title: A microdilution plating method for population analysis of antibiotic-resistant staphylococci.
abstract: 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.
authors.affiliation: Microbiology Group, Department of Biological Sciences, Illinois State University, Normal 61790-4120, USA.
authors.forename: R F
authors.initials: RF
authors.lastname: Pfeltz
authors.forename: J L
authors.initials: JL
authors.lastname: Schmidt
authors.forename: B J
authors.initials: BJ
authors.lastname: Wilkinson
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology; D015169:Colony Count, Microbial; D003470:Culture Media; D004351:Drug Resistance; D008826:Microbial Sensitivity Tests / Q000379:methods*; D010068:Oxacillin / Q000494:pharmacology; D010406:Penicillins / Q000494:pharmacology; D011153:Population; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000254:growth & development; D014640:Vancomycin / Q000494:pharmacology
chemical_list: D000900:Anti-Bacterial Agents; D003470:Culture Media; D010406:Penicillins; D014640:Vancomycin; D010068:Oxacillin