Diagnostic Laboratory Section

Selection of antibiotic disc

SELECTION OF ANTIBIOTIC DISCS

• In order to make the routine susceptibility test relevant, practical and meaningful, the antimicrobial agents used should be limited. Though a battery of antimicrobial discs is available, a number of factors are considered selection of the disc.
• Microbiologist best make the decision in close consultation with practitioners and pharmaceuticals industries. The choice of antimicrobial discs may be better if  only one representative of each group of related drugs with activity against a nearly identical spectrum of organisms is included and for which interpretive result would be nearly always the same.
  • A Tetracycline disc will predict the result of against all other tetracycline like Oxytetracyclines and Chlortetracyclines.
  • Sulphasoxazole is a suitable representative for all the Sulphonamide group of drugs
  • Erythromycin will predict the result against result of all other macrolides
  • Clindamycin disc will predict the result for Lincomycin.
  • Aminoglycosides and Quinalones should be tested separately
  • Cholramphenicol, Vancomycin, Nitrfurantoin, Trimethoprim, Sulphamethaxazole are tested separately
  • Streptococci should be tested against either Penicillin G or Ampicillin against both not necessary.
  • Staphylococci should be tested against Penicillin G and is usually susceptible to Cephalosporins.
• The disc should be stored at 4⁰C . Longer storage should be at -14⁰ c or lower. Disc crossed the expiry date should be discarded.
• Potency of new batch of discs should be tested against control bacteria.