Examples of Gram-negative bacteria species include Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Vibrio cholerae and Neisseria gonorrhoeae. Escherichia coli is a Gram-negative bacterium that causes food-borne sickness and most urinary tract infections. Pseudomonas aeruginosa is responsible for many cases of pneumonia, especially among cystic fibrosis patients. Klebsiella pneumoniae is also responsible for some cases of pneumonia as well as bloodstream and urinary tract infections. Vibrio cholerae is a waterborne Gram-negative bacteria that causes cholera, and Neisseria gonorrhoeae causes gonorrhea.
Gram-negative bacteria are of special concern in health care because many species exhibit resistance to antibiotics. Doctors treat some Gram-negative infections with broad-spectrum antibiotics. However, certain species are resistant to these antibiotics as well, limiting doctors to drugs that have harsh side effects, such as colistin. Gram-negative bacteria can spread readily in health care settings because they are able to enter the body through open wounds, ventilators and catheters.
Scientists distinguish between Gram-negative and Gram-positive bacteria based on the type of stain they produce when performing a Gram stain test. Gram-negative species appear pinkish, while Gram-positive species produce a purple stain. The color difference arises due to differences in cell wall composition. Gram-positive bacteria have a cell wall that contains a high amount of peptidoglycan, which more readily retains the purple dye applied to the bacteria during the test.