Gram-negative bacteria are a group of bacteria that do not retain the crystal violet dye used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall sandwiched between an inner cytoplasmic cell membrane and a bacterial outer membrane. Gram-negative bacteria are found everywhere in nature and include many pathogenic species, such as Escherichia coli, Salmonella, Vibrio cholerae, and Pseudomonas aeruginosa.