Characterization of bacteria according to their appearance after Gram staining, which reflects differences in the structure of their cell walls.

The cell envelopes of most bacteria fall into one of two major groups. In addition to a cytoplasmic/inner membrane, Gram-negative bacteria have a thin peptidoglycan cell wall and an outer membrane containing lipopolysaccharide. Between the two membranes is the periplasm where enzymes such as beta-lactamases are found. Gram-positive bacteria lack an outer membrane but have a much thicker peptidoglycan layer than Gram-negative organisms. There is no periplasm and enzymes e.g., beta-lactamases are excreted to the outside of the cell.

This distinction has important implications for antibiotic development. Many antibiotics target proteins involved in cell wall biosynthesis so have to traverse the outer membrane and periplasm in Gram-negative bacteria. Hence, the complex cell envelope of Gram-negative bacteria presents a significant barrier to the entry of antibiotics into the cell.

Gram-negative bacteria include many multidrug-resistant species, including all three of the highest priorities in the WHO priority pathogen list for R&D of new antibiotics.