Students review why disinfectants are desirable, then hypothesize concerning the efficacy of various common substances vs. E. coli. Using the ruler the next lab session, after the plates are incubated overnight at 37C, students measure clear or spotted halos of no growth around each filter d isk. Hydrogen peroxide is always most effective, and water has no effect. The rationale for the efficacy of hydrogen peroxide and the lack of effect of mouthwash vs E.coli is discussed by the students. Comparison of the chemicals, water and hydrogen peroxide, introduces the application of rational drug design and pharmaceutical testing in biotechnology.
Students review proper use of the microscope.
Thus, a suspension of cells or other visible particles can be sampled, and the particle density obtained. The hemocytometer is used to count the cells on the microscope. Students learn that subtle regulation of light intensity enables discernment of live and dead c ells. Data are recorded and a table or chart of temperature vs. viability is obtained. Students review the difference between temperatures of 75 F and 75 C, having felt 75 C in the waterbath.