Abstract
The microdilution plating method, using colony-forming units (CFU)/ml determinations from 10-μl droplets, was compared with the standard plate count in population analyses of methicillin-resistant and glycopeptide-intermediate Staphylococcus aureus (MRSA and GISA) strains. Efficiency of plating plots yielded similar population resistance profiles for both methods with MRSA class 1-4 strains, laboratory-selected GISA strains of varying susceptibilities, two clinical GISA strains, as well as susceptible strains. A single heterogeneous MRSA, plated by both methods in 41 trials with and without 50 μg/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 subpopulation determinations of one cell in 1.19 × 104 and 1.36 × 104, respectively. 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.
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