ISO 15214 (1998) medium: MRS

Media control Food and animal feeding stuffs

Microorganisms and performance criteria for testing selective media 'MRS' for Enumeration of Lactic acid bacteria used in Food and animal feeding stuffs microbiology according to ISO 11133.

  • Quantitative control of Productivity

  • Best Practice MRS

    Quantitative control of Productivity solid medium

    1. Use a quantitative reference material or test suspension with a known number of Lactobacillus sakei and Lactococcus lactis.
    2. Dilute this material so 50 to 120 cfu is inoculated on the MRS by spreading for example 0.1 mL.
    3. This can be a multi or single strain material.
    4. Do this also for the reference media (Media batch MRS already validated).
    5. Incubate at (30 ± 1) °C for (72 ± 3) h.
    6. Count the colonies on the medium under test (MRS) and the reference media (Media batch MRS already validated).
    7. Calculate the productivity ratio PR. The criteria is: PR ≥ 0,7.
    8. If the PR exceeds 1,4 identify the reason.
    9. For this the total count of colonies obtained on the culture medium under test (MRS) is divided by the total count of colonies obtained on the defined reference medium (Media batch MRS already validated).
    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
    Lactobacillus sakei Media batch MRS already validated (30 ± 1) °C for (72 ± 3) h PR ≥ 0,7 Characteristic colonies according to each species
    + Lactococcus lactis
    Pediococcus pentosaceus
    The best practice for Quantitative control of Productivity for media control of 'MRS' is based on the data displayed in this table according ISO 11133.
  • Qualitative control of Selectivity

  • Best Practice MRS

    Qualitative control of Selectivity solid medium

    1. Use a reference material or test suspension with a known number of Escherichia coli and Bacillus cereus.
    2. Streak the test strain (Escherichia coli and Bacillus cereus) as a single line on the media under test (MRS).
    3. Multiple strains can be streaked on the plate as long no crossing occurs.
    4. 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.
    5. Incubate at (30 ± 1) °C for (72 ± 3) h.
    6. Asses the amount of growth as, 0 for no growth, 1 for weak growth or 2 for good growth.
    7. For this medium (MRS) and standard (ISO 15214) combination the criteria is: "Total inhibition (0).

    Control strain WDCM number Ref. media Incubation Criteria Characteristic reaction
    Escherichia coli or (30 ± 1) °C for (72 ± 3) h Total inhibition (0) No growth
    + Bacillus cereus
    The best practice for Qualitative control of Selectivity for media control of 'MRS' is based on the data displayed in this table according ISO 11133.