Gram-negative bacilli are rod-shaped bacteria that have a thin layer of peptidoglycan between two membranes. They are medically significant as many strains can cause infection in humans, including the family Enterobacteriaceae, which includes Escherichia coli, Salmonella spp. and Shigella spp., states infectionNet.
During a Gram stain, crystal violet stain is applied to the bacteria sample to determine if it is positive or negative. A decolourizing agent, usually alcohol or acetone, is applied, and this tightens the peptidoglycan layer. In a Gram-negative bacterium, the outer membrane is degraded by this process, and the thin peptidoglycan layer cannot retain the crystal violet dye. After the counterstain safranin is applied, Gram-negative cells appear red, according to the Science Education Resource Center of Carleton College.
Many types of Gram-negative bacilli are harmful to humans. Enterobacteriaceae is a family of Gram-negative bacilli, containing over 14 genera of bacteria that have been found to cause infection in humans. The most significant of these is Escherichia coli, which causes most urinary tract infections and is also a factor in polymicrobial disruption of the gastrointestinal tract. Other significant genera are Salmonella, which causes gastroenteritis, Salmonella typhi, which causes enteric fever, and Shigella, a common cause of dysentery, notes infectionNet.