Epsilonproteobacteria is the smallest class of Proteobacteria. These bacteria are gram-negative and can survive with a very small amount of oxygen in the environment, hence they are called microaerophilic. Campylobacter and Helicobacter are the two clinically relevant pathogenic genera in this class. Campylobacter can cause food poisoning that manifest as small intestine inflammation. This illness is caused by the C. jejuni species which is common in developed countries. C. jejuni bacteria are often found in contaminated chicken meat.
Helicobacter pylori has been recognized as a helpful member of the gastric microbiota, but it is also the most common cause chronic gastritis and gastric ulcers. Research studies have shown that H. pylori may cause stomach cancer. H. pylori can survive the acidic environment of the stomach because it secrete urease and other enzymes to make the environment less acidic.
OpenStax Microbiology. Epsilonproteobacteria. Accessed November 25, 2019