Bacteria are single-celled microorganisms. They are found on Earth in nearly all locations. Some species have a symbiotic relationship with other forms of terrestrial life, including humans. Martian colonists will inevitably carry bacteria with them in the gut. Other species of bacteria are pathogenic, causing disease. Certain foods require bacteria to be manufactured; such as cheese, sauerkraut and yogurt. Such uses originated as food preservation techniques. Bacteria are a common subject of bioengineering research. Bacteria may be engineered to produce chemicals on an industrial scale.