Comparison of Cefoxitin and Oxacillin Disk Diffusion Methods for Detection of mecA -Mediated Resistance in Staphylococcus aureus in a Large-Scale Study

Abstract

The recommended breakpoints for the cefoxitin disk diffusion test for Staphylococcus aureus were recently modified. In this large-sample study, cefoxitin sensitivity and specificity compared to those of oxacillin were 97.3% and 100%, respectively. This study validated the new cefoxitin breakpoints for the detection of mecAmediated resistance in S. aureus.Staphylococcus aureus is a serious current health care concern. Both community-associated methicillin-resistant S. aureus (MRSA) and health care-associated MRSA are growing threats to the immunocompromised, as well as to the general public. Accurate detection of methicillin resistance in S. aureus is of the utmost importance to ensure effective treatment for the affected patient and to prevent further transmission.The mecA gene confers resistance to methicillin in S. aureus. The gene is located on the staphylococcal chromosome cassette mec and encodes penicillin binding protein 2a (PBP2a). PBP2a is located in the bacterial cell wall and has a low binding affinity for ␤-lactams (1, 2). This study evaluated the new Clinical Laboratory Standards Institute (CLSI) breakpoints for the cefoxitin disk test for determining mecA-mediated resistance in S. aureus (4).CLSI recommends usage of cefoxitin instead of oxacillin when using the disk diffusion method to determine resistance against methicillin for S. aureus (4). Cefoxitin results are easier to interpret and are thus more sensitive for the detection of mecA-mediated resistance than oxacillin results (5, 6, 7, 9-11). The recommended resistance and susceptibility breakpoints for the 30-g cefoxitin disk test used to detect mecA-mediated resistance in S. aureus were changed in January 2007 by CLSI from Յ19 mm and Ն20 mm to Յ21 mm and Ն22 mm, respectively. This study sampled a large number of recent S. aureus isolates to compare the performance of the cefoxitin disk test at the new breakpoints to that of the 1-g oxacillin disk test.Between August and September 2007, a total of 1,611 nonduplicate S. aureus isolates were collected by 53 Wisconsin clinical laboratories (up to 20 consecutive MRSA and methicillin-susceptible S. aureus isolates per laboratory) and submitted to the Wisconsin State Laboratory of Hygiene. Species identification was confirmed by colony morphology, coagulase slide test, subsequent tube test, and biochemicals. The isolation sites of the S. aureus isolates collected were as follows: skin and soft tissue (1,159 isolates; 71.9%), urine (136 isolates; 8.4%), respiratory tract (130 isolates; 8.1%), bloodstream (65 isolates; 4.0%), and other (121 isolates; 7.5%).Susceptibility to antimicrobial agents for confirmed S. aureus isolates was evaluated by the CLSI disk diffusion method on Mueller-Hinton agar (3). All agar plates and antibiotic disks were obtained from Remel (Lenexa, KS). A direct colony suspension of each S. aureus isolate was prepared to a 0.5 McFarland standard and plated on Mueller-Hinton agar. The zones of inhibition were measured using the Biomic V 3 system

Comparison of five methods for oxacillin susceptibility testing of Staphylococcus aureus isolates from cystic fibrosis patients

“…Other available routine tests include epsilometer test and oxacillin agar screening plate. Although recommended by guidelines and validated in multiple studies [1][2][3][4][5][6]8,[12][13][14][15]17,18 , there is a lack of data about the accuracy of these tests in isolates from such a specific population as cystic fibrosis patients.…”

“…There are several studies in the biomedical literature regarding different oxacillin phenotypic susceptibility tests for S. aureus 1,2,[4][5][6]8,[12][13][14][15]17,18 , but, to our knowledge, none has included only cystic fibrosis isolates. Thus, the aim of the present study was to determine the accuracy of phenotypic oxacillin susceptibility methods in Staphylococcus aureus cystic fibrosis isolates with different SCCmec types.…”

Investigation of Methicillin Resistance in Staphylococcus aureus Isolates by Using Disc Diffusion, Chromogenic Media, Gradient Diffusion Test, Oxacillin Agar Screening Methods

“…CLSI önerisiyle metisilin direncinin konvansiyonel yöntemlerle belirlenmesinde metisilin diskine göre hem daha stabil hem de MRSA tanı-sında daha yüksek özgüllüğe sahip bir diğer penisilin türevi olan oksasilin kullanılmaktay-dı (12) . Fakat heterojen direnç sonucu yalnızca oksasilin kullanılan tarama testleri MRSA -MSSA ayrımını yeterli düzeyde yapamamaktadır (13,14) .…”

“…Sefoksitin özellikle mecA düzenleyici sistemin potent indükleyicisidir ve yapılan çalışmalar sefoksitin diskinin moleküler yöntemlere alternatif olabileceğini göstermiştir (12,13) . Dünyada yapılan bazı çalışmalar disk difüzyon testinde oksasilin yerine sefoksitin diski kullanmanın MRSA tespitinde daha duyarlı (%100'e karşın %80-89.5) olduğunu göstermiştir (15,16) .…”

Difference on Cefoxitin and Oxacillin Disk Test on In Vitro MRSA Detection (Meticillin Resistant Staphylococcus aureus) Experimental Study on Microbiology Laboratorium of Medicine Faculty of UNISSULLA

“…Accuracy and reliability to detect meticilin resistance is the most important key to confirm antibiotic treatment for infected patient and to control MRSA staphylococci around hospital environment (Velasco, et al, 2005). MRSA resistance detection can be conducted by using oxacilin or cefoxitin diffusion method (Van Leeuwen WB, 2003;Broekema NM, et al, 2009).…”