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(8) Isolated colonies develop after incubation neither practical, nor particularly relevant, to determine the relative size of the cells in a given population, microbial growth is defined as an increase in the number of cells in a population.

The time it takes for a population to double in number is the generation or doubling time. This varies greatly depending on the species of the organism and the conditions in which it is grown. Some common organisms, such as Escherichia coli, can double in approximately 20 minutes; others, such as the causative agent of tuberculosis, Mycobacterium tuberculosis, require at least 12 to 24 hours to double even under the most favorable conditions. The environmental and nutritional factors that affect the rate of growth will be discussed shortly.

The exponential multiplication of bacteria has important health consequences. For example, a mere 10 cells of a food-borne pathogen in a potato salad, sitting for 4 hours in the warm sun at a picnic, may multiply to more than 40,000 cells. A simple equation expresses the relationship between the number of cells in a population at a given time (Nt), the original number of cells in the population (N0), and the number of divisions those cells have undergone during that time (n). If any two values are known, the third can be easily calculated from the equation:

In this example, let us assume that we know that 10 cells of a disease-causing organism were initially added to the potato salad and we also know that the organism has a generation time of 20 minutes. The first step is to determine the number of cell divisions that will occur in a given time. Because the organism divides every 20 minutes, 3 times every hour, we know that in 4 hours it will divide 12 times. Now that we know the original number of cells and the number of divisions, we can solve for Nt:

Thus, after 4 hours the potato salad in the example will have 40,960 cells of our pathogen. Keep this in mind, and your potato salad in a cooler, the next time you go to a picnic!

Chapter 4 Dynamics of Prokaryotic Growth

DNA attached to cytoplasmic membrane

Cell enlarges and DNA duplicates

Cross wall forms

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