Learn about the difference between Gram positive and Gram negative bacteria , including why you need to be able to tell them apart. Gram-positive bacterium , any of various types of bacteria that are characterized by having a thick peptidoglycan cell wall and by the absence of an outer membrane composed of lipopolysaccharide. The structural features that define Gram-positive and Gram -negative bacteria are also the very signals that alert the immune system to their presence. Teichoic acids in Gram-positive bacteria and LPS in Gram -negative bacteria are recognized by toll-like receptors—molecular sentinels that trigger inflammation and immune activation. Principle of Gram Staining When the bacteria is stained with primary stain Crystal Violet and fixed by the mordant, some of the bacteria are able to retain the primary stain and some are decolorized by alcohol. The cell walls of gram positive bacteria have a thick layer of protein-sugar complexes called peptidoglycan and lipid content is low.
