Comparison of a commercial disk test with vancomycin and colimycin susceptibility testing for identification of bacteria with abnormal gram staining reactions

In an effort to identify bacteria that fail to give the expected Gram react ion, thus leading to misidentification, two nonstaining tests for Gram reac tion, vancomycin and colimycin susceptibility testing and the Gram-Sure tes t (Remel, USA), were employed on 145 strains from 42 gram-negative and gram -positive genera with contradictory Gram stain results. The Gram-Sure test is a commercially available disk that detects the presence of L-alanine-ami nopeptidase, an enzyme usually found only in the cell wall of gram-negative bacteria. In this test, aminopeptidase activity is detected using a substr ate that can be hydrolyzed to produce a fluorescent compound under long-wav e UV light. The commercial disk test and vancomycin plus colimycin suscepti bility testing appeared to perform equally well except in the identificatio n of Erysipelothrix and Lactobacillus, for which the commercial disk test w as better, and Moraxella, for which vancomycin and colimycin susceptibility testing was more helpful. An advantage of the commercial disk test is that it can be performed in 10 min, whereas vancomycin and colimycin susceptibi lity testing requires at least 18 h. The commercial disk test is also less expensive than vancomycin and colimycin susceptibility testing. However, si nce the same results can be obtained with the 5 mu g and 30 mu g vancomycin disks, it is possible to use only one vancomycin disk, with the cost then being equivalent to that of the commercial disk test. The major inconvenien ce of the commercial disk test is the requirement of a UV ray. However, thi s test could be a useful tool for the identification of unusual organisms.