Gram-negative bacteria are bacteria that have a phospholipid bilayer on the outside of the cell, protecting the peptidoglycan cell wall. This factor can be determined by the simple process of gram-staining, which was developed by Hans Christian Gram. Because Of this outer layer Crystal violet dye will not saturate the peptidoglycan layer when Gram staining and decolorization will wash away any purple coloration, allowing safranin counter stain to show through. Gram negative bacteria respond differently to many Antibiotics. Because the outer membrane is semipermeable the bacterial cells can keep Beta-Lactamases within the zone and can easily degrade antibiotics such as Penicillin.
E. coli are the most heavily studied model organism, and are found almost universally in the intestines of mammals.