Microorganisms and performance criteria for testing selective media 'MWY' for Enumeration of Legionella used in Water quality microbiology by comparing with a non-selective reference medium according to ISO 11731.
ISO 11731 (2017) medium: MWY
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1 Quantitative control of Productivity
- Use a quantitative reference material or test suspension with a known number of Legionella pneumophila.
- Dilute this material so 50 to 120 cfu is inoculated on the MWY.
- Use the same method of inoculation as described in the specific individual ISO 11731.
- Do this also for the reference media (BCYE).
- Incubate at (36 ± 2) °C for 2d to 5d.
- Count the colonies on the medium under test (MWY) and the reference media (BCYE).
- Calculate the productivity ratio PR. The criteria is: PR ≥ 0,5.
- If the PR exceeds 1,4 identify the reason.
- For this the total count of colonies obtained on the culture medium under test (MWY) is divided by the total count of colonies obtained on the defined reference medium (BCYE).
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2 Quantitative control of Productivity opt
- Use a quantitative reference material or test suspension with a known number of Legionella anisa.
- Dilute this material so 50 to 120 cfu is inoculated on the MWY.
- Use the same method of inoculation as described in the specific individual ISO 11731.
- Do this also for the reference media (BCYE).
- Incubate at (36 ± 2) °C for 5d to 10d.
- Count the colonies on the medium under test (MWY) and the reference media (BCYE).
- Calculate the productivity ratio PR. The criteria is PR ≥ 0,5.
- If the PR exceeds 1,4 identify the reason.
- For this the total count of colonies obtained on the culture medium under test (MWY) is divided by the total count of colonies obtained on the defined reference medium (BCYE).
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1 Qualitative control of Selectivity
- Use a reference material or test suspension with a known number of Enterococcus faecalis.
- Streak the test strain (Enterococcus faecalis) as a single line on the media under test (MWY).
- Use a 1 µl loop in case of a concentration of at least 10000 cfu or a 10 µl loop in case of a concentration between 1000 and 10000 cfu.
- Incubate at (36 ± 2) °C for 3d.
- Asses the amount of growth as, 0 for no growth, 1 for weak growth or 2 for good growth.
- For this medium (MWY) and standard (ISO 11731) combination the criteria is: "Total inhibition.
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2 Qualitative control of Selectivity
- Use a reference material or test suspension with a known number of Escherichia coli.
- Streak the test strain (Escherichia coli) as a single line on the media under test (MWY).
- Use a 1 µl loop in case of a concentration of at least 10000 cfu or a 10 µl loop in case of a concentration between 1000 and 10000 cfu.
- Incubate at (36 ± 2) °C for 3d.
- Asses the amount of growth as, 0 for no growth, 1 for weak growth or 2 for good growth.
- For this medium (MWY) and standard (ISO 11731) combination the criteria is: "Total inhibition (0 to 1)".

Best Practice MWY
Quantitative control of Productivity solid medium
Calculate the productivity ratio PR as follows: | PR | NS |
N0 | ||
Ns = total count test medium | ||
N0 = total count reference medium |
Control strain | WDCM number | Ref. media | Incubation | Criteria | Characteristic reaction |
---|---|---|---|---|---|
Legionella pneumophila | BCYE | (36 ± 2) °C for 2d to 5d | PR ≥ 0,5 | White-grey-blue-purple colonies with an entire edge and exhibiting a characteristic groundglass appearance | |
The best practice for Quantitative control of Productivity for media control of 'MWY' is based on the data displayed in this table according ISO 11731. |
Best Practice MWY
Quantitative control of Productivity solid medium
Calculate the productivity ratio PR as follows: | PR | NS |
N0 | ||
Ns = total count test medium | ||
N0 = total count reference medium |
Control strain | WDCM number | Ref. media | Incubation | Criteria | Characteristic reaction |
---|---|---|---|---|---|
Legionella anisa | BCYE | (36 ± 2) °C for 5d to 10d | PR ≥ 0,5 | White-grey-blue-purple colonies with an entire edge and exhibiting a characteristic groundglass appearance | |
The best practice for Quantitative control of Productivity for media control of 'MWY' is based on the data displayed in this table according ISO 11731. |
Best Practice MWY
Qualitative control of Selectivity solid medium
Control strain | WDCM number | Ref. media | Incubation | Criteria | Characteristic reaction |
---|---|---|---|---|---|
Enterococcus faecalis | or | (36 ± 2) °C for 3d | Total inhibition | No growth | |
The best practice for Qualitative control of Selectivity for media control of 'MWY' is based on the data displayed in this table according ISO 11731. |
Best Practice MWY
Qualitative control of Selectivity solid medium
Control strain | WDCM number | Ref. media | Incubation | Criteria | Characteristic reaction |
---|---|---|---|---|---|
Escherichia coli | or | (36 ± 2) °C for 3d | Total inhibition (0 to 1) | No growth | |
The best practice for Qualitative control of Selectivity for media control of 'MWY' is based on the data displayed in this table according ISO 11731. |