A microdilution plating method for population analysis of antibiotic-resistant staphylococci

The microdilution plating method, using colony-forming units (CFU)/ml deter minations from 10-mul droplets, was compared with the standard plate count in population analyses of methicillin-resistant and glycopeptide-intermedia te Staphylococcus aureus (MRSA and GISA) strains. Efficiency of plating plo ts yielded similar population resistance profiles for both methods with MRS A class 1-4 strains, laboratory-selected GISA strains of varying susceptibi lities, two clinical GISA, strains, as well as susceptible strains. A singl e heterogeneous MRSA, plated by both methods in 41 trials with and without 50 mug/ml oxacillin present, demonstrated no significant difference between the results,of the two methods of colony counting (p > 0.05, and r = 0.67) . Standard plating and microdilution plating produced mean resistant subpop ulation determinations of one cell in 1.19 x 10(4) and 1.36 x 10(4), respec tively Population analyses carried out by,microdilution plating require one -fourth or fewer of the plates used for standard plating, and both plating and colony counting required less time to perform.