What does the term "Gram negative" refer to in bacteria?

The term "Gram negative" refers specifically to bacteria that do not absorb the Gram stain during the laboratory procedure developed by Hans Christian Gram. When subjected to the Gram staining process, these bacteria retain a counterstain (often safranin), which gives them a pink or red appearance under a microscope, distinguishing them from Gram-positive bacteria that absorb the primary stain (crystal violet) and appear purple.

The structural characteristics of Gram-negative bacteria include a thinner peptidoglycan layer and an outer membrane that contains lipopolysaccharides. This outer membrane not only influences staining properties but also contributes to the bacteria's pathogenicity and their defenses against certain antibiotics, making them a significant concern in medical microbiology. Understanding this distinction is crucial for treatment decisions, as it often impacts susceptibility to antibiotics and the approach in microbial remediation efforts.