Abstract
The microdilution plating method, using colony-forming units (CFU)/ml determinations from 10-
μ
l 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
μ
g/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 × 10
4
and 1.36 × 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.