Accuracy of the disk method for determining antimicrobic susceptibility of common Gram-negative bacilli |
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Authors: | Arthur L Barry Lee J Effinger |
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Institution: | (1) Microbiology Laboratories, University of California Davis Medical Center, 95817 Sacramento, California, USA;(2) Departments of Pathology and Internal Medicine, School of Medicine, University of California, 95616 Davis, California, USA |
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Abstract: | A standardized disk susceptibility test was evaluated by comparing results with minimal inhibitory concentrations obtained
with agar dilution methods. The agar overlay method was used to test 152 Gram-negative bacilli against eight different antimicrobial
agents. One to 3% of the isolates were resistant to an antimicrobic by the MIC method, but appeared to be susceptible by the
disk method. Most very major discrepancies involved disk tests withProteus sp., a microorganism notoriously difficult to test reproducibly.Serratia sp. vs. the polymyxins andKlebsiella sp. vs. nitrofurantoin accounted for most other major discrepancies. With other microorganism-drug combinations, the disk
test was a reasonably accurate technique for classifying bacteria into resistant or susceptible categories. Gentamicin disk
tests were unsatisfactory, but when an intermediate zone category of 13–16 mm was applied, the false susceptible test results
were reduced to 2.6%. Intermediate zone sizes were obtained with 6% of the disk tests; most of those isolates were resistant
or susceptible but not intermediate in susceptibility. About 11% of the strains had intermediate MICs (5–20% with different
drugs), but most of those strains were fully susceptible by the disk technique. *** DIRECT SUPPORT *** A01R4011 00007 |
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