Lecture notes Microbial Diagnostics

1. Review antimicrobial resistance results.

Disk diffusion tests: Antibiotics diffuse from the discs into the agar, producing a zone of inhibition in which sensitive
organisms fail to grow. Use a ruler to measure the diameter of each zone of inhibition, recording your data below.
Use the following table to determine whether or not each organism should be classified as sensitive, intermediate or resistant
to each antibiotic. You should check that zones of inhibition for your control organism fall within the acceptable range
indicated.

Cefoxitin Erythromyci Fusidic Gentamici Rifampicin Ciprofloxacin
n acid n
Control 24-30 23-29 26-32 19-25 30-36 21-27
A/C B A/C B A/C B
Test mm S =>2 >25 =>21 =>24 =>1 =>2 =>26 =>50 =>50
2 8 2
Test mm I 18-20 23-25
Test mm R <22 <25 <18 <24 <18 <22 <23 <21 <24
*All diameters are in millimetres

Susceptible or Resistant?
Cefoxitin Erythromyci Fusidic acid Gentamici Rifampicin Ciprofloxacin
n n
Control 30 mm 30mm 30mm 25 mm 30 mm 27 mm
Diameter
Test A 27 mm 25mm 35 mm 25 mm 37 mm 27 mm
Diameter
Test A S S S S S R
Susc. or
Res?
Test B 30 mm * 30 mm
Diameter
Test B S S R S
Susc. or
Res?

, Test C * * 35 mm * 30 mm
Diameter
Test C R R S R R
Susc. or
Res?