'Break-point Checkerboard Plate' for screening of appropriate antibiotic combinations against multidrug-resistant Pseudomonas aeruginosa.

Increase of multiple drug resistant Pseudomonas aeruginosa (MDRP) is becoming a serious problem in the clinical setting. Although the checkerboard method to determine FIC index and synergistic effects of antibiotic combinations is useful, it is not well adapted to a routine test, mainly because of its time-consuming and labor-intensive nature. Here we report 'Break-point Checkerboard Plate', in which breakpoint concentrations, such as 'S' (sensitive) and 'I' (intermediate), were combined in a microtiter plate with 8 antibiotics, including carbapenem, aminoglycoside and fluoroquinolone. The results obtained from 12 strains of MDRP demonstrated a strong synergistic effect of some antibiotic combinations at clinically relevant concentrations. Our data suggest a usefulness of 'Break-point Checkerboard Plate' to screen appropriate antibiotic combinations against drug resistant organisms, including MDRP.