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An evaluation of susceptibility testing methods for ampicillin‐sulbactam using a panel of β‐lactamase‐producing bacteria
Author(s) -
Siu L. K.,
Lo J. Y. C.,
Cheng W. L.,
Ho P. L.,
Ng W. S.,
Chau P. Y.
Publication year - 1999
Publication title -
apmis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.909
H-Index - 88
eISSN - 1600-0463
pISSN - 0903-4641
DOI - 10.1111/j.1699-0463.1999.tb01464.x
Subject(s) - sulbactam , ampicillin , agar dilution method , agar dilution , bacteria , microbiology and biotechnology , dilution , agar , agar diffusion test , biology , chemistry , antibiotics , minimum inhibitory concentration , antibiotic resistance , antibacterial activity , genetics , physics , thermodynamics , imipenem
Bacteria possessing TEM‐1‐like β‐lactamases are generally regarded as susceptible to ampicillin‐sulbactam (SAM), while those harboring OXA‐1 enzymes are considered resistant. The current study was undertaken to compare susceptibility testing using various combinations of ampicillin and sulbactam to improve clinical correlation. Members of the Enterobaeteriaceae family harboring TEM‐1, SHV‐1 or OXA‐1‐like β‐lactamases were tested using the agar dilution method. A substantial proportion of strains harboring OXA‐1‐like β‐lactamases showed false susceptibility to SAM at the 1:1 ratio or fixed sulbactam concentration of 8 μg/ml. At a fixed sulbactam concentration of 4 μg/ml, the activity of ampicillin‐sulbactam appeared to be reduced, with large numbers of TEM‐1 producers becoming frankly resistant. Results obtained with the 2:1 ratio exhibited the closest correlation with that obtained by the currently recommended disk diffusion test. However, very major errors were still found between the disk diffusion test and agar dilution test, suggesting the necessity for consideration of a change in criteria for interpretation of disk diffusion test results. In conclusion, SAM susceptibility testing by agar dilution using other than a 2:1 ratio is not recommended and results should be interpreted with caution.