In this issue of Math in the News, we look at the listeria bacteria to see why it is so dangerous. For more media resources, go to http://www.media4math.com.

1. 11/6/11

2. Bacterial Growth
Bacterial growth is a function of time. Under the right conditions, an initial
population of bacteria will grow exponentially.

3. Bacterial Growth
Bacterial growth is also a function of temperature. Within the range of 40°F and
140·F, bacteria will thrive.

4. Bacterial Growth
As a result, we can define a piecewise function of two variables that account for
different temperatures and can be graphed for different values of t.

5. Bacterial Growth
Here is an example of both variables at work. Two tomatoes, one refrigerated and
one at room temperature, are observed over several days. The tomato at room
temperature rots, while the other tomato is essentially the same.

6. Bacterial Growth
This graph is a model for a scenario in which a food item is refrigerated up to time
t0, after which it is brought to room temperature. The food-borne bacteria grow
exponentially after t0.

7. Bacterial Growth
With listeria the situation is different. This type of bacteria will continue to grow even
when refrigerated.

8. Bacterial Growth
This results in a different piecewise function.

9. Bacterial Growth
Let’s look at two situations. In each case a canteloupe is refrigerated with the
understanding that this will keep bacteria from growing. But in one case, the
canteloupe is infected with listeria.

10. Bacterial Growth
The flat-line graph represents the canteloupe without listeria, while the other graph
shows the dramatic growth for the other canteloupe.

11. Listeria
• The danger of listeria is
that it does its work
(growing exponentially)
under conditions that
we think are having the
opposite effect.

12. Bacterial Growth
For more information on listeria and other harmful bacteria, go to FoodSafety.gov.