Question

(2 points) Klebsiella is tested by disk diffusion assay (Kirby-Bauer test). A reference table is shown below to interpret the zones of inhibition.

Antibiotic         Resistant in mm      Intermediate(mm)      Susceptible (mm)

Cefotaxime                11                                 12-13                                      ________

A. What would the minimum zone diameter be in this test that would indicate the organism is susceptible?   ___________

B. If a tech used too high of an inoculum, for example used a solution of 10 million bacteria per ml instead of 100,000 per ml, would you expect an incorrect result of Resistant or Susceptible? __________

Answer 1

A. The minimum zone diameter according to the given table for the organism to be susceptible should be 14mm.

B. If the inoculum used is 100 times more concentrated than the one used to determine the table data, the zone of inhibition will become smaller for the resistant or susceptible result. This is because the antibiotic concentration is the same while the bacterial concentration is increased. Hence the effect of antibiotic will be to a lesser extent. This may result in an incorrect result of resistant or susceptible.