title: Utilization of fluorogenic assay for rapid detection of Escherichia coli in acidic fruit juice.
abstract: This study was undertaken to investigate interference by acids commonly found in fruit juice in Escherichia coli assays involving the use of 4-methylumbelliferyl-beta-D-glucuronide (MUG) as a fluorogenic substrate for enzyme reaction. Fluorescence intensity was negatively correlated (P < 0.001) with the volume of fresh citrus juice tested by the lauryl tryptose broth (LST)-MUG assay, and the permissible sample sizes were limited to 0.3 and 0.5 ml for fresh citrus juices with pHs of 3.3 and 3.9, respectively. In addition, false-negative results were visually observed under UV light when the E*Colite assay was used to test large volumes (5 to 10 ml per test) of fresh citrus juice or when the test broth used for the LST-MUG assay was supplemented with citric, malic, or tartaric acid at 2 to 4 g/liter. These results suggest that the size and pH of acidic samples should be controlled in MUG-based fluorogenic assays. The inhibitory effect on fluorescence was due to high acidity, which reduces fluorescence from 4-methylumbelliferone. Buffering improved the assays. When sodium bicarbonate was incorporated in the enrichment broth at 10 g/liter, the permissible sample sizes for fresh grapefruit juice (pH 3.1) increased from 0.3 to 1 ml for the LST-MUG (with 9.9 ml of broth) assay and from 3 to 10 ml for the E*Colite (with 99 ml of broth) assay.
authors.affiliation: Virginia State University, Agricultural Research Station, P.O. Box 9061, Petersburg, Virginia 23806, USA. spao@vsu.edu
authors.forename: Steven
authors.initials: S
authors.lastname: Pao
authors.forename: Craig L
authors.initials: CL
authors.lastname: Davis
authors.forename: Loretta M
authors.initials: LM
authors.lastname: Friedrich
authors.forename: Mickey E
authors.initials: ME
authors.lastname: Parish
mesh_terms: D001431:Bacteriological Techniques; D001628:Beverages / Q000032:analysis / Q000382:microbiology*; D004926:Escherichia coli / Q000201:enzymology / Q000302:isolation & purification*; D005188:False Negative Reactions; D005456:Fluorescent Dyes; D005516:Food Microbiology; D006863:Hydrogen-Ion Concentration; D006923:Hymecromone / Q000031:analogs & derivatives*; D018401:Sample Size
chemical_list: D005456:Fluorescent Dyes; D006923:Hymecromone; C034888:4-methylumbelliferyl glucuronide