Comparison of conventional direct and enrichment culture methods for Erysipelothrix spp. from experimentally and naturally infected swine

The objective of the current study was to compare the diagnostic performance of a direct isolation method for Erysipelothrix spp. with a broth-based enrichment technique. Samples were obtained from three sources: 1) experimentally inoculated pigs, 2) porcine tissue samples submitted to the Iowa State University Veterinary Diagnostic Laboratory (Ames, IA), and 3) tissues from condemned carcasses at an abattoir. Culture plates from direct isolation and broth-based technique were evaluated for growth at 24 and 48 hr. Results indicated that the broth enrichment method was markedly more sensitive for the isolation of Erysipelothrix spp. To the authors' knowledge, this is the first comparison of direct culture and broth-based enrichment methods for the isolation of Erysipelothrix spp. Interestingly, in several samples, a Gram-positive bacterium with almost identical growth characteristics to Erysipelothrix spp. was detected and identified as a Vagococcus sp. through 16S ribosomal RNA gene sequencing. The results of this study indicate that the brothbased enrichment method should be used for the isolation of Erysipelothrix spp. from clinical samples with a history suggestive of erysipelas and that Vagococcus spp. is potentially an important differential diagnosis.