Bacteria exist in some of the most inhospitable environments imaginable to humans. Oxygen does not get used by anaerobic bacteria. Aerobic bacteria use oxygen as the final electron acceptor to complete this process. In the case of anaerobic bacteria, oxygen either kills the bacteria, or simply is not useful to the bacteria in the process of energy generation. Anaerobic bacteria use molecules other than oxygen to complete energy production.
Stages of Growth
Anaerobic bacteria populations, like aerobic bacteria, have four stages of growth. Bacteria begin in lag phase, where they get situated in the new environment and begin to acquire nutrients required for growth. During this phase, the bacteria begin to multiply at a given rate and double in population during a given time. In stationary phase, nutrients begin to get depleted by the growing bacteria. They still grow and continue with cell division, but in stationary phase, an equal number die. Nutrient depletion is complete in death phase, and the bacterial population begins to die at an exponential rate.
Conditions in Humans
According to the University of Maryland Medical Center, the presence of anaerobic bacteria in the body result in conditions such as appendicitis, diverticulitis and perforation of the bowel. While much of the human body gets oxygenated by blood, some micro-environments may lack oxygen for a small or extended period of time. Anaerobic bacteria get introduced through food that has not been completely digested or broken down in the stomach.
Since they don't require oxygen, anaerobic bacteria need another way to create energy for cell processes. The electron transport chain, which produces adenosine triphosphate (ATP), requires a final electron acceptor to complete the process. In aerobic bacteria, oxygen accepts the final electrons to drive the process. In anaerobic bacteria, many different compounds are capable of accepting electrons. Nitrate and sulfate are two of the most common electron acceptors for anaerobic bacteria. Anaerobic bacteria may also use fermentation to create energy, where organic molecules like sugars are used by the bacteria.
Presence in Food
Presence of anaerobic bacteria in food depends on the oxygen availability of the food. Foods like honey, jams and jellies where low or no oxygen is present, can harbor anaerobic bacteria. The warning against giving infants honey is a common warning, due to the possible presence of Clostridium botulinum, an anaerobic bacteria that causes botulism.
Presence in the Environment
The oxygenated environment may look like hostile territory to an anaerobic bacteria, but many micro-environments exist which can enable anaerobic bacteria to exist in nature. Anaerobes are found in dirt, lake sediment, brush piles and even deep in the ocean where little or no oxygen is present.
Types of Anaerobic Bacteria
Species of the genus Bacteroides require the absence of oxygen, and are present in the gastrointestinal tract. As an opportunistic pathogen, members of Bacteroides normally don't cause infection, but can if a person is otherwise immunocompromised. Another opportunistic pathogen in humans, members of the genus Actinomyces, causes abscesses in the GI tract.