Bacteria reproduce by binary fission, doubling their population each generation.  Unlike the species in graph (a) that reproduces arithmatically, adding one cell at a time for each generation, the species in graph (b) doubles its population, such that numbers can be calculated as 2n, wherein the value of n represents the number of generations.  If Species B had begun with more than one cell, to determine the population after a given period of time, one would use the formula:

Nf = Ni X 2n

Where Nf is the final number of cells, Ni is the initial number of cells and n is the number of generations.

For example,

A fresh, sterile culture medium is inoculated with 10 E. coli bacteria.  The medium is placed in an incubator at 37 oC for 2 hours.  Given that E. coli can divide every 20 minutes under ideal conditions, how many cells will be present in the broth at the end of the incubation period?

Nf = 10 X 26 since there are 6 20-minute periods in 48 hours (3 generations/1 hour)

Nf = 10 X 64 = 640 bacteria