EN ISO 11133 and Water for the Preparation and Performance Testing of Microbiological Culture Media

Riche E (PhD)¹, Corpinot M (PhD)¹, Plouhinec J², Flint T¹, Fischer S (PhD)³, Gerten B³, Hemy JC¹, Airaud A¹

¹Lab Water Solutions, Merck, Guyancourt, France, ²Assystem Care, Courbevoie, France, ³LS-OII-Q Quality Specialties and BioMonitoring, {hcompany], Darmstadt, Germany

Centrally purified water vs. an autonomous water purification system

• EN ISO 11133 and the quality control of microbiological culture media
• Centrally purified water vs. water from a Milli-Q^® IX system and culture media performance
• Water sources used to prepare culture media
• Culture media preparation
• Culture media evaluation: physical and chemical quality control
• Performance testing of prepared microbiological culture media
• Suitability of the Milli-Q^® IX water purification system to prepare and performance test microbiological culture media
• Design of the Milli-Q^® IX system ensures constant quality pure water
• Benefits of the Milli-Q^® IX water purification system to prepare and performance test microbiological culture media

EN ISO 11133 and the quality control of microbiological culture media

Microbiology testing is critical to guarantee the safety and quality of food. Laboratories performing these tests are required to deliver accurate and reliable results and meet regulatory requirements while under time pressure. Many tests and procedures depend upon culture media being capable of providing consistent and reproducible results.

• Webinar: Guidance to ISO 11133 Compliance: Water Sourcing for Culture Media
• Application Note: ISO 11133 and Water for Culture Media

The EN ISO 11133 standard establishes the context that ensures the quality of culture media and specifies the requirements for media preparation when used for the microbiological analysis of food, animal feed and water.¹ In order to be compliant with the standard, each element of the process must meet specific requirements.

Water can potentially affect culture media performance since it is the largest component of microbiological media by volume. The purpose of the present study was to prepare culture media and to test their performance according to EN ISO 11133 using water from two alternative sources.

Centrally purified water vs. water from a Milli-Q^® IX system and culture media performance

This study investigated whether a modern water purification system, the Milli-Q^® IX pure water system, installed in a microbiology laboratory could advantageously replace centrally produced purified water obtained from a distribution loop when preparing and testing microbiological media. Solid and liquid culture media for important pathogens and hygiene indicators were included, such as Listeria, Salmonella, Escherichia coli serogroup O157 and coliforms, as well as culture media for yeast, molds and lactic acid bacteria.

Water sources used to prepare culture media

Each medium was prepared in duplicate by dissolving dehydrated medium in water from two different sources:

• Purified water readily available in the QC laboratory: The purified water was centrally produced by a combination of reverse osmosis (RO), ion exchange and a bactericidal UV lamp, and delivered via a distribution loop. The water conductivity remained below 2 µS/cm throughout the study and the microbial level was < 10² cfu/mL, as required by the EN ISO 11133 standard (Table 1).
• Water from a Milli-Q^® IX 7003 water purification system: This system was installed in the lab and directly connected to tap water. It combined several water purification technologies to deliver pure water: RO, Elix^® electrodeionization and bactericidal UVC LED lamps. Freshly purified water was collected from the system’s E-POD^® dispenser, which was fitted with a Millipak^® 0.22 µm filter to ensure low microbial levels. This system is validated to deliver purified water with a conductivity < 0.2 µS/cm, typically 0.1 µS/cm at 25 °C (equivalent to a resistivity > 5 MΩ.cm, typically 10-15 MΩ.cm), and microbial level < 10² cfu/mL, and fits the requirements of the ISO 11133 standard (Table 1).

Table 1. Compliance of the two types of water tested with EN ISO 11133 requirements.¹

	EN ISO 11133	Centrally purified water in the QC laboratory	Water from Milli-Q® IX system fitted with a 0.22 µm filter
Water purity	For the preparation of culture media, use only purified water, i.e. distilled, demineralized, deionized or produced by reverse osmosis, or of equivalent quality free from substances likely to inhibit or influence the growth of the microorganisms under the test conditions e.g. traces of chlorine, traces of ammonia and traces of metal ions.	Water is purified by a combination of reverse osmosis, ion-exchange and bactericidal UV lamp.	The system contains a combination of purification technologies: reverse osmosis, Elix® electrodeionization, bactericidal UVC LED, 0.22 µm filtration. Data obtained during the system’s validation process: · Free chlorine < 0.02 ppm · Ammonia ≤ 0.00003% · Heavy metals ≤ 0.00001%
Storage	The purified water shall be stored in tightly closed containers made from an inert material (neutral glass, polyethylene, etc.) which shall be free from all inhibitory substances. It is however recommended that the water is used as soon as produced.	Not applicable	Water is stored in a polyethylene tank, which is protected against airborne contaminants by a vent filter. An ergonomic E-POD® water dispenser provides easy delivery of pure water.
Microbial contamination	Microbial contamination should not exceed 103 cfu/mL and preferably be below 102 cfu /mL	Microbial contamination ≤ 102 cfu/mL	Data obtained from an independent laboratory during the system’s validation process: microbial contamination ≤ 102 cfu/mL
Conductivity	The conductivity shall be no more than 25 μS/cm (equivalent to resistivity ≥ 0.04 MΩ cm), preferably below 5 μS/cm (grade 3 water, ISO 3696) at 25 °C. The conductivity should be checked before use.	Conductivity typically ≤ 2 µS/cm	System validated to deliver water with conductivity < 0.2 μS/cm; typically 0.1 μS/cm. The system is designed to meet or exceed requirements as described by EN ISO 3696, Grade 2 water. Last produced water resistivity is displayed on the instrument.

Culture media preparation

Seven different dehydrated culture media were selected (Table 2) to obtain representative results. All media are compliant with the corresponding EN ISO/FDA Bacteriological Analytical Manual (BAM)/USDA-FSIS/APHA and other standards and methods were involved in performance testing. The standards specifying the media and the ones specifying their relative performance testing are reported in Table 3, as well as the control stains, and the WDCM and ATCC^® (American Type Culture Collection) numbers.

The media were prepared following manufacturers’ instructions. Specifically, the required mass of the dehydrated media was added to 500 mL of centrally purified water or Milli-Q^® IX water. All media were dissolved, heated and, if required, autoclaved as needed according to manufacturer’s instructions.

The study was performed by the Life Science QC laboratory for microbiological products at Merck, Darmstadt, Germany. The lab is accredited by the German accreditation authority DAkkS as registered test laboratory D-PL-15185-01-00 according to DIN EN ISO/IEC 17025² for the performance testing of media for microbiology according to DIN EN ISO 11133.

Table 2. Application of culture media used in this study and the standards specifying them.

Culture medium	Application of the culture medium	Standard specifying the medium
Listeria agar according to Ottaviani and Agosti	For the isolation and differentiation of Listeria monocytogenes and other Listeria spp. from food and animal feed, environmental samples in the area of food production and food handling, and other materials.	ISO 11290, FDA-BAM, APHA
Violet red bile lactose (VRBL) agar	For the detection and colony counting of coliform bacteria from food and animal feed, water and other materials.	ISO 4832, FDA-BAM, APHA
Malt extract agar	For the detection, isolation and enumeration of yeasts and molds in food products and other materials, and for their maintenance.	-
de Man, Rogosa and Sharpe (MRS) agar	For the isolation, enumeration and cultivation of Lactobacillus spp. and other mesophilic lactic acid bacteria from all types of materials.	ISO 15214, APHA
Rappaport-Vassiliadis medium with soya (RVS) broth	For the selective enrichment of Salmonella spp. from food and animal feed, water and other materials.	ISO 6579-1, ISO 19250
Brilliant green bile lactose (BRILA) broth	For the selective enrichment, enumeration and confirmation of Escherichia coli and other fecal coliform organisms from food and animal feed, water and other materials.	ISO 4831, ISO 4832, FDA-BAM, APHA
Modified tryptone soya broth (mTSB) with novobiocin	For the selective enrichment of Escherichia coli serogroup O157 in food and feed samples.	ISO 16654

Table 3. List of control strains for performance testing of culture media in the present study in accordance with the List of control strains for performance testing of culture media and reagents from published standards from ISO/TC 34/SC 9 & SC 5 (food microbiology)³ and ISO/TC 147/SC 4 (water microbiology)⁴.

^bStrain to be used as minimum; ^cSome national restrictions and directions may require the use of a different serovar. Refer to national requirements relating to the choice of Salmonella serovars; ^dStrain free of choice; one of the positive strains has to be used as a minimum; ^gStrain optional. (Only the required footnotes are reported here).

Culture medium	Standard specifying the medium	Standard specifying the performance testing	Control strains specified by the standard	WDCM numbers specified by the standard	WDCM numbers used in this study
Listeria agar according to Ottaviani and Agosti	ISO 11290-1:2017 ISO 11290-2:2017	ISO 11290-1:2017 ISO 11290-2:2017	Listeria monocytogenes serovar 4b	00021b	00021
			Listeria monocytogenes serovar 1/2a	00109g	00109
			Escherichia coli	00012d 00013d	00012 00013
			Enterococcus faecalis	00009d 00087d	00009 00087
			Listeria innocua	00017b	00017
Violet red bile lactose (VRBL) agar	ISO 4832:2006	EN ISO 11133:2014	Escherichia coli	00012b 00013g	00012 00013
			Enterococcus faecalis	00009d 00087d	00009 00087
			Pseudomonas aeruginosa	00025b	00025
Malt extract agar	-	-	-	-	-
de Man, Rogosa and Sharpe (MRS) agar	ISO 15214:1998	EN ISO 11133:2014	Lactobacillus sakei	00015b	00015
			Lactococcus lactis	00016b	00016
			Pediococcus pentosaceus	00158g	00158
			Escherichia coli	00012d 00013d	00012 00013
			Bacillus cereus	00001g	00001
Rappaport-Vassiliadis medium with soya (RVS) broth	ISO 6579-1:2017	ISO 6579-1:2017	SalmonellaTyphimurium/ Enteritidis	00030c,d 00031c,d	00030 00031
			Escherichia coli	00012d 00013d	00012 00013
			Pseudomonas aeruginosa	00025b	00025
			Enterococcus faecalis	00009d 00087d	00009 00087
Brilliant green bile lactose (BRILA) broth	ISO 4831:2006 ISO 4832:2006	EN ISO 11133:2014 / Amd 2:2020	Escherichia coli	00012d 00013d 00090d 00179d	00012 00013
			Enterococcus faecalis	00009d00087d 00176d	00009 00087
Modified tryptone soya broth (mTSB) with novobiocin	ISO 16654:2001	-	-	-	-

Culture media evaluation: physical and chemical quality control

The seven culture media studied were prepared with water from the two alternative sources. Each prepared culture medium was assessed by visual inspection to ensure that it conformed to stated recommendations, e.g. appearance, color, homogeneity, gel consistency and pH.

Table 4 describes the physical parameters and the expected specifications, as well as the results obtained in both conditions. In each case, the media prepared with water from the Milli-Q^® IX system matched the required specifications and gave similar outcomes as media prepared with the centrally produced purified water of the QC laboratory.

Table 4. Physical parameters and specifications of the seven culture media used in the study, as well as results obtained with either centrally purified water or water from a Milli-Q^® IX system.

Media	Physical parameters	Specifications	Purified water results	Milli-Q® IX water results
Listeria agar according to Ottaviani and Agosti	Appearance (clarity)	slightly opalescent to opalescent	opalescent	opalescent
	Appearance (color)	yellowish	yellowish	yellowish
	pH value (at 25 °C)	7.0 – 7.4	7.1	7.1
Violet red bile lactose (VRBL) agar	Appearance (clarity)	clear	clear	clear
	Appearance (color)	red	red	red
	pH value (at 25 °C)	7.2 – 7.6	7.4	7.4
	Solidification behavior (2 h at 45 °C)	liquid	liquid	liquid
Malt extract agar	Appearance (clarity)	clear to slightly opalescent	almost clear	almost clear
	Appearance (color)	brown	brown	brown
	pH value (at 25 °C)	5.4 – 5.8	5.5	5.5
de Man, Rogosa and Sharpe (MRS) agar	Appearance (clarity)	clear	clear	clear
	Appearance (color)	brown	brown	brown
	pH value (at 25 °C)	5.6 – 5.8	5.6	5.6
	Solidification behavior (2 h at 45 °C)	liquid	liquid	liquid
Rappaport-Vassiliadis medium with soya (RVS) broth	Appearance (clarity)	clear	clear	clear
	Appearance (color)	dark blue	dark blue	dark blue
	pH value (at 25 °C)	5.0 – 5.4	5.4	5.4
Brilliant green bile lactose (BRILA) broth	Appearance (clarity)	clear	clear	clear
	Appearance (color)	green	green	green
	pH value (at 25 °C)	7.0 – 7.4	7.2	7.2
Modified tryptone soya broth (mTSB) with novobiocin	Appearance (clarity)	clear	clear	clear
	Appearance (color)	yellowish brown	yellowish brown	yellowish brown
	pH value (at 25 °C)	7.2 – 7.6	7.3	7.2

Performance testing of prepared microbiological culture media

Growth and inhibition were assessed by either quantitative or qualitative methods as described in the EN ISO 11133 standard.

• Productivity is the level of recovery of a target microorganism from the culture medium under defined conditions.
• Selectivity is the degree of inhibition of a non-target microorganism on or in a selective culture medium under defined conditions.
• Specificity is the demonstration, under defined conditions, that non-target organisms, if a grow on the medium, do not show the same visual characteristics as target microorganisms.

1. Bacteria, yeasts and mold preparation and inoculation

Control strains were obtained from ATCC^® and were selected according to the requirements of the EN ISO 11133 standard and/or other specific standards. They were diluted to the desired number of organisms for inoculation for testing on productivity, specificity and selectivity. Performance evaluation and interpretation of the results were obtained following the specifications given by EN ISO 11133.

2. Performance testing

• Quantitative methods: Quantitative productivity for solid culture media was measured by quantitative methods as described in EN ISO 11133.
• Qualitative methods: Productivity, specificity and selectivity were determined qualitatively using the methods as described by EN ISO 11133.

All the culture media prepared and tested met the required criteria for productivity, selectivity and specificity, when applicable. Quantitative productivity tests met the required criteria: productivity ratio (PR) ≥ 50% for selective media and PR ≥ 70% for non-selective media. Table 5 and Table 6 report the results obtained in the study. Qualitative and quantitative tests of the culture media led to results in accordance with the standard requirements with purified water used in the QC laboratory as well as with water delivered by the Milli-Q^® IX system.

Table 5. Results of the performance tests of agar-based media prepared with either centrally purified water or water from a Milli-Q^® IX system.

*Additional strains used in the experiments of the study. Strains tested for malt extract agar were chosen by the QC laboratory.

Media	Function / Method of control	Strains tested	Specifications	Purified water results	Milli-Q® IX water results
Listeria agar according to Ottaviani and Agosti	Productivity / Quantitative	Listeria monocytogenes ATCC® 13932™ [WDCM 00021] Listeria monocytogenes ATCC® 35152™ [WDCM 00109]	Recovery rate ≥ 50%	L. monocytogenes ATCC® 13932™: 117%, Blue green colonies with opaque halo L. monocytogenes ATCC® 35152™: 74%, Blue green colonies with opaque halo PASS	L. monocytogenes ATCC® 13932™: 124%, Blue green colonies with opaque halo L. monocytogenes ATCC® 35152™: 83%, Blue green colonies with opaque halo PASS
	Selectivity / Qualitative	Escherichia coli ATCC® 8739™ [WDCM 00012] Escherichia coli ATCC® 25952™ [WDCM 00013] Enterococcus faecalis ATCC® 29212™ [WDCM 00087] Enterococcus faecalis ATCC® 19433™ [WDCM 00009]	Total inhibition	Total inhibition PASS	Total inhibition PASS
	Specificity / Qualitative	Listeria innocua ATCC® 33090™ [WDCM 00017]	No limit	Growth, blue-green colonies without opaque halo PASS	Growth, blue-green colonies without opaque halo PASS
Violet red bile lactose (VRBL) agar	Productivity / Quantitative	Escherichia coli ATCC® 8739™ [WDCM 00012] Escherichia coli ATCC® 25922™ [WDCM 00013] Enterobacter cloacae ATCC® 13047™ [WDCM 00083]*	Recovery rate ≥ 50%	E. coli ATCC® 8739™: 82% E. coli ATCC® 25922™: 105% E. cloacae: 85% All: purplish-red colonies with or without precipitation halo PASS	E. coli ATCC® 8739™: 103% E. coli ATCC® 25922™: 102% E. cloacae: 93% All: purplish-red colonies with or without precipitation halo PASS
	Selectivity / Qualitative	Enterococcus faecalis ATCC® 29212™ [WDCM 00087] Enterococcus faecalis ATCC® 19433™ [WDCM 00009]	Total inhibition	Total inhibition PASS	Total inhibition PASS
	Specificity / Qualitative	Pseudomonas aeruginosa ATCC® 27853™ [WDCM 00025]	Growth, no limit; colorless to beige colonies	Good growth, colorless to beige colonies PASS	Good growth, colorless to beige colonies PASS
Malt extract agar	Productivity / Quantitative	Candida albicans ATCC® 10231™ [WDCM 00054] Saccharomyces cerevisiae ATCC® 9763™ [WDCM 00058] Saccharomyces cerevisiae ATCC® 9080™ Rhodotoula mucilaginosa DSM 70403™	Recovery rate ≥ 70%	C. albicans: 82% S. cerevisiae ATCC® 9763™: 107% S. cerevisiae ATCC® 9080™: 87% R. mucilaginosa: 113% PASS	C. albicans: 82% S. cerevisiae ATCC® 9763™: 100% S. cerevisiae ATCC® 9080™: 102% R. mucilaginosa: 116% PASS
	Specificity / Qualitative	Geotrichum candidum DSM 1240™ Penicillium commune ATCC® 10428™ Aspergillus brasiliensis ATCC® 16404™ [WDCM 00053]	Good to very good growth	Very good growth PASS	Very good growth PASS
		Trichophyton ajelloi ATCC® 28454™	Moderate to good growth	Good growth PASS	Good growth PASS
de Man, Rogosa and Sharpe (MRS) agar	Productivity / Quantitative	Lactobacillus acidophilus ATCC® 4356™ [WDCM 00098]* Lactobacillus sakei ATCC® 15521™ [WDCM 00015] Lactococcus lactis ATCC® 19435™ [WDCM 00016] Pediococcus pentosaceus ATCC® 33316™ [WDCM 00158] Pediococcus damnosus ATCC® 29358™ [WDCM 00022]*	Recovery rate ≥ 70%	L. acidophilus: 96% L. sakei: 106% L. lactis: 97% P. pentosaceus: 91% P. damnosus: 102% PASS	L. acidophilus: 106% L. sakei: 100% L. lactis: 99% P. pentosaceus: 88% P. damnosus: 108% PASS
	Selectivity / Qualitative	Escherichia coli ATCC® 25922™ [WDCM 00013] Escherichia coli ATCC® 8739™ [WDCM 00012] Bacillus cereus ATCC® 11778™ [WDCM 00001]	Total inhibition	Total inhibition PASS	Total inhibition PASS
		Bifidobacterium bifidum ATCC® 11863™*	Good growth	Good growth PASS	Good growth PASS

Table 6. Results of the performance tests of broth prepared with either centrally purified water or water from a Milli-Q^® IX system.

*Additional strains used in the experiments of the study. Strains tested for mTSB were chosen by the QC laboratory.

Media	Function / Method of control	Strains tested	Specifications	Purified water results	Milli-Q® IX water results
Rappaport-Vassiliadis medium with soya (RVS) broth	Productivity / Qualitative	Salmonella Typhimurium ATCC® 14028™ [WDCM 00031]+ Escherichia coli ATCC® 25922™ [WDCM 00013]+ Pseudomonas aeruginosa ATCC® 27853™ [WDCM 00025]	> 10 colonies with black center on XLD agar	> 10 colonies with black center on XLD agar PASS	> 10 colonies with black center on XLD agar PASS
		Salmonella Enteritidis ATCC® 13076™ [WDCM 00030]+ Escherichia coli ATCC® 8739™ [WDCM 00012]+ Pseudomonas aeruginosa ATCC® 27853™ [WDCM 00025]	> 10 colonies with black center on XLD agar	> 10 colonies with black center on XLD agar PASS	> 10 colonies with black center on XLD agar PASS
	Selectivity / Qualitative	Escherichia coli ATCC® 8739™ [WDCM 00012] Escherichia coli ATCC® 25922™ [WDCM 00013]	Partial inhibition ≤ 100 colonies on Tryptic Soy Agar (TSA)	For both, partial inhibition: ≤ 100 colonies on TSA; PASS	For both, partial inhibition: ≤ 100 colonies on TSA PASS
		Enterococcus faecalis ATCC® 29212™ [WDCM 00087] Enterococcus faecalis ATCC® 19433™ [WDCM 00009]	< 10 colonies on Tryptic Soy Agar (TSA)	For both, < 10 colonies on TSA PASS	For both, < 10 colonies on TSA PASS
Brilliant green bile lactose (BRILA) broth	Productivity / Qualitative	Escherichia coli ATCC® 25922™ [WDCM 00013] Escherichia coli ATCC® 8739™ [WDCM 00012] Citrobacter freundii ATCC® 43864™ [WDCM 00006]*	Turbidity and gas	For all 3: turbidity and gas PASS	For all 3: turbidity and gas PASS
	Selectivity / Qualitative	Enterococcus faecalis ATCC® 29212™ [WDCM 00087] Enterococcus faecalis ATCC® 19433™ [WDCM 00009]	Partial inhibition, no gas production	For both, partial inhibition, no gas production) PASS	For both, partial inhibition, no gas production PASS
		Staphylococcus aureus ATCC® 6538™ [WDCM 00032]* Bacillus cereus ATCC® 11778™ [WDCM 00001]*	Total inhibition, no gas production	For both, total inhibition, no gas production PASS	For both, total inhibition, no gas production PASS
Modified tryptone soya broth (mTSB) with novobiocin	Productivity / Qualitative	Escherichia coli ATCC® 35150™ + Staphylococcus aureus ATCC® 25923™ [WDCM 00034] Escherichia coli ATCC® 700728™ [WDCM 00014] + Staphylococcus aureus ATCC® 25923™ [WDCM 00034]	> 10 yellow-brown colonies on CT-SMAC Agar	For all, > 10 yellow-brown colonies on CT-SMAC Agar PASS	For all, > 10 yellow-brown colonies on CT-SMAC Agar PASS
		Escherichia coli ATCC® 25922™ [WDCM 00013]+ Staphylococcus aureus ATCC® 25923™	> 10 pink colonies on CT-SMAC agar	> 10 pink colonies on CT-SMAC agar PASS	> 10 pink colonies on CT-SMAC agar PASS
	Selectivity / Qualitative	Staphylococcus aureus ATCC® 25923™ [WDCM 00034]	Total inhibition on Tryptic Soy Agar (TSA)	Total inhibition on TSA PASS	Total inhibition on TSA PASS

Selecting a water solution for media preparation

When preparing culture media, it is important to select the correct water quality since water may contain impurities that can affect media productivity or cause abnormalities, such as incorrect pH, wrong color or precipitation, as indicated in Annex H of the EN ISO 11133 standard. The water quality requirements of the standard are shown in Table 1. An additional note in the standard indicates that water which has been passed through an ion exchange resin (demineralized or deionized water) can have a very high microorganism content. It is therefore advised not to use water from these sources without first checking microbial content. The standard warns that highly contaminated demineralized water, even if sterilized by filtration, can still contain substances that are inhibitory for the growth of certain microorganisms.

Suitability of the Milli-Q^® IX water purification system to prepare and performance test microbiological culture media

This study demonstrates that culture media prepared with water from a Milli-Q^® IX system has equivalent characteristics and performance as media prepared with the centrally purified water routinely used in the accredited QC lab. This indicates that water from this system can be used with confidence when preparing media according to the EN ISO 11133 standard. The Milli-Q^® system can be used to prepare media manually or in combination with innovative automated instruments, such as the ReadyStream^® system which simplifies the process by preparing and dispensing preheated culture media.

Water purified by the Milli-Q^® IX water purification system, fitted with a 0.22 µm final filter, is validated to deliver water meeting or exceeding the water quality requirements of the EN ISO 11133 standard (Table 1). Other standards and methods, such as the American Public Health Association (APHA) Standard Methods,⁵ make specific recommendations regarding water quality and purification technologies, to which the Milli-Q^® IX system abides by its design and performance. As recommended, it includes a combination of technologies to produce pure water, resistivity monitoring, as well as a storage tank equipped with UV irradiation. The Milli-Q^® IX system delivers water with a conductivity below 0.2 µS/cm, microbial levels below 10² cfu/mL, and free from traces of contaminants likely to inhibit the growth of microorganisms, such as chlorine, ammonia and metals. Since the system delivers water that suits EN ISO 11133 water quality requirements, it will help to ensure a lab’s regulatory compliance. However, it is important to also ensure that this water is well suited for the preparation of microbiological culture media.

Design of the Milli-Q^® IX system ensures constant quality pure water

Using water of consistent quality to prepare culture media is an important factor in ensuring accurate and reproducible test results. The Milli-Q^® IX system is designed to produce constant quality pure water and to safeguard stored water purity via several key features:

• Water purified by the system is stored in a high-quality polyethylene tank, which does not release extractables into water
• The tank is protected against airborne contaminants by a vent filter.
• Reliably low microbial levels are ensured by automatic recirculation of the pure water over a UVC LED bactericidal lamp, as well as the 0.22 µm screen filter placed at the system’s point-of-delivery.
• The Elix^® electrodeionization module included in the system delivers constant water quality, especially regarding conductivity⁶

The low level of extractables from the storage tank, the low microbial level in the water, as well as consistently low water conductivity are important water quality parameters highlighted in the EN ISO 11133 standard. Having a reliable and independent source of freshly purified water prevents any risk of unexpected changes in water quality or punctual unavailability of water from a central loop due to manufacturing constraints or maintenance.

Benefits of the Milli-Q^® IX water purification system to prepare and performance test microbiological culture media

Using a Milli-Q^® IX water purification system offers many benefits to labs performing microbiology testing.

• According to the EN ISO 11133 standard, water conductivity (or its reciprocal, resistivity) should be checked before use. The Milli-Q^® IX water system lets users accurately measure water resistivity on-line and displays it on the interface of the E-POD^® dispenser, allowing a convenient monitoring solution. Checking water quality parameters is easy to perform via the intuitive touchscreen compatible with gloves.
• The E-POD^® dispenser allows time gain and precision as the flow rate can easily be adapted to the needs of the experiment, and the screen lets the user program a specific volume to be dispensed. Moreover, the dispenser brings flexibility as it can be used over the sink or rested on its arm to fill larger containers.
• A foot pedal can be added to facilitate serial dispensing while freeing the user’s hands.
• Regular system maintenance is easily done by any member of the lab, or can be delegated to a service engineer to ensure the system operates optimally.
• The system stores all data related to water quality parameters, as well as information such as consumable changes and other service activities, thus providing data traceability and paperless data management. Easy-to-retrieve data can simplify the lab’s audit preparation. An online service is available to store all documentation and enable online service contract management.
• For additional efficiency, the system can be used as a direct water source for the ReadyStream^® system, to conveniently prepare large amounts (up to 100 L) of preheated media right where it is needed in only 15 minutes. The purified water delivered by the system can also be used for other tasks, such as glassware rinsing, and to feed autoclaves or glassware washers. This makes the Milli-Q^® IX system a comprehensive solution for microbiology laboratories.

The results of this study revealed that culture media prepared with water from the Milli-Q^® IX water purification system performed similarly as the culture media prepared with the lab’s routinely used centrally purified water. This Milli-Q^® water purification system enabled the lab to prepare media performing as required by the standard, while bringing additional benefits, such as ease of use, convenience, data traceability and autonomy. Combined, these features allow for efficient water system management, confidence in water purity and improved lab efficiency, which can keep scientists working at maximum productivity.

References

1. EN ISO International Standardization Organization. Microbiology of food, animal feed and water - Preparation, production, storage and performance testing of culture media + Amendment 1 + Amendment 2. EN ISO 11133:2014/Amd1:2018/ Amd2:2020.. [Internet]. Available from: https://www.iso.org/standard/70354

2. ISO/IEC 17025:2017 - General requirements for the competence of testing and calibration laboratories. . [Internet]. Available from: http://santia.co.uk/wp-content/uploads/2017/10/P405-ISO-IEC-17025-2005-Standard.pdf

3. ISO/TC 34/SC 9 Food products – Microbiology. Available from: https://committee.iso.org/sites/tc34sc9/home/general-standards/content-left-area/culture-media/iso-11133-on-culture-media.html

4. ISO/TC 147 Water quality: Available from: https://www.iso.org/committee/52834.html

5. 2017. 9020 Quality assurance/Quality control; Standard Methods for the Examination of Water and Wastewater, 23rd Edition. [Internet]. Washington, D.C.: American Public Health Association. Available from: https://www.standardmethods.org/doi/10.2105/SMWW.2882.180

6. Dimitrakopoulos T, Feuillas E, Darbouret D, Mabic S. Merck, Darmstadt, Germany. R&D Notebook 10.. Electrodeionization: technology and applications.