Not dependent on oxygen, and may die or become inactive in its presence. Clostridium group of bacteria (that causes the deadly food poisoning in improperly sterilized cans) is anaerobic. Bacteria in heavily polluted water (with no dissolved oxygen) which degrade sewage from septic systems, live and function only in the absence of oxygen, and so does the bacteria that ferment organic material into beer, rum, whisky, wines, and other alcoholic beverages. Anaerobic organisms are the oldest life-form on earth which appeared about 4.5 billion years ago. Also spelled as anerobic. See also aerobic.