Commission Regulation (EC) No 273/2008 (repealed)Show full title

DETECTION OF ANTIMICROBIAL RESIDUES IN SKIMMED MILK POWDER

A microbial inhibitor screening test using Geobacillus stearothermophilus var. calidolactis ATCC 10149 (identical to strain C953) as test micro-organism and being sufficiently sensitive to detect 4 μg benzylpenicillin per kg milk and 100 μg sulfadimidine per kg milk shall be used. Commercial tests kits are available and can be used if they have the required sensitivity for benzylpencillin and sulfadimidine.

For the test, reconstituted skimmed milk powder (1 g powder 9 ml aqua dest) is used. The test is carried out as described in ISO/TS 26844:2006 Milk and milk products — Determination of antimicrobial residues — Tube diffusion test IDF — Bulletin No 258/1991, section 1, Chapter 2, or according to the instructions of the test kit manufacturer(1).

Positive results are to be interpreted as follows:

1.

The presence of β-lactams can be confirmed by repeating the test adding penicillinase to the test system(2):

Negative result: Inhibiting substance is a β-lactam antibiotic.

Positive result remains: Inhibiting substance cannot be identified by this procedure, continue with 2.

2.

The presence of sulfonamides can be confirmed by repeating the test adding p-amino benzoic acid to the test system:

Negative result: Inhibiting substance is a sulfonamide.

Positive result remains: Inhibiting substance cannot be identified by this procedure, continue with 3.

3.

The presence of a combination of a β-lactam and sulfonamide can be confirmed by repeating the test adding pencillinase + p-amino benzoic acid to the test system:

Negative result: Inhibiting substances are a β-lactam antibiotic and a sulfonamide.

Positive result: Inhibiting substance cannot be identified by this procedure.

(1)

Important notice: False-positive results may be obtained, when skimmed-milk powder is analysed. It is important, therefore, to verify that the test system used does not yield false-positive results.

(2)

Some β-lactams are less sensitive to β-lactamase. In such cases an additional pre-treatment of the sample, (1 ml of test sample with 0,3 ml of penase concentrate at 37 °C for 2 h) is recommended.