Utilization of fluorogenic assay for rapid detection of Escherichia coli in acidic fruit juice.
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.
Pao
Steven
S
Virginia State University, Agricultural Research Station, P.O. Box 9061, Petersburg, Virginia 23806, USA. spao@vsu.edu
Davis
Craig L
CL
Friedrich
Loretta M
LM
Parish
Mickey E
ME
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
D005456:Fluorescent Dyes; D006923:Hymecromone; C034888:4-methylumbelliferyl glucuronide