Evaluation of oxacillin and cefoxitin disks for detection of resistance in coagulase negative staphylococci

Abstract

(Beekmann et al. 2003). Healthcare-associated infections caused by these microorganisms are responsible for high mortality rates worldwide. The emergence of CoNS populations with heterogeneous resistance to oxacillin causes a great difficulty to detect them in clinical routine laboratories (Cauwelier et al. 2004). In Brazil, oxacillin resistance may be present in over 80% of isolates in some health institutions (Sader et al. 2001). On this account, vancomycin has been widely used and it is a major cause for the emergence of glycopeptide-resistant isolates (Oliveira et al. 2001, Nunes et al. 2006.Recently, Clinical and Laboratory Standards Institute (CLSI) standardized a disk-diffusion method to predict resistance mediated by the mecA gene in Staphylococcus spp., using a cefoxitin disk (30 µg). In fact, studies indicate that this is the best phenotypic test to predict resistance to beta-lactam agents among CoNS (Felten et al. 2002, Pottumarthy et al. 2005. Regardless of the CLSI guidelines, the detection of this resistance through phenotypic methods remains a challenge for clinical laboratories of microbiology (Sejas et al. 2003). Several discrepancies were observed among the laboratories that participated in the Antimicrobial Surveillance Program (SENTRY), suggesting non-observance of the interpretation criteria currently recommended (Mendes et al. 2003). The quality of the antimicrobial disks may affect the results, with failure to detect hetero-resistance (Sejas et al. 2003). This is especially important in the case of oxacillin and cefoxitin, which predict susceptibility to a large group of antimicrobial agents.The aim of this study was to evaluate two different brands of oxacillin (1 µg) and cefoxitin (30 µg) disks that are commonly used in clinical laboratories of microbiology. MATERIALS AND METHODSIsolates -CoNS isolated from blood cultures maintained in "skim-milk" (Difco, Detrot) at -20°C were studied. For the tests, the isolates were cultivated onto agar plates supplemented with sheep blood at 5% (Trypticasein agar -Oxoid, UK), incubated for 24 h at 35°C. Identification of CoNS -Characterization was made by a set of phenotypic tests (Bannerman 2003): coagulase, catalase, alkaline phosphatase, ornithine decarboxylase, urease, PYR, and utilization of carbohydrates (trehalose, mannitol, mannose, sucrose, maltose, lactose, cellobiose). Anaerobic growth in thioglicolate and susceptibility to novobiocin (> 16 mm), to polymyxin B (≥ 10 mm), and to bacitracin (> 9 mm) by disk diffusion tests were also evaluated. Other tests useful to discriminate between S. epidermidis and S. hominis were susceptibility to desferrioxamine (≥ 20 mm) and fosfomycin (> 30 mm) (Lindsay et al. 1993, Rosco 2000. Mcfarland standard suspension of each isolate was inoculated onto a Mueller Hinton agar plate (Oxoid), and the disks were distributed maintaining a distance of 30 mm edge to edge. The diameters of the inhibition zones were interpreted according to the criteria recommended by the CLSI (2006), considering a breakpoint diameter...

Bacteremias por Staphylococcus coagulase negativos oxacilina resistentes em um hospital escola na cidade de Santa Maria, Estado do Rio Grande do Sul

“…A resistência à oxacilina sem a presença do gene mecA pode ser devida a hiperprodução de penicilinase ou por modificação nas proteínas de ligação da penicilina (PBP 1, 2 e 4) 14 . Em relação à sensibilidade aos glicopeptídeos, todos os isolados mostraram-se sensíveis à vancomicina.…”

“…Contudo, a emergência de cepas com resistência heterogênea MÉTODOS RESULTADOS dificulta a interpretação dos testes de difusão do disco na rotina laboratorial 9,13,14 De acordo com a sensibilidade a agentes antimicrobianos, efetuada pelo método de difusão do disco (Tabela 1), as cepas de SCN apresentaram resistência significativa a várias classes de antimicrobianos. As maiores sensibilidades obtidas foram frente aos antimicrobianos rifampicina (43 de 60/72%) e tigeciclina (60 de 60/100%).…”

Oxacillin Resistance and Antimicrobial Susceptibility Profile of Staphylococcussaprophyticus and Other Staphylococci Isolated from Patients with Urinary Tract Infection

“…Resistance to β-lactam antibiotics among S. saprophyticus isolates has been reported in Japan [42], Canada [31], United States [56], and Brazil [57]. However, studies investigating resistance mediated by the mecA gene in S. saprophyticus , particularly in urinary isolates, are scarce.…”

Oxacillin Resistance of <i>Staphylococcus aureus</i> Isolated from the University Hospital of Botucatu Medical School in Brazil

“…The quality of the impregnated cefoxitin disks is a relevant factor for the detection of oxacillin resistance by oxacillin and cefoxitin disk diffusion methods. Souza Antunes et al [29] reported differences in the detection of oxacillin resistance between different brands of oxacillin and cefoxitin disks.…”