Comparative analysis of a modified rapid presence/absence test and the standard MPN method for detectingEscherichia coli in orange juice

A modified rapid presence/absence test was evaluated and compared to the standard most probable number (MPN) method for detecting Escherichia coli in artificially contaminated orange juice. In each of the four experiments conducted, pasteurized and unpasteurized orange juice samples were seeded with one of the three different strains ofE. coli , at levels ranging from 0·4 to 6·5 cfu ml−1. The samples were also seeded with 360–510 cfu ml−1 of other enteric bacteria to simulate background flora. Samples were analysed by the MPN method for E. coli and by the modified ColiComplete (CC) presence/absence test in E. coli (EC) broth at 44·5°C, after pre-enriching 10 ml of juice samples in Universal Pre-enrichment Broth for 24 h (modified CC method). Of the 12 comparative analyses performed, E. coli was detected in all 12 tests by the modified CC method and, furthermore, showed the presence of E. coli in 59 of the 60 (98·3%) orange juice replicates that were examined. In contrast, the standard MPN method was only able to quantify detectable levels of E. coli in eight of 12 tests. The modified CC procedure was faster, required less media and reagents, enabled analysis of 10 ml samples and was more reliable than the standard MPN method for determining the presence or absence of E. coli in artificially contaminated orange juice.