A sandwich enzyme-linked immunosorbent assay (ELISA) for detection of Pseudomonas fluorescens and related psychrotrophic bacteria in refrigerated milk

Polyclonal antibodies against protein F from the cell envelope of Pseudomonas fluorescens for the detection of psychrotrophic bacteria in refrigerated meat using an indirect ELISA

An indirect enzyme-linked immunosorbent assay (ELISA) has been developed for detection of Pseudomonas fluorescens and related psychrotrophic bacteria in refrigerated meat. The ELISA uses polyclonal antibodies raised in rabbits against protein F from the cell envelope of Pseudomonas fluorescens AH-70. The anti-protein F antibodies were recovered from the crude antiserum by ammonium sulfate precipitation and conjugated to biotin. Commercial Extr Avidin-peroxidase conjugate was used to detect the biotinylated antibodies bound to their specific antigens. Subsequent enzymatic conversion of substrate gave distinct absorbance differences when assaying meat samples containing P. fluorescens strains of different origin as well as related psychrotrophic microorganisms. The detection threshold for the ELISA assay developed in this work was 10(4)-10(5) cfu cm(-2).

Outer Membrane Protein Heterogeneity within Pseudomonas fluorescens and P. putida and Use of an OprF Antibody as a Probe for rRNA Homology Group I Pseudomonads

The electrophoretic patterns of outer membrane proteins of strains representing the biovars of Pseudomonas fluorescens and Pseudomonas putida were analyzed by gel electrophoresis. The outer membrane protein profiles were variable, and they were not useful for assigning strains to a specific biovar. However, three or four predominant outer membrane proteins migrating at 42 to 46 kDa, 33 to 38 kDa, and 20 to 22 kDa were conserved among the strains. They could be tentatively identified as OprE (44 kDa), OprF (38 kDa), OprH (21 kDa), and OprL (20.5 kDa), which are known proteins from Pseudomonas aeruginosa. A 37-kDa OprF-like protein was purified from P. fluorescens DF57 and used to raise a polyclonal antibody. In Western blot (immunoblot) analysis, this antibody reacted with OprF proteins from members of Pseudomonas rRNA homology group I but not with proteins from nonpseudomonads. The heterogeneity in M(infr) of OprF was greater among P. fluorescens strains than among P. putida strains. Immunofluorescence microscopy of intact cells demonstrated that the antibody recognized epitopes that were accessible only after unmasking by EDTA treatment. The antibody was used in a colony blotting assay to determine the percentage of rRNA homology group I pseudomonads among bacteria from the rhizosphere of barley. The bacteria were isolated on 10% tryptic soy agar, King's B agar, and the pseudomonad-specific medium Gould S1 agar. The estimate of OprF-containing CFU in rhizosphere soil obtained by colony blotting on 10% tryptic soy agar was about 2 and 14 times higher than the values obtained from King's agar and Gould S1 agar, respectively, indicating that not all fluorescent pseudomonads are scored on more specific media. The colonies reacting with the OprF antibody were verified as being rRNA homology group I pseudomonads by using the API 20NE system.

Culturability and Expression of Outer Membrane Proteins during Carbon, Nitrogen, or Phosphorus Starvation of Pseudomonas fluorescens DF57 and Pseudomonas putida DF14

Changes in culturability and outer membrane protein profiles were investigated in Pseudomonas fluorescens DF57 and Pseudomonas putida DF14 during starvation for carbon, nitrogen, and phosphorus. P. fluorescens DF57 remained fully culturable for 4 days in all starvation regimes. The cell mass increased during starvation for nitrogen and phosphorus, indicating the accumulation of storage compounds, whereas it decreased slightly in carbon-starved cells. P. putida DF14 lost culturability during phosphorus starvation, and the mass of phosphate-starved cells did not increase. Analysis of additional P. fluorescens and P. putida strains, however, showed that the ability to preserve culturability during phosphorus starvation was not species but strain dependent. In DF57, an outer membrane protein of 55 kDa appeared during starvation for phosphorus, while another protein of 63 kDa was seen during all starvation conditions. DF14 induced two outer membrane proteins of 28 and 29 kDa during starvation for carbon and nitrogen, but no phosphorus-specific starvation protein could be detected. Therefore, starvation-induced outer membrane proteins do not seem to be conserved among the fluorescent pseudomonads and a unique starvation response might be found in individual strains.