DISINFECTANT SUSCEPTIBILITY

STUDENT INSTRUCTIONS

• Section three agar-filled Petri dishes into three sections each.
• Choose three disinfectants for testing and label the sections accordingly.
• With a swab dipped in E. coli broth, lawn the surface of one agar plate.
• Repeat using B. sub. on another, and Staph sp. on the last.
• Dip filter paper disks into the disinfectants chosen, let air dry, then place in appropriately labeled section of dish.
• Incubate and observe for regions of no growth after 48 hours.

    NEXT LAB:

• From the bottom of the plate, measure the zone of inhibition from the edge of the disk to the edge of the growth with a millimeter rule, and double. Compare figures between the various disinfectants tested.  Realize that the results are influenced by the disinfectant’s ability to diffuse through the agar.
• Answer these questions:

  Were some disinfectants more effective than others?

   

          If so, which were most effective?

   

          Least?

   

  Did you find differences in susceptibility among the three organisms?         Explain.

   

   

  What factors determine the best choice when choosing a disinfectant?

ANTIBIOTIC SENSITIVITY

STUDENT INSTRUCTIONS

• Make bacterial lawns in three agar plates, using E. coli, Staph sp., and B. sub., respectively.
• From the automatic dispensers, add six different antibiotic disks to each plate (using the same 6 for each plate).  Be sure to make notations about the identity of each antibiotic chosen as well as its concentration given.
• Incubate for 48 hours

    NEXT LAB:

• Observe for regions of no growth.  From the bottom of the plate, measure the zone of inhibition from the edge of the disk to the edge of the growth with a millimeter rule, and double. Compare figures between the various antibiotics and concentrations tested.
• Answer these questions:

         Were some antibiotics more effective than others?
         

                If so, which were most effective?
           

                Least?

                Explain.

        Which antibiotics could be described as narrow spectrum based on your results?

        Which antibiotics could be described as broad spectrum based on your results?

• Compare your plates to the demonstration plates. In hospital or research labs, under very controlled conditions, areas of no growth can be quantified and related to effectiveness at a tested concentration against a tested organism.