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Evaluating vancomycin susceptibility in Staphylococcus aureus

Authors Mimica M, Navarini A

Received 14 June 2016

Accepted for publication 20 June 2016

Published 26 September 2016 Volume 2016:9 Pages 239—241

DOI https://doi.org/10.2147/IDR.S114942

Checked for plagiarism Yes

Editor who approved publication: Professor Suresh Antony

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Marcelo J Mimica, Alessandra Navarini

Department of Pathology, Division of Microbiology, Santa Casa de São Paulo School of Medicine, São Paulo, Brazil

We read the report by Phillips et al1 with great interest and would like to discuss it in comparison with our previous published data on the subject.2,3
    We have also studied a number of Staphylococcus aureus clinical isolates (n=125), comparing different vancomycin susceptibility tests, including microdilution, Etest® (bio-Mérieux, Marcy-l’Étoile, France), and brain heart infusion vancomycin screening plates. We found only one isolate with reduced susceptibility with a minimum inhibitory concentration (MIC) =4 mg/L when tested with Etest and 2 mg/L when tested with microdilution.2,3 Our results showed a tendency of higher lethality when higher MICs were present, even within the susceptible range,3 as some previous studies have shown.4,5


View original paper by Phillips et al.

Dear editor

We read the report by Phillips et al1 with great interest and would like to discuss it in comparison with our previous published data on the subject.2,3

We have also studied a number of Staphylococcus aureus clinical isolates (n=125), comparing different vancomycin susceptibility tests, including microdilution, Etest® (bioMérieux, Marcy-l’Étoile, France), and brain heart infusion vancomycin screening plates.

We found only one isolate with reduced susceptibility with a minimum inhibitory concentration (MIC) =4 mg/L when tested with Etest and 2 mg/L when tested with microdilution.2,3 Our results showed a tendency of higher lethality when higher MICs were present, even within the susceptible range,3 as some previous studies have shown.4,5

Concordant to Phillips et al1 and other authors,6,7 we also reported a poor correlation between different tests. Comparing Etest and microdilution (approximating an Etest MIC value between two twofold dilutions up to the highest value), 58% of the isolates had similar MICs, whereas 38% had an MIC by Etest one dilution higher than microdilution. One isolate had an Etest MIC twofold higher and four isolates an Etest MIC onefold lower than microdilution.2

However, in our study, a brain heart infusion screening plate with 2.0 mg/L of vancomycin showed a sensitivity of 100% to detect isolates with an MIC ≥2.0 by Etest and 91% to detect an MIC ≥2.0 by microdilution, making this test an interesting option for initial screening of S. aureus isolates for reduced vancomycin susceptibility. Specificities were 63% and 38%, respectively, which would still make necessary the further testing with an MIC method, but in a much smaller number of isolates.2 This approach would be suitable for a large number of laboratories throughout the world where the routine MIC testing of all S. aureus isolates is not feasible.

Disclosure

The authors report no conflicts of interest in this communication.

References

1.

Phillips CJ, Wells NA, Martinello M, Smith S, Woodman RJ, Gordon DL. Optimizing the detection of methicillin-resistant Staphylococcus aureus with elevated vancomycin minimum inhibitory concentrations within the susceptible range. Infect Drug Resist. 2016;9:87–92.

2.

Navarini A, Martino MD, Sasagawa SM, Massaia IF, Mimica MJ. Accuracy of a vancomycin brain heart infusion screening plate for the screening of Staphylococcus aureus isolates with increased vancomycin minimum inhibitory concentrations. New Microbiol. 2015;38(3):423–426.

3.

Sulla F, Bussius DT, Acquesta F, Navarini A, Sasagawa SM, Mimica MJ. Vancomycin minimum inhibitory concentrations and lethality in Staphylococcus aureus bacteremia. Germs. 2015;5(2):39–43.

4.

Aguado JM, San-Juan R, Lalueza A, Sanz F, Rodríguez-Otero J, Gómez-Gonzalez C, Chaves F. High vancomycin MIC and complicated methicillin-susceptible Staphylococcus aureus bacteremia. Emerg Infect Dis. 2011;17(6):1099–1102.

5.

van Hal SJ, Lodise TP, Paterson DL. The clinical significance of vancomycin minimum inhibitory concentration in Staphylococcus aureus infections: a systematic review and meta-analysis. Clin Infect Dis. 2012;54(6):755–771.

6.

Vaudaux P, Huggler E, Bernard L, Ferry T, Renzoni A, Lew DP. Underestimation of vancomycin and teicoplanin MICs by broth microdilution leads to underdetection of glycopeptide-intermediate isolates of Staphylococcus aureus. Antimicrob Agents Chemother. 2010;54(9):3861–3870.

7.

Mason EO, Lamberth LB, Hammerman WA, Hulten KG, Versalovic J, Kaplan SL. Vancomycin MICs for Staphylococcus aureus vary by detection method and have subtly increased in a pediatric population since 2005. J Clin Microbiol. 2009;47(6):1628–1630.

Authors’ reply

Cameron J Phillips1–3, Nicholas A Wells4, Marianne Martinello4, Simon Smith4, Richard J Woodman5, David L Gordon2,4

1SA Pharmacy, Flinders Medical Centre, Bedford Park, 2Department of Microbiology and Infectious Diseases, School of Medicine, Flinders University, 3School of Pharmacy and Medical Sciences, University of South Australia, Adelaide, 4SA Pathology, Microbiology and Infectious Diseases, Flinders Medical Centre, Bedford Park, 5Flinders Centre for Epidemiology and Biostatistics, School of Medicine, Flinders University, Adelaide, SA, Australia

Correspondence: Cameron J Phillips, SA Pharmacy, Flinders Medical Centre, 1 Flinders Drive, Bedford Park, SA, 5042, Australia
Email [email protected]

Dear editor

We thank Mimica and Navarini1 for their comments on our article.2 We note with interest their findings consistent with our study regarding the poor correlation between methods for determining vancomycin minimum inhibitory concentration (MIC) also reported elsewhere.3 The need to obtain good susceptibility methods that provide both high sensitivity and high specificity is indeed challenging. We have assessed diagnostic accuracy for two commonly used susceptibility methods (Etest® and Vitek®2) measured against the gold standard broth microdilution to give microbiologists and clinicians further insight into the sensitivity and specificity at incremental MICs. Employing two susceptibility methods is likely to become a more common practice when testing vancomycin MIC ≥1 and <2 µg/mL in an effort to more appropriately dose and monitor vancomycin in patients with these infections. Investigators of future laboratory and clinical studies that report MICs using two methods may also consider the reporting of diagnostic accuracy using a combined test approach, which might improve the interpretation of overall sensitivity and specificity.

Disclosure

The authors report no conflicts of interest in this communication.

References

1.

Mimica MJ, Navarini A. Evaluating vancomycin susceptibility in Staphylococcus aureus. Infect Drug Resist. 2016;4:27–32.

2.

Phillips CJ, Wells NA, Martinello M, Smith S, Woodman RJ, Gordon DL. Optimizing the detection of methicillin-resistant Staphylococcus aureus with elevated vancomycin minimum inhibitory concentrations within the susceptible range. Infect Drug Resist. 2016;9:87–92.

3.

Keel RA, Sutherland CA, Aslanzadeh J, Nicolau DP, Kuti JL. Correlation between vancomycin and daptomycin MIC values for methicillin-susceptible and methicillin-resistant Staphylococcus aureus by 3 testing methodologies. Diag Microbiol Infect Dis. 2010;68(3):326–329.

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