In vitro evaluation of two types of moxalactam disks revealed significant performance differences when Staphylococcus aureus was being tested. The differences were traced to the amount of decarboxylated moxalactam present in the disks. The decarboxylated analog was much more active than the parent compound against S. aureus, not active against Pseudomonas aeruginosa, and approximately as active as the parent compound against Escherichia coli. A nine-laboratory coordinated study was performed to establish quality control parameters for 30-micrograms moxalactam disks. Problems with the establishment of interpretive standards for moxalactam disk tests were evaluated in the light of differences between disks utilized in earlier studies and those that are now commercially available. The type of disk greatly influences standards for tests with S. aureus but has insignificant influence on testing gram-negative bacilli
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