How can I determine the generation time of bacteria belong to bacillus species?
Start with a low concentration of bacteria in a proper media (by inoculation) and measure the optical density (OD) over time. Depending on the generation time, you may need to adjust the time between each sampling but 15 minutes usually is good for fast growers.
After obtaining the ODs, plot them (y-axis, log-scale) according to time (x-axis, linear scale) and look for the region showing a linear increase in the OD over time.
This is the exponential growth phase and is used to determine the generation time. Find the time required for the OD to double.
One of the problems of working with optical density (OD) is the different shape and size of bacteria; for instance the relationship of OD and number of bacterial cells have been well establish for Salmonella in the McFarland scale but this relationship only counts for Enterobacteriaceae. Small bacteria such as Brucella will give more cells than Salmonella at the same OD whereas big bacteria such as Bacillus will give fewer cells than Salmonella at the same OD. Despite these facts, the method proposed by Robert Mitchell avoids these differences because just measures the time required for the OD to double, relating the measures to the same bacteria. On the other hand, counting viable bacteria does not consider the OD of dead cells unable to multiply.
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