Variation in the abilities of automated, commercial, and reference methods to detect methicillin-resistant (heteroresistant) Staphylococcus aureus

Abstract

The abilities of commercial MIC, automated, and reference methods for in vitro detection of methicillinresistant Staphylococcus aureus were determined on 49 strains from eight hospitals. Micro-Media, MicroScan, Sensititre, Sceptor, API Uniscept KB, Abbott MS-2, Vitek AMS, Autobac MTS, NCCLS disk diffusion, and broth microdilution antimicrobial susceptibility testing procedures were evaluated. All testing was performed by using manufacturers' or reference procedures, and results were determined at no later than 24 h of incubation at 35°C. With NCCLS disk diffusion, all strains were resistant to oxacillin (1 ,ug), 47 (96%) were resistant to methicillin (4 jig), and 48 (98%) were resistant to nafcillin (1 ,ug). The percentages of strains resistant to methicillin (>8 ,ug/mI) were 98% with API Uniscept KB, 86% with Sceptor, MicroScan, and Autobac MTS, 84% with Sensititre, 71% with Micro-Media, and 70% with NCCLS MIC. Abbott MS-2 detected 86% of strains resistant to methicillin (>5,ug/ml). With oxacillin (>2 ,ug/mI), 90% were detected with Vitek AMS and 70% were detected with NCCLS MIC. With nafcillin (>2 ,Ig), 82% were resistant with Micro-Media, 57% were resistant by NCCLS MIC, and 50% (three of six) were resistant by MicroScan. Two strains from one hospital and one strain from another gave susceptible results with all automated and commercial methods. All strains from three centers were detected by all methods. Variability also occurred among the systems with cephalothin, clindamycin, gentamicin, chloramphenicol, and trimethoprim sulfamethoxazole.

Evaluation of laboratory tests for detection of methicillin-resistant Staphylococcus aureus and Staphylococcus epidermidis

“…In our study we evaluated automated susceptibility testing systems in two ways that other studies have not: (i) all staphylococci were tested with reference procedures that use salt in the media and (ii) a large number of unique S. epidermidis isolates were examined (2,5,6,12). The AMS and the MicroScan system detected roughly 75% of the methicillin-resistant S. aureus, and the AMS detected onethird of the methicillin-resistant S. epidermidis isolates.…”

“…However, these procedures are labor intensive, and in a recent College of American Pathologists survey, less than 3% of 1,700 participating laboratories in the United States used in-house agar dilution or broth microdilution methods to detect a methicillin-resistant S. aureus strain (D. Edson, petsonal communication). Alternative screening and automated methods have been evaluated (2,3,5,11), but most of these studies focused only on testing that would detect methicillin-resistant S. aureus. It is at least as important to identify methicillin-resistant S. epidermidis since most S. epidermidis isolates which cause serious infections bear this phenotype.…”

Misclassification of Susceptible Strains of Staphylococcus aureus as Methicillin-Resistant S. aureus by a Rapid Automated Susceptibility Testing System

“…The heterogeneous resistance to methicillin and oxacillin manifested by many strains of MRSA made detection of these organisms problematic for early versions of rapid automated susceptibility testing systems (1,2,6,10,20). Early versions of the Vitek system had difficulty detecting MRSA (4,6,13,20), but modifications led to improved detection (7,8). However, some modifications of GPS-M cards led to false reports of oxacillin resistance among S. aureus strains (7).…”

Methicillin-resistant staphylococci.

“…The automated systems (e.g., oxacillin per ml, 104-10' CFU these systems the sensitivity for detection of methicillin resistance has not been acceptable (3,21,28,40,78,173). Sensitivity in detection of resistance can be strain dependent, which probably is due to differences in sizes of the resistant subpopulation within strains (99).…”