Abstract
Since 1988, when the first vancomycin-resistant enterococcus was described, several descriptions of failures of disk diffusion breakpoints to detect low-level vancomycin resistance (MICs, 8 to 32 micrograms/ml) have been published. A four-laboratory collaborative study was undertaken to establish more accurate breakpoints for the disk test. Mueller-Hinton agar was used to perform dilution testing (in three laboratories) and disk diffusion testing (in all laboratories). Results were determined at 18, 24, and 48 h, and zones of inhibition were read using both transmitted and reflected light. One hundred organisms (35 Enterococcus faecalis, 55 E. faecium, and 10 E. gallinarum or E. casseliflavus isolates) were selected to represent vancomycin-susceptible and -resistant phenotypes. Interlaboratory agreement of agar dilution MICs was better at 24 h (91 to 94% within +/- 1 dilution) than at 18 h (76% within +/- 1 dilution). Therefore, 24-h agar dilution MIC results were used as the reference. For disk diffusion, it was critical to note the presence of a haze or colonies inside the zone when interpreting the test, since this correlated better with the results of the agar dilution test. The presence of a haze or inner colonies was best detected by reading the zones with transmitted light and incubating the plates for a full 24 h. When plotted against 24-h agar dilution MICs, breakpoints of </= 14 mm (resistant), 15 to 16 mm (intermediate), and >/= 17 mm (susceptible) resulted in 58 minor errors (14.5% of total values) and 5 very major errors (2.2% of resistant values or 1.3% of total values). No major errors were seen. Results of repeat testing using a common lot of Mueller-Hinton agar showed 52 minor errors (13.3%) and 4 major errors (4.2% of susceptible values of 1.0% pf total values) but no very major errors. It is recommended that any haze or colonies within the zone be taken into account when determining zones of inhibition and that an MIC test be performed for strains with intermediate zones if vancomycin is being considered for treatment.
Full text
PDF



Selected References
These references are in PubMed. This may not be the complete list of references from this article.
- Barry A. L., Thornsberry C., Jones R. N. Evaluation of teicoplanin and vancomycin disk susceptibility tests. J Clin Microbiol. 1986 Jan;23(1):100–103. doi: 10.1128/jcm.23.1.100-103.1986. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Bingen E., Lambert-Zechovsky N., Mariani-Kurkdjian P., Cezard J. P., Navarro J. Bacteremia caused by a vancomycin-resistant Enterococcus. Pediatr Infect Dis J. 1989 Jul;8(7):475–476. doi: 10.1097/00006454-198907000-00015. [DOI] [PubMed] [Google Scholar]
- Courvalin P. Resistance of enterococci to glycopeptides. Antimicrob Agents Chemother. 1990 Dec;34(12):2291–2296. doi: 10.1128/aac.34.12.2291. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Guiot H. F., Peetermans W. E., Sebens F. W. Isolation of vancomycin-resistant enterococci in haematologic patients. Eur J Clin Microbiol Infect Dis. 1991 Jan;10(1):32–34. doi: 10.1007/BF01967094. [DOI] [PubMed] [Google Scholar]
- Kaplan A. H., Gilligan P. H., Facklam R. R. Recovery of resistant enterococci during vancomycin prophylaxis. J Clin Microbiol. 1988 Jun;26(6):1216–1218. doi: 10.1128/jcm.26.6.1216-1218.1988. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Leclercq R., Derlot E., Duval J., Courvalin P. Plasmid-mediated resistance to vancomycin and teicoplanin in Enterococcus faecium. N Engl J Med. 1988 Jul 21;319(3):157–161. doi: 10.1056/NEJM198807213190307. [DOI] [PubMed] [Google Scholar]
- Lütticken R., Kunstmann G. Vancomycin-resistant Streptococcaceae from clinical material. Zentralbl Bakteriol Mikrobiol Hyg A. 1988 Jan;267(3):379–382. doi: 10.1016/s0176-6724(88)80054-3. [DOI] [PubMed] [Google Scholar]
- Sahm D. F., Kissinger J., Gilmore M. S., Murray P. R., Mulder R., Solliday J., Clarke B. In vitro susceptibility studies of vancomycin-resistant Enterococcus faecalis. Antimicrob Agents Chemother. 1989 Sep;33(9):1588–1591. doi: 10.1128/aac.33.9.1588. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Sahm D. F., Olsen L. In vitro detection of enterococcal vancomycin resistance. Antimicrob Agents Chemother. 1990 Sep;34(9):1846–1848. doi: 10.1128/aac.34.9.1846. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Shlaes D. M., Al-Obeid S., Shlaes J. H., Boisivon A., Williamson R. Inducible, transferable resistance to vancomycin in Enterococcus faecium, D399. J Antimicrob Chemother. 1989 Apr;23(4):503–508. doi: 10.1093/jac/23.4.503. [DOI] [PubMed] [Google Scholar]
- Shlaes D. M., Bouvet A., Devine C., Shlaes J. H., al-Obeid S., Williamson R. Inducible, transferable resistance to vancomycin in Enterococcus faecalis A256. Antimicrob Agents Chemother. 1989 Feb;33(2):198–203. doi: 10.1128/aac.33.2.198. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Swenson J. M., Hill B. C., Thornsberry C. Problems with the disk diffusion test for detection of vancomycin resistance in enterococci. J Clin Microbiol. 1989 Sep;27(9):2140–2142. doi: 10.1128/jcm.27.9.2140-2142.1989. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Uttley A. H., Collins C. H., Naidoo J., George R. C. Vancomycin-resistant enterococci. Lancet. 1988 Jan 2;1(8575-6):57–58. doi: 10.1016/s0140-6736(88)91037-9. [DOI] [PubMed] [Google Scholar]
- Uttley A. H., George R. C., Naidoo J., Woodford N., Johnson A. P., Collins C. H., Morrison D., Gilfillan A. J., Fitch L. E., Heptonstall J. High-level vancomycin-resistant enterococci causing hospital infections. Epidemiol Infect. 1989 Aug;103(1):173–181. doi: 10.1017/s0950268800030478. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Vincent S., Knight R. G., Green M., Sahm D. F., Shlaes D. M. Vancomycin susceptibility and identification of motile enterococci. J Clin Microbiol. 1991 Oct;29(10):2335–2337. doi: 10.1128/jcm.29.10.2335-2337.1991. [DOI] [PMC free article] [PubMed] [Google Scholar]