The growth of microbial populations can be measured in many ways Some methods measure cell numbers and other methods track the population s total mass which is often directly proportional to cell numbers But the most frequently used method for measuring bacterial population is the plate count One of the important advantages of this method is that it determines the number of viable cells This is in contrast to for example estimating cell numbers by measuring turbidity which does not distinguish between viable and dead cells Bacterial growth normally refers to increase in bacterial numbers rather than an increase in size Most bacteria divide by binary fission so that under ideal circumstances the bacterial population doubles every generation given that every bacterium divides into two bacteria Doubling varies greatly among bacteria but can occur as quickly as every 20 minutes and after 20 generations a single cell can grow to over 1 million in number It is difficult to graph population changes of such enormous magnitude by using arithmetic numbers A logarithmic scale is preferable in visualizing bacterial growth Generation time G is the time it takes for the population to double in number The equation t is the time and n is the number of generations The average generation time is between 30 and 60 minutes under optimal conditions Here is an example about calculating generation time What is the generation time of a bacterial population that increases from 10 000 to 10 000 000 cells in four hours of growth The formula to use would be G t 3 3log a A a number of bacteria at the beginning of a time interval A number of bacteria at the end of the time interval G 1 3 3log a A G 240 minutes 3 3log 107 104 G 24 minutes If the generation time for two different populations of bacteria is identical it is indicative of similar growth rates Control 500 ml of cottage cheese was inoculated with a 2 ml culture of Pseudomonas aeruginosa and incubated at 25 C Five hours after inoculation in a standard plate count there were 200 bacterial cells ml After 29 hours at 25 C there were 1 000 000 cells ml Experiment 500 ml of cottage cheese containing the preservative was inoculated with a 2 ml culture of Pseudomonas aeruginosa After 6 hours of incubation at 25 C a standard plate count was performed There were 700 bacterial cells ml After 38 hours there were 61 000 000 bacterial cells ml Number 1 2 ch 6 24 32 200 700 1 00 x 10 6 10 x 10 6 10 x 10 Log 0 00 0 30 0 70 0 78 1 38 1 51 2 30 2 85 6 00 6 79 7 79 1 Why were plate counts used for analyzing bacterial growth instead of direct microscopic counts or turbidity measurements here and the experiment cottage cheese differ What was the independent variable and dependent variable