Gram staining
![A Gram stain of mixed Staphylococcus aureus (S. aureus ATCC 25923, gram-positive cocci, in purple) and Escherichia coli (E. coli ATCC 11775, gram-negative bacilli, in red), the most common Gram stain reference bacteria](/Images/godic/202501/16/Gram_stain_015947.jpg")
Gram staining, also called Gram's method, is a method of staining used to differentiate bacterial species into two large groups (gram-positive and gram-negative). The name comes from the Danish bacteriologist Hans Christian Gram, who developed the technique.