Gram-negative rods, or bacilli, are rod-shaped bacteria that give a negative result with Gram staining. These bacteria have a thin peptidoglycan layer sandwiched between the inner and outer cell membrane. This layer is removed when the bacterial cells are washed with an alcohol solution so they do not retain the crystal violet stain first applied. Instead, the cells turn red or pink with the safranin or fuchsine counterstain.
In contrast, gram-positive bacteria have a thick peptidoglycan layer and retain the crystal violet stain, even after washing the cells with an alcohol solution. Some examples of gram-negative rods are the Salmonella, Escherichia, Pseudomonas, Vibrio, Campylobacter and Shigella species. These type of bacteria are known to cause pneumonia, infections in the urinary tract, gastrointestinal tract, bloodstream and other parts of the body. Escherichia coli are commonly found in the gastrointestinal tract of people and cause urinary tract and gastrointestinal tract infections. Salmonellosis is the diarrhea caused by Salmonella bacteria. The common sources of this bacteria are eggs, dairy and poultry products. A particular species, Salmonella typhi, however, does not cause diarrhea but rather enteric fever. Another gram-negative bacilli that are causing a growing concern regarding the incidence of pneumonia are Klebsiella pneumonia. Because of the resistance of these bacteria to nearly all modern antibiotics, they are more difficult to control and are spreading worldwide.