Scientist Hans Christian Gram (1853-1938) invented the dyeing method. In this method bacteria are divided into two groups.
1. Gram positive bacteria: Those bacteria that retain the dye (blue, purple) when washed with crystal violet dye are called gram positive bacteria. Their cell walls are thick. Its cell wall consists of peptidoglycan, teichoic acid and thin lipopolysaccharide. Such as Clostridium, Staphylococcus, Streptococcus, Bacillus, Actinobacteria.
2. Gram negative bacteria: Those bacteria which cannot retain the dye after washing with crystal violet dye are called gram negative bacteria. Their cell walls are thin. Its cell wall consists of peptidoglycan and thick lipopolysaccharide. E. coli, Salmonella typhi, Shigella, Proteus, Rhizobium, Vibrio cholerae, Neisseria meningitidis, Cyanobacteria.