanding this characteristic is essential for selecting appropriate treatments in clinical settings.