Abstract
Background
Cinical and Laboratory Standards Institute (CLSI) recommends the use of cefoxitin disks instead of long-used oxacillin disks for screening methicillin-resistant isolates of staphylococci. The frequency of discrepant results and accuracy of the tests were evaluated by detecting mecA gene.
Methods
A total of 3,123 Stapylococci isolates from patients in Severance Hospital were tested during September 2005 to August 2006 by the CLSI-recommended test using both cefoxitin and oxacillin disks. The mecA gene was detected by PCR and the oxacillin minimum inhibitory concentration (MIC) was determined by using agar dilution method for the isolates with discrepant tests.
Results
Among 1,915 S. aureus islolates tested, one isolate was resistant to oxacillin disk but susceptible to cefoxitin disk; the isolate did not have mecA gene. Another isolate susceptible to oxacillin but resistant to cefoxitin had mecA gene. Among 1,208 coagulase-negative staphylococcal isolates, 15 isolates were resistant to oxacillin disk but susceptible to cefoxitin disk; the isolates did not have mecA genes. Two isolates susceptible to oxacillin disk but resistant to cefoxitin disk had mecA genes. Among the 16 Staphylococcus isolates that did not have mecA gene, 15 isolates had the oxacillin MICs of ≤2 μg/mL and were considered as methicillin-susceptible, while 1 isolate with the MIC of 4 μg/mL was considered as methicillin-resistant.
Conclusions
Overall, 1.9% of staphylococcal isolates showed discrepant results when the screening tests were performed by using oxacillin and cefoxitin disks. None of the isolates resistant to oxacillin disk but susceptible to cefoxitin disk had mecA gene. In conclusion, the cefoxitin disk test is more reliable than oxacillin disk test in screening methicillin-resistant staphylococcal isolates.
References
1. Kim JS, Kim HS, Song WK, Cho HC, Lee KM, Kim EC. Antimicrobial resistance profiles of Staphylococcus aureus isolated in 13 Korean hospitals. Korean J Lab Med. 2004; 24:223–9.
2. Swenson JM, Tenover FC, Cefoxitin Disk Study Group. Results of disk diffusion testing with cefoxitin correlate with presence of mecA in Staphylococcus spp. J Clin Microbiol. 2005; 43:3818–23.
3. Frigatto EA, Machado AM, Pignatari AC, Gales AC. Is the cefoxitin disk test reliable enough to detect oxacillin resistance in coagulase-negative staphylococci? J Clin Microbiol. 2005; 43:2028–9.
![crossref](/image/icon/bnr_ref_cross.gif)
![crossref](/image/icon/bnr_ref_cross.gif)
4. Clinical and Laboratory Standards Institute. Performances standards for antimicrobial susceptibility testing. Sixteenth Informational supplement, M100-S17. Wayne, PA: Clinical and Laboratory Standards Institute;2007.
5. Hussain Z, Stoakes L, Massey V, Diagre D, Fitzgeral V, El Sayed S, et al. Correlation of oxacillin MIC with mecA gene carriage in coagulase-negative staphylococci. J Clin Microbiol. 2000; 38:752–4.
6. Mateo M, Maestre JR, Aguilar L, Cafini F, Puente P, Sanchez P, et al. Genotypic versus phenotypic characterization, with respect to susceptibility and identification, of 17 clinical isolates of Staphylococcus lugdunensis. J Antimicrob Chemother. 2005; 56:287–91.
7. Hellbacher C, Tornqvist E, soderquist B. Staphylococcus lugdunensis: clinical spectrum, antibiotic susceptibility, and phenotypic and genotypic patterns of 39 isolates. Clin Microbiol Infect. 2006; 12:43–9.
8. Araj GF, Talhouk RS, Simaan CJ, Maasad MJ. Discrepancies between mecA PCR and conventional tests used for detection of methicillin resistance Staphylococcus aureus. Int J Antimicrob Agents. 1999; 11:47–52.
9. Velasco D, del Mar Tomas M, Cartelle M, Beceiro A, Perez A, Molina F, et al. Evaluation of different methods for detecting methicillin (oxacillin) resistance in Staphylococcus aureus. J Antimicrob Chemother. 2005; 55:379–82.
10. Cauwelier B, Gordts B, Descheemaecker P, Van Landuyt H. Evaluation of a disk diffusion method with cefoxitin (30 microg) for detection of methicillin-resistant Staphylococcus aureus. Eur J Clin Microbiol Infect Dis. 2004; 23:389–92.
11. Bannerman TL. Staphylococcus, Micrococcus and other catalase-positive cocci that grow aerobically. In. Murry PR, editor. Manual of clinical microbiology. 8th ed.Washington, DC: ASM Press;2003. p. 396–7.
12. Lee MK, Choi YS, Chong YS, Lee SY. Prevalence of Methicillin-resistant Staphylococcus aureus and comparison of susceptibility test methods for its detection. Korean J Clin Pathol. 1987; 7:265–73.
13. Lee CK, Ma KR, Lee DH, Whang SC, Kim YK, Lee KN. Detection of methicillin resistance in Staphylococcus aureus by 2% NaCl Mueller-Hinton agar and mannitol salt agar. Korean J Infect Dis. 1998; 30:539–44.
14. Kim SY, Park YJ, Kim BK. Evaluation of oxacillin-salt agar screen test for detection of methicillin-resistant Staphylococcus aureus. Korean J Clin Pathol. 2001; 21:377–80.
Table 1.
Frequency of cefoxitin and oxacillin disk tests for screening methicillin-resistant Stapylococcus
Cefoxitin |
S. aureus (N=1,915) (%) |
CNS (N=1,208) (%) |
||
---|---|---|---|---|
Oxa-R | Oxa-S | Oxa-R | Oxa-S | |
Resistant | 1,121 (58.5) | 7 (0.4) | 768 (63.6) | 6 (0.5) |
Susceptible | 14 (0.7) | 773 (40.4) | 42 (3.5) | 392 (32.4) |
Table 2.
Detection of mecA in strains with discrepant oxacillin and cefoxitin disk tests