The Effect of Non-Expired and Expired Cephalexin on the Growth of Escherichia coli

The object of this experiment was to study the effects of non-expired and expired Cephalexin on the growth of the Escherichia coli (E. coli) bacteria.  Diluted E. coli was grown in Petri dishes of laurel broth mixed with either expired or non-expired Cephalexin.  Expired Cephalexin was expired two years after the date printed on the label.  These dishes were then put in an incubator for 48 hours so the E. coli could grow and the resulting colonies were counted.  Three trials of 30 plates each (15 expired and 15 non-expired) were run.  The resulting colonies of E. coli were counted and averaged with the other trials of the same treatment to get an average number of E. coli colonies for both the expired and non-expired plates.  The hypothesis that non-expired Cephalexin would produce less E. coli growth, fewer E. coli colonies, was rejected.  A two-sample t-test showed that the average number of E. coli colonies was significantly less in expired Cephalexin than in non-expired Cephalexin.  These results can be applied to the real world because it shows that due to less E. coli growth in the expired plates than the non-expired plates, the Cephalexin is still just as effective, or even more effective, after its expiration date.  If the Cephalexin has not degraded after its expiration date, then due to its continued effectiveness, it can still be used even after the printed date.  Through this research, Hospitals and Pharmacies can keep and use antibiotics after the printed date, extending the expiration dates, instead of disposing of the expired antibiotics.

Research Done By:

Alexa Morrison
Warren Mott High School

Jessica Chapman
Warren Woods Tower High School