SIST EN ISO 9308-3:1999

SLOVENSKI STANDARD
SIST EN ISO 9308-3:1999
01-november-1999
.DNRYRVWYRGH±8JRWDYOMDQMHSULVRWQRVWLLQãWHYLOD(VFKHULFKLDFROLLQNROLIRUPQLK
EDNWHULMYSRYUãLQVNLLQRGSDGQLKYRGDK±GHO0HWRGD]PLQLDWXUL]DFLMRQDMEROM
YHUMHWQRãWHYLOR]QDVDMDQMHPYWHNRþHPJRMLãþX,62
Water quality - Detection and enumeration of Escherichia coli and coliform bacteria in
surface and wastewater - Part 3: Miniaturized method (Most Probable Number) by
inoculation in liquid medium (ISO 9308-3:1998)
Wasserbeschaffenheit - Nachweis und Zählung von Escherichia coli und coliformen
Bakterien in Oberflächenwasser und Abwasser - Teil 3: Miniaturisiertes Verfahren durch
Animpfen in Flüssigmedium (MPN-Verfahren) (ISO 9308-3:1998)
Qualité de l'eau - Recherche et dénombrement des Escherichia coli et des bactéries
coliformes dans les eaux de surface et résiduaires - Partie 3: Méthode miniaturisée
(nombre le plus probable) pour ensemencement en milieu liquide (ISO 9308-3:1998)
Ta slovenski standard je istoveten z: EN ISO 9308-3:1998
ICS:
07.100.20 Mikrobiologija vode Microbiology of water
SIST EN ISO 9308-3:1999 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------

INTERNATIONAL ISO
STANDARD 9308-3
First edition
1998-11-15
Water quality — Detection and enumeration
of Escherichia coli and coliform bacteria
in surface and waste water —
Part 3:
Miniaturized method (Most Probable Number)
by inoculation in liquid medium
Qualité de l’eau — Recherche et dénombrement des Escherichia coli
et des bactéries coliformes dans les eaux de surface et résiduaires —
Partie 3: Méthode miniaturisée (nombre le plus probable) pour
ensemencement en milieu liquide
A
Reference number
ISO 9308-3:1998(E)

---------------------- Page: 2 ----------------------

ISO 9308-3:1998(E)
Contents Page
1 Scope .1
2 Normative references .1
3 Terms and definitions .2
4 Principle.2
5 Apparatus .2
6 Sampling.3
7 Culture media and diluent.3
8 Procedure .4
9 Expression of results .6
10 Test report .7
11 Performance data.7
Annex A (informative) Example of software for statistical analysis of MPNs .8
Annex B (informative) Example of software for computation of MPN .12
Annex C (informative) Synthetic sea salt.14
(informative)
Annex D Performance characteristics of the method.16
Annex E (normative) Quality criteria for manufacturing of the medium in microtitre plates .17
Annex F (normative) Preparation of calibration microtitre plates.18
Bibliography.20
©  ISO 1998
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
ii

---------------------- Page: 3 ----------------------

© ISO
ISO 9308-3:1998(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
International Standard ISO 9308-3 was prepared by Technical Committee ISO/TC 147, Water quality,
Subcommittee SC 4, Microbiological methods.
ISO 9308 consists of the following parts, under the general title Water quality — Detection and enumeration of
Escherichia coli and coliform bacteria in surface and waste water:
 Part 1: Membrane filtration method
 Part 2: Liquid enrichment method
 Part 3: Miniaturized method (Most Probable Number) by inoculation in liquid media
Annexes E and F form a normative part of this part of ISO 9308. Annexes A to D are for information only.
iii

---------------------- Page: 4 ----------------------

INTERNATIONAL STANDARD  © ISO ISO 9308-3:1998(E)
Water quality — Detection and enumeration of Escherichia coli
and coliform bacteria in surface and waste water —
Part 3:
Miniaturized method (Most Probable Number) by inoculation in liquid
medium
1 Scope
This part of ISO 9308 specifies a miniaturized method for the detection and enumeration of Escherichia coli (E. coli)
in surface and waste water by inoculation in a liquid medium. The method is applicable to all types of surface and
waste waters, particularly those rich in suspended matter.
This method is not suitable for drinking water and any other type of water for which the guideline is less than 15
counts per 100 ml.
This method is not appropriate for enumeration and detection of coliform bacteria other than E. coli.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent editions of the normative documents indicated below. For
undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC
maintain registers of currently valid International Standards.
ISO 3951, Sampling procedures and charts for inspection by variables for percent nonconforming.
ISO 5667-1, Water quality — Sampling — Part 1: Guidance on the design of sampling programmes.
ISO 5667-2, Water quality — Sampling — Part 2: Guidance on sampling techniques.
ISO 5667-3, Water quality — Sampling — Part 3: Guidance on the preservation and handling of samples.
ISO 8199, Water quality — General guide to the enumeration of microorganisms by culture.
ISO/IEC Guide 2, Standardization and related activities — Vocabulary.
1

---------------------- Page: 5 ----------------------

© ISO
ISO 9308-3:1998(E)
3 Terms and definitions
For the purposes of this part of ISO 9308, the terms and definitions given in ISO/IEC Guide 2 and the following
apply.
3.1
Escherichia coli
E. coli
b-D-glucuronidase-positive microorganism growing at an incubation temperature of 44 °C in the specified liquid
medium containing 4-methylumbelliferyl-b-D-glucuronide (MUG)
4 Principle
The diluted sample is inoculated in a row of microtitre plate wells containing dehydrated culture medium.
The microtitre plates are examined under ultraviolet light at 366 nm in the dark after an incubation period of 36 h
minimum and 72 h maximum at 44 °C – 0,5 °C. The presence of E. coli is indicated by a blue fluorescence resulting
from hydrolysis of MUG. The results are given as most probable number (MPN) per 100 ml.
5 Apparatus
With the exception of equipment supplied sterile, the glassware shall be sterilized in accordance with the
instructions given in ISO 8199.
Usual microbiological laboratory equipment, and in particular:
5.1  Apparatus for sterilization by dry heat (oven) or steam (autoclave).
5.2  Thermostatic incubator regulated at 44 °C – 0,5 °C.
5.3  Tunnel drier or vertical laminar air flow cabinet (preferably class II).
5.4  UV observation chamber (Wood's Lamp, 366 nm).
WARNING: UV light causes irritation of eyes and skin. Use protective glasses and gloves.
5.5  Portable refractometer (optional).
5.6  pH meter with an accuracy of – 0,1
5.7  Test tubes of dimensions 16 mm · 160 mm and 20 mm · 200 mm, or flasks with similar capacity.
5.8  8-channel multipipette, adjustable or preset, or any other system suitable for measuring and distributing
200 μl per well.
5.9  Sterile tips for multipipette.
5.10  Equipment for membrane filtration in accordance with ISO 8199, including membrane filters with a nominal
pore size of 0,2 μm, for sterilization of liquid media.
5.11  Sterile microtitre plates, 96-well, 350 μl, flat bottomed, nonfluorescent.
Sterile adhesive covering strips for sealing microtitre plates.
5.12
, 90 mm in diameter.
5.13 Sterile Petri dishes
2

---------------------- Page: 6 ----------------------

© ISO
ISO 9308-3:1998(E)
6 Sampling
Take the samples and deliver them to the laboratory in accordance with ISO 8199 and ISO 5667-1, ISO 5667-2 and
ISO 5667-3.
7 Culture media and diluent
7.1 General instructions
To ensure reproducible results, prepare culture medium and diluents, using either constituents of uniform quality
and chemicals of recognized analytical grade, or a dehydrated diluent or complete medium prepared following the
manufacturer's instructions. Prepare them with demineralized or distilled water free from substances capable of
inhibiting growth under the test conditions. If the media are not used immediately, preserve them in the dark at
(5 – 3) °C for up to one month in conditions avoiding any alterations to their composition.
NOTE The use of chemicals of other grades is permissible providing they are shown to be of equivalent performance in the
test.
7.2 Diluents
7.2.1  Special diluent (SD)
1)
Synthetic sea salt 22,5 g
Bromophenol blue solution (optional) 10 ml
Demineralized or distilled water (7.2.2) 1000 ml
Sterilize in the autoclave (5.1) at 121 °C – 3 °C for 15 min to 20 min.
The bromophenol blue solution is prepared by adding 0,04 g in 100 ml of 50 % ethanol. It is only used to colour the
SD blue and avoid confusing it with demineralized or distilled water.
7.2.2  Demineralized or distilled water, free from substances inhibiting growth under the test conditions.
Sterilize in the autoclave (5.1) at 121 °C – 3 °C for 15 min to 20 min.
7.3 Culture medium: MUG/EC medium
Composition
Tryptone 40 g
Salicin 1 g

Triton X 100®  1 g
MUG (4-methylumbelliferyl-b-D-glucuronide) 100 mg
Demineralized or distilled water (7.2.2) 1000 ml

1)
  A typical analysis of a commercially available and suitable synthetic sea salt is given in annex C. Other diluents, such as
distilled water, can be used for E. coli enumeration, unless intestinal enterococci are to be enumerated from the same dilution
tubes.
3

---------------------- Page: 7 ----------------------

© ISO
ISO 9308-3:1998(E)
2)
Successively add tryptone, salicin and Triton to one litre of water, whilst maintaining a gentle heat and magnetic
stirring, then bring to the boil until completely dissolved. Allow to cool and add the fluorogenic constituent MUG,
dissolved in 2 ml of N,N-dimethylformamide.
WARNING: N,N-dimethylformamide is toxic and can cause cancer. Harmful by inhalation, in contact with
skin and if swallowed. Use in a chemicals fume hood.
Adjust the pH to 6,9 – 0,2.
Sterilize by filtration with membranes of average pore size 0,2 μm (5.10).
Distribute in 96-well microtitre plates (5.11) with a volume of 100 μl of media in each well (minimum capacity 350 μl)
and dehydrate immediately in a tunnel drier or laminar air flow cabinet (5.3).
The manufacturing of the medium shall meet the quality criteria given in annex E.
8 Procedure
8.1 Choice of dilutions
The number of dilutions to inoculate varies according to the presumed level of contamination of the water to be
tested. Table 1 gives examples.
Table 1
Measurement limits,
Origin of sample No. of dilutions No. of wells/dilution
bacteria/100 ml
Bathing water 2 64 wells to 1/2
4
15 to 3,5 · 10
32 wells to 1/20
Fresh surface water 4 24 wells to 1/2
24 wells to 1/20
6
40 to 3,2 · 10
24 wells to 1/200
24 wells to 1/2 000
Waste water and 6 16 wells to 1/2
8
treatment plants Up to 16 wells to 60 to 6,7 · 10
1/200 000
8.2 Preparation of dilutions
NOTE These procedures should be performed in a biological safety cabinet, as aerosols may be created by the diluting
and pipetting.
8.2.1 Fresh and brackish (waste) water (salinity , 30 g/kg, measured with a refractometer (5.5) or equivalent
method)
Prepare the relevant number of sterile tubes (5.7) in a rack, according to the number of selected dilutions; add 9 ml
of the special diluent (7.2.1) to each tube.
Vigorously stir the sample (clause 6) in order to obtain a homogeneous distribution of the microorganisms and using
a sterile pipette, immediately transfer 9 ml of this homogenized sample to the first tube containing 9 ml of diluent
(1/2 dilution).

2)
  Triton X 100 is an example of a suitable product available commercially. This information is given for the convenience of
users of this part of ISO 9308 and does not constitute an endorsement by ISO of this product. Equivalent products may be
used if they can be shown to lead to the same results.
4

---------------------- Page: 8 ----------------------

© ISO
ISO 9308-3:1998(E)
Using a fresh pipette, transfer 1 ml of this dilution (homogenized) to the second tube (1/20 dilution).
From the second tube (dilution 1/20 carefully homogenized) proceed, if necessary to another 1/10 dilution giving the
dilution 1/200.
Continue as above until all the dilutions have been prepared.
8.2.2 Sea water (salinity > 30 g/kg)
Prepare the relevant number of sterile tubes in a rack, according to the number of selected dilutions; add 9 ml of
demineralized or distilled water (7.2.2) to the first tube and 9 ml of the special diluent (7.2.1) to the following tubes.
Vigorously stir the sample (clause 6) in order to obtain a homogeneous distribution of the microorganisms and using
a sterile pipette, immediately transfer 9 ml of this homogenized sample to the first tube containing 9 ml of diluent
(7.2.2) (1/2 dilution).
Using a fresh sterile pipette, transfer 1 ml of this dilution (homogenized) to the second tube (1/20 dilution).
From the second tube (dilution 1/20 carefully homogenized) proceed, if necessary to another 1/10 dilution, giving
the dilution 1/200.
Continue as above until all the dilutions have been prepared.
8.3 Inoculation and incubation of microtitre plates
8.3.1 Inoculation
Transfer the contents of the first tube of dilution to an empty, sterile Petri dish of diameter 90 mm.
Using a multichannel pipette (5.8) with eight sterile tips (5.9), distribute 200 μl into each well of microtitre plate (5.11)
corresponding to this first dilution.
For subsequent dilutions (1/20, 1/200, etc.) operate in an identical manner, changing the Petri dish and the row of
eight sterile tips between each dilution.
Alternatively, any other suitable system (5.8) may be used to distribute 200 μl of each dilution per well according to
Table 1.
CAUTION: Beware of contamination via overflow from one well to another.
8.3.2 Incubation
Once the microtitre plate is inoculated, cover with the disposable sterile adhesive tape (5.12) provided for this
purpose.
Incubate the microtitre plate in an incubator (5.2) at 44 °C – 0,5 °C for a minimum of 36 h and a maximum of 72 h.
NOTE The microtitre plates should be handled with care, without tilting.

8.4 Reading of results
Place each microtitre plate, with the adhesive on, in the UV observation chamber (5.4).
Consider all wells
...