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Applied and Environmental Microbiology, February 2002, p. 539-544, Vol. 68, No. 2
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.2.539-544.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Marine Bacteria Cause False-Positive Results in the Colilert-18 Rapid Identification Test for Escherichia coli in Florida Waters

John M. Pisciotta,1 Damon F. Rath,2 Paul A. Stanek,2 D. Michael Flanery,2 and Valerie J. Harwood1*

Department of Biology, University of South Florida, Tampa, Florida 33620-5150,1 Pinellas County Health Department, Florida Department of Health, Clearwater, Florida 337642

Received 9 August 2001/ Accepted 5 November 2001

The Colilert-18 system for enumeration of total coliforms and Escherichia coli is approved by the U.S. Environmental Protection Agency for use in drinking water analysis and is also used by various agencies and research studies for enumeration of indicator organisms in fresh and saline waters. During monitoring of Pinellas County, Fla., marine waters, estimates of E. coli numbers (by Colilert-18) frequently exceeded fecal coliform counts (by membrane filtration) by 1 to 3 orders of magnitude. Samples from freshwater sites did not display similar discrepancies. Fecal coliforms, including E. coli, could be cultured from 100% of yellow fluorescent wells (denoting E. coli-positive results) inoculated with freshwater samples but could be cultured from only 17.1% of the "positive" wells inoculated with marine samples. Ortho-nitrophenyl-ß-D-galactopyranoside (ONPG)-positive or 4-methylumbelliferyl-ß-D-glucuronide (MUG)-positive noncoliform bacteria were readily cultured from Colilert-18 test wells inoculated with marine samples. Filtered cell-free seawater did not cause false positives. Coculture preparations of as few as 5 CFU of Vibrio cholerae (ONPG positive) and Providencia sp. (MUG positive) ml-1 inoculated into Colilert-18 caused false-positive E. coli results. Salinity conditions influenced coculture results, as the concentration of coculture inoculum required to cause false positives in most wells increased from about 5 CFU ml-1 in seawater diluted 1:10 with freshwater to {approx}5,000 CFU ml-1 in seawater diluted 1:20 with freshwater. Estimated E. coli numbers in various marine water samples processed at the 1:10 dilution ranged from 10 to 7,270 CFU·100 ml-1, while E. coli numbers in the same samples processed at the 1:20 dilution did not exceed 40 CFU·100 ml-1. The lower estimates of E. coli numbers corresponded well with fecal coliform counts by membrane filtration. This study indicates that assessment of E. coli in subtropical marine waters by Colilert-18 is not accurate when the recommended 1:10 sample dilution is used. The results suggest that greater dilution may diminish the false-positive problem, but further study of this possibility is recommended.

* Corresponding author. Mailing address: Department of Biology, SCA 110, University of South Florida, 4202 E. Fowler Ave., Tampa, FL 33620. Phone: (813) 974-1524. Fax: (813) 974-3263. E-mail: vharwood{at}chuma1.cas.usf.edu.

Applied and Environmental Microbiology, February 2002, p. 539-544, Vol. 68, No. 2
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.2.539-544.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Chao, K.-K., Chao, C.-C., Chao, W.-L. (2004). Evaluation of Colilert-18 for Detection of Coliforms and Eschericha coli in Subtropical Freshwater. Appl. Environ. Microbiol. 70: 1242-1244 [Abstract] [Full Text]  


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