Because bacteria can grow and thrive in a variety of environments, optimal growth temperatures may vary significantly between species. In general, most pathogenic or commensal bacterial strains grow well at body temperature (37°C). In contrast, many environmental strains thrive at lower temperatures, often
within a range of 25°C to 30°C.
Bacterial species can be categorized based on their growth temperature; these include psychrophiles (0°C to 20°C), mesophiles (25°C to 40°C), and thermophiles (45°C to 122°C). Though bacterial strains require optimal temperatures for growth and reproduction, most strains can withstand considerable drops in temperature and survive several days at 4°C. At these lower temperatures, bacterial growth and metabolism are significantly diminished. (See: NOTE 2)
Regularly calibrate the temperature control system of incubators. Use an alarm system when possible to warn against temperature increases above the optimum setting.
In addition to varying requirements for optimal growth temperatures, bacteria also differ in their use of oxygen for respiration. Aerobic organisms, such as Bacillus species, use oxygen as a terminal electron acceptor during respiration. Similarly, microaerophiles, such as Helicobacter pylori, also require the use of oxygen, but at lower levels than naturally occurring in the environment. In contrast, anaerobic organisms use electron acceptors such as nitrate or sulfate, among other inorganic acceptors. These inorganic compounds, however, have a lower reduction potential than oxygen thus resulting in less efficient respiration.
The use of oxygen and inorganic compounds by anaerobic organisms can differ greatly between species. Obligate anaerobes, such as Clostridium species, can only survive and reproduce in the absence of oxygen; these organisms are often killed by the presence of oxygen. Similarly, aerotolerant anaerobes, such as Lactobacillus species, cannot use oxygen during respiration; however, unlike strict anaerobes, these microorganisms can tolerate oxygen for short periods of time. Lastly, facultative anaerobes, such as Escherichia coli and Staphylococcus species, are able to survive in both the presence and absence of oxygen. If given the choice, these organisms prefer the use of oxygen during respiration as it has the greatest reduction potential as compared to other electron acceptors.
When working with anaerobic cultures, it is important to avoid unnecessary exposure to oxygen. Anaerobic conditions may be obtained for either transfer or incubation by the methods listed below. Unless specifically mentioned, the standard anaerobic gas mixture is 80% N2, 10% CO2, and 10% H2.
Anaerobic conditions for transfer may be obtained by either of the following:
- Use of an anaerobic gas chamber.
- Placement of test tubes under a gassing cannula system hooked to anaerobic gas.
During incubation, anaerobic conditions may be maintained by any of the following:
- Loose screw-caps on test tubes in an anaerobic chamber.
- Loose screw-caps on test tubes in an activated anaerobic GasPak™ system.
- Use of sterile butyl rubber stoppers on test tubes so that an anaerobic gas headspace is retained.