Jan 1, 2000 - Advanced Search. Issue Section: Correspondence. J Antimicrob Chemother 2000; 45: 130â131. Sir,. We read with interest the recent article by ...
JAC
Journal of Antimicrobial Chemotherapy (2000) 45, 129–134
Correspondence Heterogeneous resistance to vancomycin in Staphylococcus aureus J Antimicrob Chemother 2000; 45: 130–131 Robin A. Howea*, Mandy Woottona, Timothy R. Walshb, Peter M. Bennettb and Alasdair P. MacGowana a
Bristol Centre for Antimicrobial Research and Evaluation, Department of Microbiology, Southmead Hospital, Bristol BS10 5NB; b Department of Pathology and Microbiology, School of Medical Sciences, University of Bristol, Bristol, UK
*Corresponding author. Tel: Fax: 44-117-959-3154.
44-117-959-5651;
Sir, We read with interest the recent article by Geisel et al. concerning the emergence of heterogeneous intermediate vancomycin resistance in Staphylococcus aureus (heteroVISA) isolates in the Dusseldorf area.1 These investigators reported that 8.2% of 85 methicillin-resistant S. aureus (MRSA) isolates tested were hetero-VISA. However, we have reservations about the detection method employed in the study. Geisel et al. used the method proposed by Hiramatsu et al. when they first identified hetero-VISA.2 This involves plating high inocula of test strains on to Brain–Heart Infusion agar containing vancomycin at a concentration of 4 mg/L and then determining the MICs of vancomycin for any colonies that grow on the medium. A strain is designated hetero-VISA if it produces subclones with stable MICs of vancomycin 8 mg/L. We have evaluated this method and found that it lacks both specificity and reproducibility.3 Of 100 MRSA isolates tested in quintuplicate, 16 were identified as hetero-VISA on only one of five occasions and one additional strain was positive on two occasions. None of these strains was confirmed to be hetero-VISA when it was investigated by a population analysis technique (see below). Geisel et al. have not indicated whether they repeated the screening method and, if so, whether or not the results were consistent. An additional problem with the Hiramatsu method is that it may select for vancomycin resistance rather than simply detect it. Similar techniques have been used to select resistance to vancomycin in Staphylococcus epider -
midis and Staphylococcus haemolyticus4 and in S. aureus.5,6 In fact, Geisel et al. stated that “Hetero-VISA status was confirmed if a strain produced subclones with vancomycin MICs 8 mg/L following subculture in the presence of increasing concentrations of vancomycin. . .”. This description very closely resembles that of a classical method of selecting resistance. With regard to the cell wall analysis, it is not clear whether the strains used in the experiment had been preexposed to, or selected with, vancomycin, or if they were original parental strains. If the former, it may have been that the changes observed were the result of exposure to vancomycin—changes which would have been exhibited by any strain similarly exposed—rather than reflecting stable genetically determined factors mediating vancomycin resistance. The fact that some of the cell wall changes observed (e.g. increased amounts of glutamine-nonamidated muropeptides) were unstable and were reversed on repeated subculture suggests that they may represent a phenotypic adaptation to vancomycin exposure. We believe that population analysis profiling (PAP) is currently the only reliable method of confirming the heterogeneity of vancomycin susceptibility. Aliquots of an overnight culture are inoculated on to agar containing a range of vancomycin concentrations. After incubation for 48 h, the colonies on each plate are counted and plotted (Figure). Mu3 (the original Japanese hetero-VISA strain2) and SMH2 (a hetero-VISA strain isolated in Bristol7) are characterized by population analysis profiles as different to those of other MRSA strains, including a VISA strain.
Figure. Population analysis profiles for hetero-VISA strains Mu3 ( ) and SMH2 ( ), as well as Mu50 (a VISA strain) ( ) and two representative vancomycin-susceptible MRSA strains ( and ).
130 © 2000 The British Society for Antimicrobial Chemotherapy
Correspondence Vancomycin resistance in S. aureus is clearly a matter of concern and many workers around the world will be screening for VISA and hetero-VISA. We suggest that PAP is currently the only reliable method of identifying hetero-VISA strains. The technique is time-consuming (we are currently evaluating alternative methods suitable for use in diagnostic laboratories), but the results of epidemiological studies that do not confirm hetero-VISA status in this way should be interpreted with caution.
3. Wootton, M., Walsh, T., Howe, R., White, G., Bowker, K. E., Andrews, J. M. et al. (1998). Methodological factors in the detection of heterogeneously vancomycin-resistant Staphylococcus aureus (h-VRSA). In Program and Abstracts of the Thirty-Eighth Interscience Conference on Antimicrobial Agents and Chemotherapy, San Diego, CA, 1996. Abstract C139, p. 108. American Society for Microbiology, Washington, DC. 4. Herwaldt, L., Boyken, L. & Pfaller, M. (1991). In vitro selection of resistance to vancomycin in bloodstream isolates of Staphylococcus haemolyticus and Staphylococcus epidermidis. European Journal of Clinical Microbiology and Infectious Diseases 10, 1007–12. 5. Biavasco, F., Giovanetti, E., Montanari, M. P., Lupidi, R. & Varaldo, P. E. (1991). Development of in-vitro resistance to glycopeptide antibiotics: assessment in staphylococci of different species. Journal of Antimicrobial Chemotherapy 27, 71–9.
References 1. Geisel, R., Schmitz, F.-J., Thomas, L., Berns, G., Zetsche, O., Ulrich, B. et al. (1999). Emergence of heterogeneous intermediate vancomycin resistance in Staphylococcus aureus isolates in the Dusseldorf area. Journal of Antimicrobial Chemotherapy 43, 846–8. 2. Hiramatsu, K., Aritaka, N., Hanaki, H., Kawasaki, S., Hosoda, Y., Hori, S. et al. (1997). Dissemination in Japanese hospitals of strains of Staphylococcus aureus heterogeneously resistant to vancomycin. Lancet 350, 1670–3.
6. Daum, R. S., Gupta, S., Sabbagh, R. & Milewski, W. M. (1992). Characterization of Staphylococcus aureus isolates with decreased susceptibility to vancomycin and teicoplanin: isolation and purification of a constitutively produced protein associated with decreased susceptibility. Journal of Infectious Diseases 166, 1066–72. 7. Howe, R. A., Bowker, K. E., Walsh, T. R., Feest, T. G. & MacGowan, A. P. (1998). Vancomycin resistant Staphylococcus aureus. Lancet 351, 602.
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