STUDIES ON THE APPLICABILITY OF COMMERCIA L INHIBITOR TESTS AS SCREENING-TEST FOR THE PRESENCE OF ANTIINFECTIVA IN MILK AND MILK-BASED PRODUCTS

This paper deals with studies on the applicability of the live commerc ial microbiological inhibitor tests Brilliantblack-Reduction-Test (BRT )-LV (Landesvereiningun der Bayerischen Milchwirtschaft), BR-Test ''AS '' and BR-Test ''Blue Star'' (Laboratorium Enterotox, now: Gist-brocad es), BR-Test ''AS-Special'' and Delvotest SP (Gist-brocades) as screen ing test far the presence of antiinfectiva in pasteurized, UHT or ster ilized drinking milk and in milk-based products (cream, plain condense d milk, sweetened condensed milk, buttermilk, whey, dry milk, lactose, casein and caseinate as well as powdered dietetic infant food). About 400 samples were analyzed. 1. The preparation of a 10 % suspension by reconstitution of the tested dried foods in sterile distilled water w as adequate in most cases. Products with a pH differing from that of f resh milk should be adjusted to pH 6,7 +/- 0,1 prior to testing; for t he examination of products with a higher viscosity (e. g. condensed mi lk) a ''dilution'' with sterile distilled water may be favourable. 2. With several products the incubation time necessary for complete colou r change of the indicator and interpretation of the results was consid erably longer than that for pasteurized or raw milk. This was especial ly true for the BR-Tests ''Blue Star'', ''AS'' and ''AS-Special'', but not so often for Delvotest SP and only in the case of condensed milk for BRT-LV. In order to avoid false-positive results originating from a too short incubation time it is essential, that an antiinfectiva-fre e material identical (or at least equivalent) to the product tested is used as control substrate and not only pasteurized milk. 3. Analyzing artificially contaminated pasteurized whole milk the Maximum Residue Limit (MRL) for benzylpenicillin could be detected by all test systems , that for cloxacillin by three (BRT-LV, BR-Test ''AS-Special'' and De lvotest SP) and that for sulfadimidine only by one (BR-Test ''Blue Sta r''). The sensitivity of all test systems for dihydrostreptomycin and oxytetracycline was mostly less than the given MRLs. Comparable detect ion limits were observed with UHT milk, cream products and reconstitut ed skim milk; only with buttermilk products the sensitivity was often considerably decreased. 4. The applicability of the test systems as sc reening test for the presence of antiinfectiva is mainly determined by test period, sensitivity, specificity, and costs. The low-priced BRT- LV was the only system that could easily be used as screening test for beta-lactam-antibiotics in all analyzed samples. The long test period (at least 3 h 15 min) caused primarily by the additional pre-diffusio n of 1 h and the low sensitivity against sulfadimidine and oxytetracyc line are, however, disadvantageous for its routine use. Delvotest SP i s the most expensive of the five tested systems. but it is more sensit ive against the two last mentioned antiinfectiva and can be used for t he majority of the products without delay of the test period (at least 2 h 30 min). The BR-Test ''AS-Special'' as well as the less sensitive BR-Test, ''AS'' are not as expensive as the Delvotest SP, but require nearly always an extra incubation time compared to pasteurized milk ( at least 2 h 15 min). This is especially true for the BR-Test ''Blue S tar'', that has only the advantage of a high sensitivity against sulfa dimidine (with the exception of buttermilk products).