ANTIMICROBIAL SUSCEPTIBILITY TESTING IN SWEDEN - III - METHODOLOGY FOR SUSCEPTIBILITY TESTING

A subcommittee of the Swedish Reference Group for Antibiotics, SRGA-M, has worked with standardization of methodology for susceptibility tes ting. In vitro data obtained with the disk diffusion procedure were co llected from 5 clinical laboratories, compiled and presented as histog rams of inhibition zones, and compared with data [minimum inhibitory c oncentrations (MICs) and inhibition zones] obtained from the reference laboratory at the Swedish Institute for Infectious Disease Control on a collection of clinically relevant bacterial species. Results from t he reference collection of strains were presented as MIC histograms, a nd their corresponding inhibition zones were inserted in the compiled zone histograms as identifiable bars. These distributions formed the b asis for decisions of breakpoints. Special tests were recommended for the detection of certain resistance mechanisms, A beta-lactamase test should be used for Haemophilus influenzae, Moraxella catarrhalis, Neis seria gonorrhoeae and enterococci. Screening for beta-lactam resistanc e caused by altered penicillin binding proteins should be done by usin g oxacillin 1 mu g for Streptococcus pneumoniae and Staphylococcus aur eus (MRSA), and by phenoxymethylpenicillin 10 mu g for H. influenzae. The standardized disk diffusion procedure was helpful in detecting ent erobacteria carrying beta-lactamases with extended spectre. Registrati on of inhibition zones will provide a powerful tool for the epidemiolo gical surveillance of antibiotic resistance.