Evaluation of Current Methods for Detection of Staphylococci with Reduced Susceptibility to Glycopeptides

Abstract

The sensitivity and specificity of seven methods (agar dilution, broth microdilution, Etest at 0.5 and 2.0 McFarland (McF) inocula, two agar screening methods, and population studies [PS]) were evaluated in a double-blind study involving 284 methicillin-resistant Staphylococcus aureus (MRSA) strains and 45 Staphylococcus strains with reduced susceptibilities to vancomycin (SRSV). The results were compared to the population analysis profile-area under the curve ratio method (PAP-AUC ratio compared to that of Mu3) as described by Wootton et al. The agar screening method using brain heart infusion agar (6 g of vancomycin per ml) gave a sensitivity of 22% and a specificity of 97%. A similar method using Mueller-Hinton agar (5 g of vancomycin per ml) gave a sensitivity of 20% and a specificity of 99%. The PS method detected 34 false positives (12%) and gave a sensitivity of 71% and a specificity of 88%. Etest using 0.5 and 2.0 McF inocula gave sensitivities and specificities of 82 and 93% and of 96 and 97%, respectively. The best Etest interpretative criteria for the 2.0 McF inoculum was >8 mg of vancomycin per liter and >8 g teicoplanin per ml or >12 g of teicoplanin per ml. The direct colony suspension inoculum for this method was found to be equally accurate in detecting (hetero-) glycopeptide-intermediate S. aureus compared to the overnight broth inoculum preparation method. Agar dilution and broth microdilution using the NCCLS breakpoint criteria for vancomycin gave sensitivities and specificities of 20 and 100% and of 11 and 100%, respectively. Using the Etest with a 2.0 McF inoculum, six different media were assessed against a selection of SRSV (n ‫؍‬ 48) and MRSA (n ‫؍‬ 12). Brain heart infusion agar yielded the highest sensitivity and specificity values: 88 and 88%, respectively.

Phagocytosis and intracellular killing of heterogeneous vancomycin-intermediate Staphylococcus aureus strains

“…versus vancomycin concentration. The area under the curve (PAP-AUC) was calculated for each strain (Walsh et al, 2001). To distinguish VISA, hVISA and vancomycin-susceptible MRSA (ATCC 29213), a ratio of the AUC of the test MRSA divided by the corresponding AUC for S. aureus strain Mu3 used as a positive control was calculated.…”

“…The criteria used for detection of hVISA and VISA strains were AUC ratios of ¢0.9 and ¢1.3, respectively. The ratio of test MRSA AUC divided by the corresponding Mu3 AUC was calculated as described previously (Walsh et al, 2001;Tó th et al, 2008).…”

The Basis for Resistance to β-Lactam Antibiotics by Penicillin-binding Protein 2a of Methicillin-resistant Staphylococcus aureus

“…Also, the clinical urgency has been met in the past few years by the introduction of Synercid (a combination of quinupristin and dalfopristin) (9), daptomycin (10), and linezolid (an oxazolidinone) (11) for treatment of MRSA. However, resistance to all of these agents exists, and the recent emergence of variants of MRSA resistant to linezolid (12) and glycopeptide antibiotics (13)(14)(15)(16) has created a situation in which certain strains of S. aureus are either treatable only with a single class of antibiotics or are simply not treatable, which is a disconcerting situation clinically.…”

Methicillin, Vancomycin, Teicoplanin and Macrolide-Lincosamide-Streptogramin B (MLSB) Resistance in Staphylococcus aureus Isolated from Postmortem Samples

“…Çalışmaya alınan izolatların koyun kanlı agardaki pasajlarından, serum fizyolojik içinde 2 McFarland bulanıklığında bakteri süspansiyonları hazırlan-mıştır. Bu süspansiyonlardan "Brain Heart Infusion" agara (BHIA) yapılan sürüntü ekimleri üzerine E-test şeritleri (BioMerieux, Fransa) yerleştirilerek 37ºC'de bir gece inkübe edilmiştir (30) . İnhibisyon zonunun E-test şeridini kestiği noktada minimum inhibitör konsantrasyon (MİK) değeri olarak kaydedilmiş ve sonuç-lar CLSI önerilerine göre değerlendirilmiştir.…”