The Australian microbiologists Barry J. Marshall and J. Robin Warren became Nobel laureates for their work demonstrating that infection by the bacterium Helicobacter pylori is closely related to several gastric pathologies, specifically gastritis, peptic ulcer, and gastric carcinoma. H. pylori are a gram-negative microaerophilic bacterium that induces a chronic infectious state as a precursor of chronic gastritis, peptic ulcer, gastric and duodenal cancer, and mucosa-associated lymphoid tissue lymphoma. In addition to H. pylori, several other species of gram-negative bacteria are associated with cancer. Gram-negative bacteria are surrounded by a double membrane: a plasma membrane that is very similar to the plasma membranes of all cell systems and an additional outer membrane. Both gram-negative and gram-positive bacteria have been linked, directly or indirectly, to cancer. Among aerobic gram-negative bacteria, only one species, Neisseria gonorrhoeae, has thus far been epidemiologically related to a specific cancer, namely prostate cancer, demonstrated in a population of Mexican males.