Microorganisms and performance criteria for testing selective media 'MYP' for Enumeration of Bacillus cereus used in Food and animal feeding stuffs microbiology according to ISO 11133.
ISO 7932 (2004) medium: MYP
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Quantitative control of Productivity
- Use a quantitative reference material or test suspension with a known number of Bacillus cereus.
- Dilute this material so 50 to 120 cfu is inoculated on the MYP by spreading for example 0.1 mL.
- Do this also for the reference media (TSA).
- Incubate at (30 ± 1) °C for (24 ± 3) h to (44 ± 4) h.
- Count the colonies on the medium under test (MYP) and the reference media (TSA).
- 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 (MYP) is divided by the total count of colonies obtained on the defined reference medium (TSA).
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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 (MYP).
- 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 (30 ± 1) °C for (44 ± 4) h.
- Asses the amount of growth as, 0 for no growth, 1 for weak growth or 2 for good growth.
- For this medium (MYP) and standard (ISO 7932) combination the criteria is: "Total inhibition (0).
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Qualitative control of Specificity
- Use a reference material or test suspension with a known number of Bacillus subtilis subsp. spizizenii.
- Streak the test controlstrain (Bacillus subtilis subsp. spizizenii) on the media under test (MYP) to obtain discrete colonies.
- Use a 1 µl loop in case of a concentration of at least 1000 cfu/mL.
- Incubate at (30 ± 1) °C for (44 ± 4) h.
- Asses the presence, absence and/or grade of expression of biochemical responses and colony sizes and morphology.
- For this medium (MYP) and standard (ISO 7932) combination the characteristic reaction is: Yellow colonies without precipitation halo.

Best Practice MYP
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 |
---|---|---|---|---|---|
Bacillus cereus | TSA | (30 ± 1) °C for (24 ± 3) h to (44 ± 4) h | PR ≥ 0,5 | Pink colonies with precipitation halo | |
The best practice for Quantitative control of Productivity for media control of 'MYP' is based on the data displayed in this table according ISO 11133. |
Best Practice MYP
Qualitative control of Selectivity solid medium
Control strain | WDCM number | Ref. media | Incubation | Criteria | Characteristic reaction |
---|---|---|---|---|---|
Escherichia coli | or | (30 ± 1) °C for (44 ± 4) h | Total inhibition (0) | No growth | |
The best practice for Qualitative control of Selectivity for media control of 'MYP' is based on the data displayed in this table according ISO 11133. |

Best Practice MYP
Qualitative control of Specificity solid medium
Control strain | WDCM number | Ref. media | Incubation | Criteria | Characteristic reaction |
---|---|---|---|---|---|
Bacillus subtilis subsp. spizizenii | (30 ± 1) °C for (44 ± 4) h | growth | Yellow colonies without precipitation halo | ||
The best practice for Qualitative control of Specificity for media control of 'MYP' is based on the data displayed in this table according ISO 11133. |