define gram negative
Microscopic image of gram-negative Pseudomonas aeruginosa bacteria (pink-red rods). Gram-negative bacteria are a group of bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation, making positive identification possible.ram-negative bacteria are a group of bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation, making positive identification possible.