Analysis of Bacteroides species by pyrolysis mass spectrometry

Two methods for the classification of isolates of the Bacteroides fragilis group were compared in a study of 133 clinical isolates and 10 reference strains. There was broad similarity between the pattern of clusters in pyrolysis mass spectrometry (PyMS), which reflects whole-cell composition, and that in conventional tests, which reflect nutritional interactions with the environment. A single large cluster was resolved by both approaches; this cluster comprised mainly strains identified as B. fragilis and was surrounded by a multiplicity of small clusters (42 in PyMS and 24 in conventional tests). Individual clusters corresponding to strains identified as Bacteroides thetaiotaomicron and Bacteroides vulgatus, respectively, were found by both techniques. However, for the remainder of strains, little correlation was evident between clustering in PyMS and that in conventional tests. Further comparison with data obtained by sodium dodecyl sulfate- polyacrylamide gel electrophoresis for this collection of strains showed equal levels of disagreement between classification based on whole-cell protein and each of the classifications described here. In previous studies of other genera, PyMS-based and conventional test-based classifications showed good agreement. It appears that the B. fragilis group presents intractable problems in phenotypic characterization.