Performance of Eight Methods, Including Two New Rapid Methods, for Detection of Oxacillin Resistance in a Challenge Set of Staphylococcus aureu s Organisms

Abstract

Using a set of 55 Staphylococcus aureus challenge organisms, we evaluated six routine methods (broth microdilution, disk diffusion, oxacillin agar screen, MicroScan conventional panels, MicroScan rapid panels, and Vitek cards) currently used in many clinical laboratories and two new rapid methods, Velogene and the MRSAScreen, that require less than a day to determine the susceptibility of S. aureus to oxacillin. The methods were evaluated by using the presence of the mecA gene, as detected by PCR, as the "gold standard." The strains included 19 mecA-positive heterogeneously resistant strains of expression class 1 or 2 (demonstrating oxacillin MICs of 4 to >16 g/ml) and 36 mecA-negative strains. The oxacillin MICs of the latter strains were 0.25 to 4 g/ml when tested by broth microdilution with 2% NaCl-supplemented cation-adjusted Mueller-Hinton broth as specified by the NCCLS. However, when tested by agar dilution with 4% salt (the conditions used in the oxacillin agar screen method), the oxacillin MICs of 16 of the mecA-negative strains increased to 4 to 8 g/ml. On initial testing, the percentages of correct results (% sensitivity/% specificity) were as follows: broth microdilution, 100/100; Velogene, 100/100; Vitek, 95/97; oxacillin agar screen, 90/92; disk diffusion, 100/89; MicroScan rapid panels, 90/86; MRSA-Screen, 90/100; and MicroScan conventional, 74/97. The MRSA-Screen sensitivity improved to 100% if agglutination reactions were read at 15 min. Repeat testing improved the performance of some but not all of the systems.The oxacillin agar screen test has been used for many years to aid in the identification of oxacillin-resistant staphylococci. Recently, this test was shown to be ineffective for coagulasenegative staphylococci (20), and the NCCLS no longer recommends its use for these organisms. In a previous study, the inoculation methods for the oxacillin agar screen test were more clearly defined (19a), and recommendations were made to use a 1-l loop or an expressed swab inoculated into an area of 10 to 15 mm in diameter. In that study, a challenge group of Staphylococcus aureus isolates was tested, including several borderline oxacillin-resistant mecA-positive strains (n ϭ 19) that would be difficult to detect and mecA-negative strains (n ϭ 36) that might be falsely detected as resistant. Since several lots of oxacillin screen agar lacked specificity, the issue of the reliability of all methods for detection of oxacillin resistance was raised. Thus, using that same group of challenge organisms, we chose to examine the accuracy of the other routine methods for detecting oxacillin resistance. These methods were broth microdilution, disk diffusion, oxacillin agar screen, MicroScan conventional panels, MicroScan rapid panels, and Vitek. We also evaluated two new rapid methods, Velogene, a cycling probe assay, and MRSA-Screen, a latex agglutination method that detects the presence of PBP-2a (also known as PBP-2Ј), the penicillin-binding protein (PBP) responsible for the most common form of oxacillin resis...

Evaluation of various methods for the detection of meticillin-resistant Staphylococcus aureus strains and susceptibility patterns

“…Swenson et al (2001) noted that sensitivity decreased when heterogeneous resistant strains were tested and specificity decreased with strains having borderline MIC. Among the recently developed methods, CHROMagar showed 97.1 % sensitivity and 99.2 % specificity.…”

“…One drop of this suspension was inoculated on Mueller-Hinton agar containing 4 % NaCl and 6 mg oxacillin ml 21 (Hi-Media). Plates were incubated at 35 uC for 24 h. Any strains showing growth on the plate containing oxacillin were considered to be resistant to meticillin (Swenson et al, 2001).…”

Evaluation of MicroScan Synergies plus Positive Combo 3 Panels for Identification and Antimicrobial Susceptibility Testing of Staphylococcus aureus and Enterococcus Species

“…자동화된 항균제 감수성 장비를 이 용한 희석법은 methicillin 내성을 정확하고 신속하게 검출하고 자 다양한 개선이 이루어졌으나 표준방법만큼의 신뢰도를 보이 지 못한다는 연구가 있다 [8][9][10][11][12][13]. 따라서 임상검사실에서 MRSA를 정확히 검출하는 것은 해결해야 할 과제이다 [14].…”

Comparison of clinical methods for the phenotypic detection of Methicillin resistant Staphylococcus aureus: Disc diffusion methods with BrillianceTM MRSA agar

“…The identification of such strains is a challenge even if special phenotypic methods are used (Brown, 2001). Therefore, for the determination of methicillin resistance in Staphylococci, mecA or PBP2 detection is considered to be a gold standard (Brown, 2001;Dufour et al, 2002 ;Gosbell et al, 2001;Swenson et al, 2001). However, in Pakistan most of the laboratories do not have the economic and technical capabilities for applying all these tests on the S. aureus isolates recovered in microbiology laboratories.…”