Microorganisms and performance criteria for testing selective media 'VRBG' for Enumeration of Enterobacteriaceae used in Food chain microbiology according to ISO 21528-2.
ISO 21528-2 (2017) medium: VRBG
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Quantitative control of Productivity
- Use a quantitative reference material or test suspension with a known number of Escherichia coli.
- Dilute this material so 50 to 120 cfu is inoculated on the VRBG by pouring for example 1 mL.
- Do this also for the reference media (TSA).
- Incubate at (37 ± 1) °C for (24 ± 2) h.
- Count the colonies on the medium under test (VRBG) 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 (VRBG) 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 Enterococcus faecalis.
- Streak the test strain (Enterococcus faecalis) as a single line on the media under test (VRBG).
- 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 (37 ± 1) °C for (24 ± 2) h.
- Asses the amount of growth as, 0 for no growth, 1 for weak growth or 2 for good growth.
- For this medium (VRBG) and standard (ISO 21528-2) combination the criteria is: "Total inhibition (0).

Best Practice VRBG
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 |
---|---|---|---|---|---|
Escherichia coli | TSA | (37 ± 1) °C for (24 ± 2) h | PR ≥ 0,5 | Pink to red colonies with or without precipitation halo | |
Salmonella Typhimurium | |||||
Salmonella Enteritidis | |||||
The best practice for Quantitative control of Productivity for media control of 'VRBG' is based on the data displayed in this table according ISO 21528-2. |
Best Practice VRBG
Qualitative control of Selectivity solid medium
Control strain | WDCM number | Ref. media | Incubation | Criteria | Characteristic reaction |
---|---|---|---|---|---|
Enterococcus faecalis | or | (37 ± 1) °C for (24 ± 2) h | Total inhibition (0) | No growth | |
The best practice for Qualitative control of Selectivity for media control of 'VRBG' is based on the data displayed in this table according ISO 21528-2. |