A Microdilution Plating Method for Population Analysis of Antibiotic-Resistant Staphylococci

The microdilution plating method, using colony-forming units (CFU)/ml determinations from 10-microl 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 microg/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 x 10(4) and 1.36 x 10(4), 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.