Evaluation of antibiotic susceptibility testing by agar dilution and the Micro Media system (Fox Panel)

Abstract

The susceptibilities of 350 gram-positive cocci and 638 gram-negative bacilli to various antimicrobial agents were compared by using the Micro-Media system (MMS) (Fox Panel) (Micro-Media Systems, Inc., Potomac, Md.) and a standard agar dilution procedure. Major discrepancies occurred with enterococci, among which 48 of 53 isolates (91%) were found to be resistant to penicillin G by agar dilution and reported as susceptible by the MMS. Other large discrepancies occurred with Staphylococcus aureus and Acinetobacter calcoaceticus subsp. anitratus, among which more than 40% of the isolates were judged to be resistant to ampicillin by agar dilution and susceptible by the MMS. In terms of overall agreement in interpretation of MICs by the two systems, an agreement of greater than 84% was seen for both gram-positive and gram-negative organisms when ampicillin and cephalothin (68 and 78% agreement for gram-positive cocci, respectively) were excluded. These disagreements in MIC interpretations may result in part from the small number of organisms tested per well (4,000 CFU) in the MMS, as compared with 10,000 CFU per test in the agar dilution method.

Measurement of Antimicrobial-Resistant Escherichia coli in Pig Feces with a Hydrophobic Grid Membrane Filter Interpreter System

“…The overall observed agreements between HGMF-RES and Repliscan for AMP, SUL, and TET not only surpassed the criteria of Rand et al (23) of 0.8 for a resistance test to be considered useful but were comparable to those reported between Food and Drug Administration-approved methods (5,32). A major limitation of observed agreement as a measure of accuracy is that it does not correct for the agreement that occurs due to chance nor does it indicate a test's ability to discriminate between susceptible and resistant isolates, i.e., the test's relative specificity and sensitivity, respectively (15).…”

“…Sensitivity and specificity are used here in the epidemiological sense and describe the proportions of isolates correctly classified by HGMF-RES as resistant and nonresistant, respectively, relative to NCCLS-accepted, designated "gold standard" tests (15). Observed agreement is a parameter that is often used to measure agreements among antimicrobial susceptibility tests (3,5,22,28,32). Degener et al (5) categorized observed agreement as follows: satisfactory, Ͼ95%; moderate, 85 to 95%; insufficient, Ͻ85%.…”