The Gram Stain Preparation
The Kingdom Prokaryotae (formerly Kingdom Monera) includes the Archaebacteria (ancient forms), as well as the Eubacteria (modern bacteria). When bacteria were first discovered, the available technology revealed that all known forms had cell walls composed of a carbohydrate substance called peptidoglycan. Over time, new technology provided the means to discover that some very small Eubacteria lack cell walls, and also that some of the Archaebacteria have cell walls made of substances other than peptidoglycan.
Christian Gram’s bacterial cell wall staining process has been used since the 1800s for placing all bacteria that have cell walls made of peptidoglycan into two classification groups within their kingdom. Placement is based upon the cell's response to Gram's staining procedure. The two groups are Gram positive (Gram+ = blue/purple color), and Gram negative (Gram- = red/pink color).
Bacteria that are Gram+ have a cell wall composed of layers of peptidoglycan placed above their cell membrane. The layers of peptidoglycan are secured by amino acids. The cell walls of Gram- bacteria consist of a membrane layer, a layer of peptidoglycan, and another membrane layer.
Because of the differences in their cell wall structure, the Gram+ and Gram- bacteria will respond differently not only to Gram’s staining process, but also to various antibiotics. Hence, the Gram stain is often used in diagnosis. The exact type of bacteria causing an infection should be identified before physicians prescribe antibiotics. Gram+ and Gram- organisms will not respond to an antibiotic that cannot interact with their surface structure. Refer to the lab manual for the Gram stain procedure.