Always remember: Antibiotic Susceptibility testing should be performed on clinically significant isolates, not on all microorganisms recovered in culture.
World Health Organization (WHO) recommends routine use of modified Kirby-Bauer Method for routine antibiotic susceptibility testing. According to WHO to avoid errors following guidelines should be used:
- Antibiotic discs should be of correct diameter (6.35 mm).
- Antibiotic discs should be of correct potency
- The stock supply should be stored frozen (-20° C).
- The working supply should be kept no longer than 1 month in a refrigerator (2–8° C).
- Only Mueller–Hinton agar of performance-tested quality should be used.
- Correct pH (7.2–7.4) of the ﬁnished medium is essential for some antibiotics.
- The inoculum should be standardized against the prescribed turbidity standard
- Zone sizes should be measured exactly.
- Zone sizes should be interpreted by referring to a table of critical diameters. Zone diameters for each organism should fall within the limits
The three standard control strains are:
- Staphylococcus aureus (ATCC 25923; NCTC 6571);
- Escherichia coli (ATCC 25922; NCTC 10418);
- Pseudomonas aeruginosa (ATCC 27853; NCTC 10622).
• Tests should be carried out with the three standard strains:
- when a new batch of discs is put into use;
- when a new batch of medium is put into use;
- Once a week, in parallel with the routine antibiograms.
• Use the quality control chart for recording and evaluating performance tests.