In this study we observed the proliferation of Pseudomonas fluorescens (P. fluorescens) in mouse organ homogenates at 4°C. P. fluorescens secreted a protease possessing properties different from those of the mammalian tissue proteases. The specificity of this protease required a basic amino acid residue at the P1 position at a pH optimum of 6.0. The specificity of the protease was similar to that of trypsin, but the pH optimum was different. The protease mildly degraded elastin-Congo red; this suggests that the protease serves as an alternative for elastase in the case of P. fluorescens strains that lack virulent elastase. The protease was identified as an alkaline protease of P. fluorescens by liquid chromatography-tandem mass spectrometry analysis. Our results show that proteome analysis of the soluble proteins is useful in identifying bacterial species, particularly the bacterial contaminants in samples containing antibiotics.

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