Comparative analysis of a modified ecolite method, the colicomplete method, and a most-probable-number method for detecting Escherichia coli in orange juice.

Abstract

The Ecolite High Volume Juice (HVJ) presence-absence method for a 10-ml juice sample was compared with the U.S. Food and Drug Administration Bacteriological Analytical Manual most-probable-number (MPN) method for analysis of artificially contaminated orange juices. Samples were added to Ecolite-HVJ medium and incubated at 35 degrees C for 24 to 48 h. Fluorescent blue results were positive for glucuronidase- and galactosidase-producing microorganisms, specifically indicative of about 94% of Escherichia coli strains. Four strains of E. coli were added to juices at concentrations of 0.21 to 6.8 CFU/ ml. Mixtures of enteric bacteria (Enterobacter plus Klebsiella, Citrobacter plus Proteus, or Hafnia plus Citrobacter plus Enterobacter) were added to simulate background flora. Three orange juice types were evaluated (n = 10) with and without the addition of the E. coli strains. Ecolite-HVJ produced 90 of 90 (10 of 10 samples of three juice types, each inoculated with three different E. coli strains) positive (blue-fluorescent) results with artificially contaminated E. coli that had MPN concentrations of <0.3 to 9.3 CFU/ml. Ten of 30 E. coli ATCC 11229 samples with MPN concentrations of <0.3 CFU/ml were identified as positive with Ecolite-HVJ. Isolated colonies recovered from positive Ecolite-HVJ samples were confirmed biochemically as E. coli. Thirty (10 samples each of three juice types) negative (not fluorescent) results were obtained for samples contaminated with only enteric bacteria and for uninoculated control samples. A juice manufacturer evaluated citrus juice production with both the Ecolite-HVJ and Colicomplete methods and recorded identical negative results for 95 20-ml samples and identical positive results for 5 20-ml samples artificially contaminated with E. coli. The Ecolite-HVJ method requires no preenrichment and subsequent transfer steps, which makes it a simple and easy method for use by juice producers.