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Project: Comparison among IDEXX, Membrane Filtration, and Multiple Tube Fermentation Bacterial Indicator Analysis Methods

Background and Objectives

Indicator bacteria have historically been measured using either membrane filtration (MF) or multiple tube fermentation (MTF). In the late 1990s, IDEXX Laboratories introduced chromogenic substrate (CS) methods that had the allure of providing results in as little as 18 hours compared to the 24 to 96 hours required for traditional methods. These were easier to implement and had the additional advantage of being less expensive than traditional methods.

To assess whether all three methods could be used interchangeably, SCCWRP coordinated a series of intercalibration studies involving 26 microbiological water quality laboratories in Southern California. These studies were conducted as part of the Bight Regional Monitoring Program quality assurance efforts.

Membrane filtration is one traditional method for enumerating fecal indicator bacteria, where the water sample is filtered and the filter placed on a growth medium for cultivation and visible quantification of bacterial colonies.


These studies were conducted between 1997 and 2004.


Three separate method evaluation studies were done, each involving redistribution of the same samples among multiple laboratories. Each laboratories received the sample in the morning and began processing it at the same time to eliminate differences due to holding time. Samples were analyzed for total coliform, fecal coliform, Escherichia coli, and enterococci, using the method or methods routinely performed by each laboratory. Only three types of methods were used: MF, MTF, and CS. Each laboratory analyzed multiple dilutions of each sample to minimize the number of samples occurring outside of a quantifiable range. All analyses were performed in triplicate. This study design allowed comparison of (1) within-laboratory/within-method variability, (2) within-laboratory/across-method variability and (3) across-laboratory/across-method variability.


• CS results were found to be highly correlated with both MF and MTF regardless of where and when samples were collected.

• No significant differences among methods were observed for enterococci, although variability was considerably higher for MTF than for the other two methods.

• Fecal coliform values were on average 12% lower when measured with CS, reflecting specialization of the CS method for E. coli rather than the entire fecal coliform group.

• Though correlated, total coliform values were typically higher using the CS method, consistent with other studies that have demonstrated false positives with this method.

• The differences among methods for some indicators were small enough that, when assessed categorically, there was more than 90% agreement as to whether State of California Beach Water Quality Standards were met between CS methods and either MF or MTF methods.

• Within-laboratory/within-method variance associated with these methods was considerably larger than any among-method bias.


Los Angeles Department of Public Works
Heal the Bay
Malibu Bacteria Source Identification Task Force

For more information on Comparison among IDEXX, Membrane Filtration and Multiple Tube Fermentation Bacterial Indicator Analysis Methods, contact John Griffith at (714) 755-3228.
This page was last updated on: 7/2/2014