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Journal of Clinical Microbiology, October 2001, p. 3781-3784, Vol. 39, No. 10
Nosocomial Pathogens Laboratory Branch,
Centers for Disease Control and Prevention, Atlanta, Georgia
30333,1 and Department of
Microbiology, Massachusetts General Hospital, Boston, Massachusetts
021142
Received 5 April 2001/Returned for modification 1 June
2001/Accepted 18 July 2001
To define more precisely the inoculation methods to be used in the
oxacillin screen test for Staphylococcus aureus, we
tested agar screen plates prepared in house with 6 µg of oxacillin/ml and 4% NaCl using the four different inoculation methods that would
most likely be used by clinical laboratories. The organisms selected
for testing were 19 heteroresistant mecA-producing
strains and 41 non-mecA-producing strains for which
oxacillin MICs were near the susceptible breakpoint. The inoculation
method that was preferred by all four readers and that resulted in
the best combination of sensitivity and specificity was a 1-µl
loopful of a 0.5 McFarland suspension. A second objective of the study
was to then use this method to inoculate plates from five different
manufacturers of commercially prepared media. Although all commercial
media performed with acceptable sensitivity compared to the reference
lot, one of the commercial lots demonstrated a lack of specificity.
Those lots of oxacillin screen medium that fail to grow
heteroresistant strains can be detected by using S.
aureus ATCC 43300 as a positive control in the test and
by using transmitted light to carefully examine the plates for any
growth. However, lack of specificity with commercial lots may be
difficult to detect using any of the current quality control organisms.
0095-1137/01/$04.00+0 DOI: 10.1128/JCM.39.10.3781-3784.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Optimal Inoculation Methods and Quality Control for the
NCCLS Oxacillin Agar Screen Test for Detection of
Oxacillin Resistance in Staphylococcus
aureus
*
Corresponding author. Mailing address: CDC, Mailstop
G08, 1600 Clifton Rd., Atlanta, GA 30333. Phone: (404) 639-0196. Fax: (404) 639-1381. E-mail: jswenson{at}cdc.gov.
Journal of Clinical Microbiology, October 2001, p. 3781-3784, Vol. 39, No. 10
0095-1137/01/$04.00+0 DOI: 10.1128/JCM.39.10.3781-3784.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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