Collaborative clinical laboratory study of a broth-disk test for determination of bacterial susceptibility to beta-lactams in combination with amdinocillin.

Methodology for the performance of synergistic antibiotic susceptibility studies has not been standardized. We addressed this problem collaboratively with combinations of amdinocillin and select other beta-lactam antibiotics by using a simple broth-disk test compared with a microdilution approach. Each method used the same drugs singly and in combination. The broth-disk test evaluated each agent and the combinations at concentrations that reflected the breakpoints for each drug; the same ratios of beta-lactam to amdinocillin were used in doubling dilutions with the microdilution method. Initially, each participant studied the same 50 members of the family Enterobacteriaceae; each bacterium was studied on three occasions. Thereafter, 500 representatives of Enterobacteriaceae isolated recently from clinical specimens were studied. Designated strains served as controls. Reproducibility between the two approaches studied in phase 1 of the investigation indicated good agreement between the methods, ranging from 87 to 100%. Agreement between the microdilution and broth-disk tests for the 2,551 clinical isolates ranged from 86 to 95%, with slightly better correlations between combination results than with the single agents. The findings indicate that antibiotic disks used routinely in the clinical laboratory can be used in a simple elution test to determine susceptibility of organisms to beta-lactam antibiotics alone and in combination with amdinocillin.