Gram+ Gram￾Schematic diagram of bacterial cell walls Bacteria can be divided into two major groups, called gram-positive and gram-negative. The original distinction between gram-positive and gram-negative was based on a special staining procedure, the Gram stainGram+ Gram￾Schematic diagram of bacterial cell walls Bacteria can be divided into two major groups, called gram-positive and gram-negative. The original distinction between gram-positive and gram-negative was based on a special staining procedure, the Gram stain