Passage II

Scientists conducted 3 experiments to study the trans­fer of bacteria from one surface to another by 2 species of flies: Musca domestica and Sarcophaga carnaria.

Experiment 1

A group of 10 M. domestica was tested using this procedure:

1. Each fly was placed in a separate enclosure containing Escherichia coli (a type of bacteria) and allowed to walk on the E. coli for 5 min.

2. Each fly was then immediately placed in a separate petri dish containing sterile nutrient agar. Five minutes later, the flies were removed from the dishes.

3. The dishes were incubated at 37°C for 24 hr so that each E. coli cell on the dish divided to form a separate colony, and then the number of E. coli colonies on each dish was counted.

4. The average number of colonies per dish was calculated.

This procedure was also used to test a group of 10 S. carnaria. The results are shown in Figure 1.

Experiment 2

The procedure from Experiment 1 was repeated with each of 3 groups of 10 S. carnaria except that the flies in each group were allowed to walk on the E. coli for a different period of time一5 min, 30 min, or 60 min一before each fly was placed in a separate petri dish containing nutrient agar. The results are shown in Figure 2.

Experiment 3

The procedure from Experiment 1 was repeated with each of 3 groups of 10 S. carnaria except that, after Step 1, the flies in each group were allowed a different period of time一0 min, 30 min, or 60 min一to clean themselves before each fly was placed in a separate petri dish containing sterile nutrient agar. The results are shown in Figure 3.

Figures adapted from Julie J. Shaffer, Kasey Jo Warner, and W. Wyatt Hoback, "Filthy Flies? Experiments to Test Flies as Vectors of Bacterial Disease." ©2007 by National Association of Biology Teachers.