As mentioned above, bacterial growth rates during the phase of exponential growth, under standard nutritional conditions (culture medium, temperature, pH, etc.), define the bacterium's generation time. Generation times for bacteria vary from about 12 minutes to 24 hours or more. The generation time for E. coli in the laboratory is 15-20 minutes, but in the intestinal tract, the coliform's generation time is estimated to be 12-24 hours. For most known bacteria that can be cultured, generation times range from about 15 minutes to 1 hour. Symbionts such as Rhizobium tend to have longer generation times. Many lithotrophs, such as the nitrifying bacteria, also have long generation times. Some bacteria that are pathogens, such as Mycobacterium tuberculosis and Treponema pallidum, have especially long generation times, and this is thought to be an advantage in their virulence. Generation times for a few bacteria are are shown in Table 2.
Table 2. Generation times for some common bacteria under optimal conditions of growth.
|Bacterium |Medium |Generation Time (minutes) |
|Escherichia coli |Glucose-salts |17 |
|Bacillus megaterium |Sucrose-salts |25 |
|Streptococcus lactis |Milk |26 |
|Streptococcus lactis |Lactose broth |48 |
|Staphylococcus aureus |Heart infusion broth |27-30 |
|Lactobacillus acidophilus |Milk |66-87 |