Escherichia coli are Gram negative rod-shaped organisms found naturally in the lower intestines of warm blooded organisms. Most serotypes of this organism are relatively harmless, making up a small percentage of bacterial colonization in the gut. These serotypes prevent the establishment of pathogenic strains.
E. coli is one of the most common bacteria found in the gut of animals. This includes humans. There are other animal species that contain this organism in the gut including reptiles and fish. E. coli colonizes the gut and can cause infection in the urinary tract and brain stem (meningitis) as well as intestinal diseases referred to as gastroenteritis. There are five classes of E. coli that produce disease. The most serious disease is the Enterohemorrhagic (EHEC) class. These organisms can cause diarrhea distinct from some others (including Shigella) in that there is copious bloody discharge and no fever. The life threatening situation is its toxic effects on the kidneys (hemolytic uremia).
Why Test for the Presence of E. coli?
E. coli is often used as indicator organisms to test the effectiveness of effluent disinfection in a wastewater treatment plant, on animal products as well as in nutraceutical and pharmaceutical products. While these organisms are generally harmless, they do live under the same conditions that human pathogens live. Since we cannot test for every pathogen, we test for easily detectable indicator organisms. The assumption is that if we kill the indicator organisms then we most likely kill the pathogens during effluent disinfection. E. coli has reemerged as an indicator, partly facilitated by the introduction of newer methods that can rapidly identify E. coli.
The current methodology can take anywhere from 3-7 days and includes multiple broths, agars,
transfers and temperatures. Different methodologies are utilized by the various industries. Below
are some examples:
Nutraceutical and Dietary supplements: The protocol described in USP <2022> requires
absence of the organisms in 10 grams of product. A 1:10 dilution of product is made into TSB or
another enrichment medium. This mixture is incubated for 24 to 48 hours at 30°C to 35°C, and
then 1.0 mL is transferred to 10 mL of MacConkey broth. This mixture is incubated for 24 to
48 hours at 42°C to 44°C. A loop from the MacConkey broth is transferred to MacConkey agar
and the plate is incubated for 18 to 24 hours at 30°C to 35°C. If typical colonies appear, these
colonies are then transferred to Levine Eosin Methylene Blue agar and incubated 24 to 48 hours
at 30°C to 35°C. If none of the colonies exhibit green metallic sheen under reflected light or if
none of the colonies exhibit a blue-black appearance under transmitted light, the sample meets
the requirement for the absence of Escherichia coli. Because results can sometimes be misread
due to interpretation of plates, identification may be run on the sample(s) adding another 2-3
days of testing.
Pharmaceutical: The protocol described in USP <62> again requires the absence of E. coli in
10 grams of product. A 1:10 dilution is made and incubated for 18 to 24 hours at 30°C to 35°C.
From this mixture, 1.0 mL is transferred to 100 mL of MacConkey broth and incubated 24 to 48
hours at 42°C to 44°C. A loop is transferred to MacConkey agar and this plate is incubated 18 to
72 hours at 30°C to 35°C. Growth of typical colonies indicates the presence of E. coli which is
confirmed with an identification test.
Food Testing: The food industry follows Bacteriological Analytical Manual (BAM). In most
cases, the Most Probable Number (MPN) method is utilized. This is labor intensive, multi-
step assay consists of presumptive (in LST tubes), confirmed (in BGLB tubes), and completed
phases (in EC tubes). In the assay, serial dilutions of a sample are inoculated into broth media.
Analysts score the number of gas positive (fermentation of lactose) tubes, from which the other 2
phases of the assay are performed and then uses the combinations of positive results to consult a
statistical table. From this table, the analyst is able to estimate the number of organisms present.
The 3-tube MPN test is used for testing most foods. The 5-tube MPN is used for water, shellfish
and shellfish harvest water testing, and there is also a 10-tube MPN method that is used to test
bottled water or samples that are not expected to be highly contaminated. Positive EC tubes
must be transferred to L-EMB agar and if typical colonies are observed, they must be further
The BioLumix E. coli Vial
The E. coli vial, or EC vial, is a membrane vial that is monitor by the fluorescent
signal in the instrument. The membrane separates the incubation zone from the
reading zone. If E. coli is present, it utilizes MUG (4-Methylumbelliferyl-3-D-
Glucuronide) through an enzymatic reaction to create fluorescence.
A 1:10 dilution is made in TSB or a similar growth medium and this mixture is
incubated for 18-24 hours at 35°C. From the TSB dilution, 0.1 mL is added to an
EC vial containing the MUG supplement and then tested in the BioLumix instrument. The EC
assay runs for 18 hours in the instrument. An
example of E. coli curves is shown in the graph. If
detection occurs, a confirmation test is performed.
From the time the sample is prepared to the time
the confirmation is complete, the EC test takes
only 2 days, saving valuable time.
The BioLumix assay is much simpler to perform, requiring less labor and disposables as any to
the standard method. It is faster (completed in 30 hours), accurate, automated, and it eliminates
any product interference due to the two zone vial.
USP <2021> MICROBIAL ENUMERATION TESTS—NUTRITIONAL AND DIETARY
USP <2022> MICROBIOLOGICAL PROCEDURES FOR ABSENCE OF SPECIFIED
MICROORGANISMS—NUTRITIONAL AND DIETARY SUPPLEMENTS
USP <62> MICROBIOLOGICAL EXAMINATION OF NONSTERILE PRODUCTS: TESTSFOR SPECIFIED MICROORGANISMS
Peter Feng, Stephen D. Weagant, Michael A. Grant, 2002. Bacteriological AnalyticalManual BAM). Chapter 4: Enumeration of Escherichia coli and the Coliform Bacteria