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False-positive fluorescence by pink salmon tissue and staphylococci in a rapid test for Escherichia coli
B H Himelbloom1, R C Pfutzenreuter
1University of Alaska Fairbanks Fishery Industrial Technology Center, Alaska 99615-7401, USA. ffbhh@uaf.edu
Abstract:
Fluorescence from 4-methylumbelliferyl-beta-D-glucuronide (MUG) hydrolysis is a common, rapid method for determining Escherichia coli in water and food. False-positive fluorescence occurred when either pink salmon fillets were tested or beta-glucuronidase-positive Staphylococcus species were present in other fish products. Salmon fillet, E. coli, S. xylosus, and S. warneri produced 2, 17, 39, and 43 nmol of 4-methylumbelliferone per ml, respectively, in a one-step detection broth (lauryl salts tryptose broth with MUG) for E. coli after 48 h at 35 degrees C. These false-positive reactions need to be considered when testing fish products, especially those contaminated through human handling.
Insights
The 4-methylumbelliferyl-beta-D-glucuronide (MUG) test for Escherichia coli can yield false positives in fish products. Beta-glucuronidase-positive Staphylococcus species in fish can interfere with accurate E. coli detection.
Area of Science:
- Food Microbiology
- Analytical Chemistry
- Environmental Science
Background:
- The 4-methylumbelliferyl-beta-D-glucuronide (MUG) assay is a standard method for detecting Escherichia coli.
- This fluorogenic substrate is hydrolyzed by the enzyme beta-glucuronidase, produced by E. coli.
- Potential interferences in complex matrices like fish products require investigation.
Purpose of the Study:
- To evaluate the specificity of the MUG assay for Escherichia coli detection in fish products.
- To identify potential sources of false-positive results in the MUG assay when applied to seafood.
Main Methods:
- Testing of pink salmon fillets using the MUG assay.
- Inclusion of beta-glucuronidase-positive Staphylococcus species (S. xylosus, S. warneri) in the detection broth.
- Quantification of 4-methylumbelliferone production under specific incubation conditions (48h, 35°C).
Main Results:
- False-positive fluorescence was observed with pink salmon fillets.
- Beta-glucuronidase-positive Staphylococcus species also induced false-positive reactions.
- Specific levels of 4-methylumbelliferone were produced by S. xylosus and S. warneri, comparable to E. coli.
Conclusions:
- The MUG assay may produce false positives when analyzing fish products, particularly those contaminated via human handling.
- Beta-glucuronidase activity from non-E. coli bacteria, such as Staphylococcus species, can lead to inaccurate results.
- Careful consideration of potential interferences is crucial for reliable E. coli testing in seafood.