Precision of Vancomycin and Daptomycin MICs for Methicillin-Resistant Staphylococcus aureus and Effect of Subculture and Storage

Abstract

dThe reproducibility of vancomycin and daptomycin MICs, measured by broth microdilution (BMD) and Etest, was prospectively assessed for 10 methicillin-resistant Staphylococcus aureus (MRSA) isolates from the blood samples from patients on vancomycin therapy. The isolates were tested at the time of isolation from blood and following 5, 10, and 20 subcultures and at 1, 3, 6, and 12 months of storage at ؊70°C. The MICs were determined by Etest and BMD using two different manufacturers (BBL and Difco) of cation-adjusted Mueller-Hinton broth (CA-MHB), and using three different drug powders: vancomycin from Sigma, vancomycin from Novation, and daptomycin from Cubist. The antimicrobial concentrations tested were 0.25, 0.5, 0.75, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 g/ml. Two isolates were vancomycin intermediate and daptomycin nonsusceptible, and two isolates had reduced susceptibility to vancomycin (BMD MIC, 1.5 or 2.0 g/ml). The vancomycin MICs were significantly higher in the BBL CA-MHB than those in the Difco CA-MHB, and with Sigma versus Novation vancomycin powder. The daptomycin MICs were also significantly higher in the BBL CA-MHB. The Etest MICs were significantly higher than those obtained by BMD for vancomycin but not for daptomycin. The average precision of the vancomycin BMD MICs when analyzing 20 results was ؎1.10-fold log 2 dilutions, and it was ؎1.67-fold for daptomycin (10 results). The average precision for Etest was ؎1.11-fold for vancomycin and ؎1.16-fold for daptomycin. No significant change in MICs was noted following 5, 10, or 20 subcultures or at up to 6 months of frozen storage. However, the vancomycin MICs alone were significantly lower (0.74-fold) following 12 months of frozen storage. From these data, despite variations in CA-MHB and antimicrobial powder, the MIC result precision was <0.5 log 2 dilutions in a single laboratory, suggesting that testing interdilution MICs (e.g., MICs between serial 2-fold dilutions) is a possibility. A more accurate method for measuring vancomycin MIC results is thus possible, but further standardization of BMD testing would be required to achieve this goal.

CLSI Methods Development and Standardization Working Group Best Practices for Evaluation of Antimicrobial Susceptibility Tests

“…For example, colistin MICs for P. aeruginosa, Klebsiella pneumoniae, and Acinetobacter baumannii have been shown to decrease from a resistant/non-wild-type phenotype to a susceptible/wild-type phenotype after frozen storage (8). Similarly, vancomycin MICs for Staphylococcus aureus have been shown to decrease after 1 year of frozen storage (9). Details to be included in Materials and Methods for the evaluation, regarding how the cAST was performed, are outlined in Table 2.…”

Antimicrobial hydroxyapatite–gelatin–silica composite pastes with tunable setting properties

“…On one hand, to deepen this question, we performed the antibacterial testing of the composite pastes in vitro . On the other hand, with the release totaling ~ 2 % of the total drug content in the first 24 h, equaling 2 mg/g of the paste, it would take only 1 g/dm 3 of the paste to reach the uppermost end of the MIC for vancomycin against methicillin-resistant S. aureus (2 μg/ml) 49 in this period of time. Such an absence of the burst release indicates that the antibiotic is almost completely confined to the interior of the hardened paste and that the degradation of the paste in an aqueous environment will precondition its antibacterial activity.…”

Does sub-culturing of positive MRSA blood cultures affect vancomycin MICs?

“…In MRSA strains with heterogenous resistance, after separation from blood and sub-culture on drug-free, nutrient-rich medium, the lower MICs observed in isolates after sub-culturing of positive blood cultures may reflect a change in the MRSA population being tested. Although a previous study reported no differences in VAN MICs after repeated sub-cultures, that study did not compare sub-cultured MRSA and direct VAN MIC testing [11].…”