To do a statistical analysis, there are a couple of different methods.
I'd first suggest graphing the numbers of bacteria over time on Excel or a similar program. That should give you an idea of how the numbers changed over time.
Then, I'd suggest doing a regression; this is essentially a best-fit line or curve. In Excel, you can add a trendline; searching "add a trendline" should tell you how to do this if you haven't before. These can be linear, exponential, or polynomial, and each regression will also give you a correlation (R squared). The higher the correlation, the better the fit. If you try most of the options for a trendline and choose the one with the greatest correlation, you should be able to find the best fit. If it's linear, the slope will give you an idea of how fast the bacteria grow, and if it's exponential, the numbers in the formula will do the same.
You can also use the average number of bacteria over the seven-day experiment as a measure of how much the bacteria grew.
Nature uses only the longest threads to weave her patterns, so that each small piece of her fabric reveals the organization of the entire tapestry.