Coliforms, E. coli
Coliforms, E. coli
DOC316.53.01211
modified m-TEC prepared Agar1
Method 8367
Membrane Filtration
Scope and application: For potable water, nonpotable water, recreation water and wastewater.
1
USEPA accepted.
Test preparation
Before starting
Set the temperature of the incubator to 35 ± 0.5 °C (95 ± 0.9 °F). Let the incubator temperature become stable, then add the
samples.
Wash hands thoroughly with soap and water.
Use a germicidal cloth, bactericidal spray, weak bleach solution or weak iodine solution to clean the work area.
Make sure that all of the materials that come in contact with samples are sterile.
During filtration, remove the vacuum as soon as the funnel is empty so that the membrane filter does not become dry.
As an alternative to the filter assembly with flask, use a sterile, disposable filter unit.
Items to collect
Description
Quantity
m-TEC, prepared agar plates
1
Sterile buffered dilution water
1
Membrane filter, 0.45 micron
1
Filtration apparatus with aspirator or pump
1
Forceps, sterilized
1
Incubator
1
Microscope, low-power
1
Pipet(s) for dilution or for sample volumes less than 100 mL, if necessary
1
Refer to Consumables and replacement items on page 5 for order information.
1
Sample collection
•
•
•
•
•
•
•
Use a sterile glass or plastic container such as a Whirl-Pak bag that contains
sterilized sodium thiosulfate. The sodium thiosulfate is not necessary if the sample
does not contain a residual disinfectant.
Open the sample containers immediately before collection and close immediately
after collection. Do not put the lid or cap down. Do not touch the lip or inner surfaces
of the container. Do not rinse the containers before use.
To collect a potable water sample from a faucet, spigot, hydrant or pump, let the
water flow at a moderate rate for 2–3 minutes. Remove the screens or aerators. Do
not use faucets or spigots that have a bad seal or that show a leak between
components.
To collect a non-potable sample from a river, lake or reservoir, hold the container
below the water surface, then remove the cap. As an alternative, remove the cap and
push the container, mouth down, below the water surface to prevent the collection of
surface scum. Put the mouth of the container into the current. Fully fill the container
below the water surface.
Collect a minimum of 100 mL of sample. Keep a minimum of 2.5 cm (1 inch) of air
space in the container.
Write the sample information on the container and start the analysis as soon as
possible.
If immediate analysis is not possible, keep the sample at or below 10 °C (50 °F) for a
maximum of 8 hours. Do not let the sample freeze.
Sample volumes
Use a sample volume that is applicable to the sample type. For samples with a low level
of bacteria such as finished, potable water, use 100 mL of sample. Use less sample for
non-potable water or water that contains more bacteria.
When the approximate bacteria level is unknown, analyze three different sample
volumes. Use the results from the sample volume that shows approximately 20 to
200 colonies for each membrane filter.
When the sample volume is less than 20 mL (diluted or undiluted), add 10 mL of sterile
buffered dilution water to the filter funnel before the vacuum is applied. The additional
dilution water helps to apply the bacteria equally across the membrane filter.
Sample dilution
Dilute samples that contain a high level of bacteria so that approximately 20 to
200 bacteria colonies grow on the membrane filter. Use the steps that follow to make
serial dilutions of the sample.
1.
2.
3.
4.
5.
Wash hands thoroughly with soap and water.
Invert the sample container for 30 seconds (approximately 25 times).
Open a bottle of sterile buffered dilution water.
Use a sterile pipet to add 11 mL of sample into the dilution water bottle.
Put the cap on the dilution water bottle and invert for 30 seconds (25 times). This is a
10x dilution (sample is diluted by a factor of 10).
6. Add 11 mL of the 10-fold dilution to another dilution bottle (100x dilution). Mix well.
7. Add 11 mL of the 100-fold dilution to the third bottle (1000x dilution). Mix well.
8. If necessary, continue to dilute the sample.
2
Coliforms, E. coli, modified m-TEC
Membrane filtration test procedure
1. Set up the membrane
filtration apparatus. Use a
sterile forceps to put a
membrane filter in the
assembly. Make sure that
the grid side is up.
2. Invert the sample or the
diluted sample for
30 seconds (25 times) to
make sure that the sample
is mixed well.
3. Pour or use a pipet to
add the sample into the
funnel. If the volume is less
than 20 mL, add 10 mL of
sterile buffered dilution
water to the funnel.
4. Apply the vacuum until
the funnel is empty. Stop the
vacuum.
5. Rinse the funnel with 20
to 30‑mL of sterile buffered
dilution water. Apply the
vacuum. Rinse the funnel
two more times.
6. Stop the vacuum when
the funnel is empty. Remove
the funnel from the filter
assembly. Use sterile
forceps to lift the membrane
filter.
7. Put the membrane filter
on the prepared agar plate.
Let the membrane filter
bend and fall equally across
the agar to make sure that
air bubbles are not caught
below the filter.
8. Put the lid on the petri
dish and invert the petri
dish.
9. Incubate the inverted
petri dish at 35 (± 0.5) °C
(95 (± 0.9) °F) for 2 hours,
then at 44.5 °C (112.1 °F)
for 22 hours.
10. Remove the petri dish
from the incubator. Use a 10
to 15x microscope to count
the number of bacteria
colonies on the membrane
filter. Refer to Interpret and
report the coliform results
on page 4.
Coliforms, E. coli, modified m-TEC
3
Interpret and report the coliform results
Report the coliform density as the number of colonies in 100 mL of sample. For total
coliforms, use a sample volume that gives 20–80 coliform colonies on the membrane
filter. For fecal coliforms, use a sample volume that gives 20–60 fecal coliform colonies
on the membrane filter.
If there are more than 200 colonies, dilute the sample and use the diluted sample in the
test procedure. Use the sample volume before dilution in the coliform density
determination.
1. Use the microscope to look at the colonies on the membrane filter. Count the number
of isolated coliform colonies.
2. Determine the coliform density as follows:
Membrane filter(s)
Coliform density determination
One membrane
filter
Coliform colonies in 100 mL = Coliform colonies counted ÷ mL sample
× 100
Example: 50 coliform colonies were counted. The sample volume was
20 mL. The coliform density is 50 ÷ 20 mL × 100 = 250 coliforms in
100 mL of sample.
Multiple filters,
dilutions or
duplicates for
each sample
Average coliform colonies in 100 mL = Sum of coliform colonies in all
samples ÷ sum of mL sample × 100
Example: Two 50-mL samples gave 5 colonies on one filter and
9 colonies on another filter. The coliform density is (5 + 9) ÷ (50 + 50) ×
100 = 14 coliforms in 100 mL of sample.
3. If colonies are not isolated or if there are more than 200 colonies of all types:
a. Report the results as “Confluent growth with or without coliforms” when the
bacteria grows together across some or all of the membrane filter.
b. Do the test procedure again with half the sample volume. If the total number of
colonies (coliforms plus non-coliforms) is more than 200 for each membrane or
the colonies are not isolated, report the results as “Too numerous to count”
(TNTC).
c. Do the test procedure again with a dilution that gives approximately 50 coliform
colonies and not more than 200 colonies of all types.
Bacteria disposal
Make sure to kill the cultured bacteria before disposal. Refer to Bacteria disposal
on page 5 to sterilize with a microwave or an autoclave.
Use one of the methods that follow to kill the cultured bacteria before disposal:
•
•
4
Hypochlorite (bleach) solution can also be used. Add 1–2 mL of hypochlorite (bleach)
solution to each test container. If a container has a lid, do not close it too tightly. Put
the container in the microwave at 70–80 °C (158–176 °F) for 50 seconds. Wait 10 to
15 minutes. Pour the liquid down the drain.
Kill the cultured bacteria with autoclave pressure. Put the used test containers in a
contaminated items bag or biohazard bag to prevent leaks. Do not seal the bag. Put
the bag in the autoclave at 121 °C (250 °F) for 30 minutes at 15 lb of pressure. When
the bag is cool, seal it and put it into a garbage bag. Make sure to tie the garbage bag
tightly.
Coliforms, E. coli, modified m-TEC
Figure 1 Bacteria disposal
Summary of method
This method uses modified m-TEC Agar to find E. coli bacteria in water samples. The
modified m-TEC Agar has an enzymatic substrate, which is bonded to form colonies. Red
or magenta colonies show that E. coli is in the sample. A confirmation procedure is not
necessary with this method.
The membrane filtration procedure is used for samples that are low in turbidity and have
low bacteria counts. The sample is poured through a membrane filter. The bacteria in the
sample stays on the membrane filter. The membrane filter is moved to a petri dish that
contains a nutritional broth or agar. During incubation, the bacteria grow and form
colonies on the membrane filter. After incubation, the filter is examined with a microscope
for bacteria colonies.
Consumables and replacement items
Required reagents
Description
Quantity/test
Unit
Item no.
m-TEC, modified, prepared agar plates
1
15/pkg
2811815
Dilution water, buffered, 99 mL, sterile1
1
25/pkg
1430598
Description
Unit
Item no.
Membrane filter holder, magnetic, 300-mL funnel
each
1352900
Filter pump, aspirator
each
213100
Flask, filtering, glass, 1000 mL
each
54653
Forceps, stainless steel
each
2141100
Membrane filter, 0.45 micron, 47 mm diameter, sterile
200/pkg
1353001
Membrane filter, 0.45 micron, 47 mm diameter, sterile EO (ethylene oxide)
150/pkg
2936100
each
2947050
Required apparatus
Microscope, compound
1
Buffered dilution water is prepared with magnesium chloride and potassium dihydrogen phosphate.
Coliforms, E. coli, modified m-TEC
5
Required apparatus (continued)
Description
Unit
Item no.
25/pkg
209798
each
1465100
each
1970010
Pipet tips, TenSette, 1.0–10.0 mL, sterile, individually wrapped
50/pkg
2558996
Stopper, rubber, size 8, for filtration assembly
6/pkg
211908
3.66 m (12 ft)
56019
Description
Unit
Item no.
Laboratory incubator, culture, 110 VAC
each
2619200
Laboratory incubator, culture, 230 VAC
each
2619202
Portable incubator with 12 VDC power socket
each
2569900
AC power supply for portable incubator, 110–240 VAC
each
2968100
Battery pack, rechargeable, for portable incubator 12 VDC
each
2580300
Portable incubator rack, general purpose/petri dish
each
2580502
Unit
Item no.
Sampling bags, Whirl-Pak® with dechlorinating reagent, 177 mL
100/pkg
2075333
Sampling bags, Whirl-Pak without dechlorinating reagent, 207 mL
100/pkg
2233199
Sampling bottles, sterilized, with dechlorinating agent, 100-mL sample
100/pkg
8888006
Sampling bottles, sterilized, without dechlorinating reagent, 100-mL sample
12/pkg
2495012
Sampling bottles, sterilized, without dechlorinating reagent, 100-mL sample
50/pkg
2495050
each
2568700
Pipet, serological, 10–11 mL, sterile, disposable
Pipet filler, safety bulb
®
Pipet, TenSette , 1.0–10.0 mL
Tubing, rubber, 7.9 mm (5/16-in.) inside diameter
Incubators
Sample collection
Description
Sample transport kit, includes 100 sample bags with dechlorinating agent, refrigerant
pack, rack and 9-L cooler
FOR TECHNICAL ASSISTANCE, PRICE INFORMATION AND ORDERING:
In the U.S.A. – Call toll-free 800-227-4224
Outside the U.S.A. – Contact the HACH office or distributor serving you.
On the Worldwide Web – www.hach.com; E-mail – [email protected]
©
Hach Company/Hach Lange GmbH, 2007–2017. All rights reserved.
HACH COMPANY
WORLD HEADQUARTERS
Telephone: (970) 669-3050
FAX: (970) 669-2932
04/2017, Edition 10
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