Coagulase-Negative Staphylococci: Comparison of Phenotypic and Genotypic Oxacillin Susceptibility Tests and Evaluation of the Agar Screening Test by Using Different Concentrations of Oxacillin

Abstract

This study evaluated the oxacillin susceptibilities of 152 coagulase-negative staphylococcal (CoNS) strains of 12 species by disk diffusion; agar dilution; E-test; the slide latex agglutination test (Slidex MRSA Detection test; bioMérieux S/A, Paris, France); the agar screening test with 1, 2, 4, or 6 g of oxacillin per ml and incubation for 24 or 48 h; and detection of the mecA gene by PCR. The results revealed that the agar screening test with 4 g of oxacillin per ml and incubation for 48 h was superior to any single phenotype-based susceptibility assay, presenting a sensitivity and a specificity of 100% each. For the different methods evaluated, the sensitivities and specificities were as follows: for disk diffusion, 94.2 and 91.8%, respectively; for the agar dilution test 100 and 73.5%, respectively; for E-test, 100 and 71.4%, respectively; and for the slide latex agglutination test, 97.1 and 98%, respectively. A good correlation was observed between oxacillin susceptibility testing results and PCR results for Staphylococcus epidermidis, S. haemolyticus, S. hominis subsp. hominis, and all mecA-positive strains. However, at least 60% of the mecA-negative isolates of the species S. saprophyticus, S. cohnii subsp. urealyticum, S. lugdunensis, and S. sciuri were erroneously classified as oxacillin resistant by the agar dilution test. Conversely, the slide latex agglutination test presented a high sensitivity (97.1%) and a high specificity (98%) for all CoNS species. Our results demonstrated the accuracy of the agar screening test with 4 g of oxacillin per ml and incubation for 48 h and the slide latex agglutination test for the appropriate detection of the oxacillin susceptibilities of CoNS isolates. Both assays are technically simple and can be easier to perform in routine laboratories than PCR.

Identification and detection of methicillin resistance in Non-Epidermidis coagulase-negative staphylococci

“…We evaluated a higher number of isolates of SCoNne than other studies performed in Brazil, and we found high rates of resistance to oxacillin by PCR (71%) in these isolates, though earlier studies also included isolates of S. epidermidis [26][27][28]. In our study, among the SCoNne isolates, 100 were identified as S. haemolyticus, with 91% presenting a mecAgene-positive PCR result.…”

“…Detection of oxacillin resistance in SCoN is a challenge for clinical laboratories because many false negative results can be attributed to heterogeneous resistance to oxacillin expressed by this type of organism [26,29,33]. The cefoxitin disk test is considered a better predictor for resistance than oxacillin, especially when there is heteroresistance to oxacillin.…”

Cefoxitin disc diffusion test for detection of meticillin-resistant staphylococci

“…Phenotypic expression of resistance can vary depending on the growth conditions (e.g. temperature, osmolarity and culture medium supplements such as NaCl or sucrose) (Ferreira et al, 2003;Sakoulas et al, 2001), making susceptibility testing by standard microbiological methods potentially problematic.…”

Antimicrobial susceptibility profile and research of mec A and erm genes in coagulase-negative staphylococci isolated from platelet concentrates bags

“…The microtube was subjected to heating shock (on ice) for 5 minutes and centrifuged at 11,000 rotations per minute (rpm) for 20 minutes. The supernatant (containing the DNA) was transferred to another microtube which was frozen at -20 °C (Nogueira et al, 2004;Freschi, Carvalho, Oliveira, 2005;Scheffer, 2008;Aguilar, 2009). DNA extraction by heating shock was confirmed by the 16S rRNA gene amplification, obtaining a high quality DNA.…”

“…Studies show that there has been a substantial increase in MRCoNS isolation rates in hospital infections, including Brazil, where resistance rates are among 50-80%, varying according to bacterial species and patient's demographic data (Marshall et al, 1998;Hussain et al, 2000;Ferreira et al, 2003;Palazzo, Darini, 2006).…”