Chromogenic selective medium for Listeria monocytogenes isolation and differentiation (ISO 11290)

DESCRIPTION
O.A. LISTERIA AGAR is a chromogenic selective and differential medium for Listeria monocytogenes identification and isolation from foodstuffs and other samples and it is recommended in ISO Draft 11290 Part 1 and 2.

PRINCIPLE
The selectivity of the medium is due to lithium chloride and to the addition of antimicrobial selective mixture containing ceftazidime, polymyxin B, nalidixic acid and cycloheximide. The differential activity is due to the presence in the medium of the chromogenic compound X-glucoside as a substrate for the detection of β-glucosidase enzyme, common to all Listeria species. The specific differential activity is obtained by means a substrate (L-α-fosphatidylinositol) for a phospholipase C enzyme that is present in Listeria monocytogenes.

TYPICAL FORMULA (g/l)

PREPARATION
Suspend 35.35 g of powder in 500 mL of distilled or deionized water. Heat to boiling with frequent agitation. Sterilize in autoclave at 121°C for 15 minutes. Cool to 48-50°C and add the contents of one vial of O.A. LISTERIA ENRICHMENT Supplement (ref. 81074), prewarmed to 48-50°C, and one vial of O.A. LISTERIA SELECTIVE Supplement (ref. 81074) reconstituted with 5 mL of ethanol/sterile water (1:1). Mix well and dispense in final petri dishes.

TECNIQUE
This medium can be used according to the usual methods for the isolation of Listeria monocytogenes after 2 steps or 1 step enrichment. Make a 1:9 dilution of the sample in Listeria Fraser Broth (code 20131) and incubate at 30°C for 24 hours. Streak 0.1 ml of this suspension onto O.A. LISTERIA AGAR plate and incubate at 36+/-1°C for 24-48 hours.

INTERPRETATION OF RESULTS
Thanks to the combination of two substrates, it is possible to differentiate the colonies of Listeria spp, which grow with a green-blue colour, from the colonies of Listeria monocytogenes which grow with green-blue colonies surrounded by an opaque halo.

STORAGE
The powder is very hygroscopic: store the powder at 10-30°C, in a dry environment, in its original container tightly closed and use it before the expiry date on the label or until signs of deterioration or contamination are evident. Store prepared media at 2-8°C.

WARNING and PRECAUTIONS
The product is not classified as hazardous by current legislation. It is neverthless recommended that the safety data sheet be consulted on its correct use. The product must be used only by properly trained operators.

DISPOSAL of WASTE
Disposal of waste must be carried out according to national and local regulations in force.

PRESENTATION
Dehydrated culture medium

STORAGE
10-30°C

APPEARANCE of the MEDIUM
Homogeneously turbid, withish-beige in color.

SHELFLIFE
4 years

PACKAGING

QUALITY CONTROL

1. Control of general characteristics, label and print
2. Sterility control
   7 days at 25 ± 1°C, in aerobiosis
   7 days at 36 ± 1°C, in aerobiosis
3. Microbiological control
   Inoculum for productivity: 10-100 UFC/ml
   Inoculum for selectivity: 10⁴-10⁵ UFC/ml
   Inoculum for specificity: 10⁴ UFC/ml
   Incubation conditions: 24-48 h at 36 ± 1°C

REFERENCES

1. Artault, S., Bind, J.L., Delaval, Y., Dureuil, N., Gillard, N., (2000) AFNOR Validation of the ALOA method for the detection of Listeria monocytogenes in foodstuffs. Colloque de la Societè Francaise de Microbiologie, Paris 19-20 Octobre, 2000.
2. ISO 11290 1/2 (Draft, May 2002) Microbiology of food and animal feeding stuffs – Horizontal method for detection and enumeration of Listeria monocytogenes.
3. Ottaviani, F., Ottaviani, M., Agosti, M., (1997) Esperienze su un agar selettivo e differenziale per Listeria monocytogenes. Industrie alimentari, XXXVI, luglio-agosto, 888.

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