Gram-negative bacteria do not retain the crystal violet stain used in the Gram staining method of bacteria identification and absorb the red counterstain, according to MedicineNet. Gram-negative bacteria have cell walls made of peptidoglycan.
Types of Gram-negative bacteria include Escherichia coli, which is a food borne illness, the water borne Vibrio cholerae, which causes cholera, and the plague causing Yersinia pestis, according to the National Institute of Allergy and Infectious Diseases. Gram-negative bacteria commonly spread through person-to-person contact, especially in health care settings where patients can contract infections through wounds, ventilators and catheters. Health care practitioners have difficultly treating infections caused by Gram-negative bacteria because this type of bacteria is resistant to several classes of antibiotics.