The estimation of the hygienic quality of feed components using an enterobacteriaceae enrichment test

The observed lack of decrease in Salmonella contamination of slaughter animals makes it necessary to take preventive measures. One of the causes of this observation is the use of contaminated feeds. Therefore, either prevention of the use of contaminated components in mixed feeds, or else making some terminal treatment of the mixed feeds compulsory, so as to guarantee the absence of salmonellae, seems required. It has become clear that no reliable opinion on the efficacy of such a system can be obtained in the laboratory by examining feeds or feed components exclusively for the presence or absence of salmonellae. The possibilities for the evaluation of the sanitary condition of such materials with the aid of Enterobacteriaceae as indicator organisms was therefore studied. On the basis of the results obtained, the following procedure is recommended for routine laboratory use.
Out of a consignment of the order of one hundred tons, 20 bags are chosen at random according to an adequate scheme of random selection. From each of these 20 bags a random sample of 100 g. is taken. Of these, ten are first examined for Enterobacteriaceae. For this purpose the sample is well homogenized and then an opening is made with aseptic precautions in the plastic bag containing the sample. Of this material, 50 g. is introduced directly into bottles containing 750 ml. of EE-broth. After 18–24 hours' incubation at 37° C, the broth is streaked in a straight line onto plates of violet-red-bile-glucose-agar; after 18–24 hours' incubation at 37° C, this agar is checked for the presence of more than one single violet colony with a violet halo, which result is taken as positive. If less than two out of ten samples contain Enterobacteriaceae the consignment is accepted; when more than two samples are positive the consignment is rejected. When exactly two out of the first ten samples are positive ten more are examined for Enterobacteriaceae. If less than two of these give positive results, the lot is accepted; if two or more do, the consignment is rejected. This system will allow acceptance of consignments of which 5% of bags are contaminated in 98% of cases; however, lots with 25% bags contaminated will be rejected in about 70% of cases.
As large-scale examination of dry materials of this type might be feared to lead to cross-contamination, bacteriological examination of the air was carried out during routine examination of 120 samples of strongly contaminated fishmeal, i.e. meal with an Enterobacteriaceae count of the order of 0.3 × 103/g. These experiments showed that there is no reason to fear cross-contamination in the routine examination of such samples, provided that the measures usually employed in the laboratory to guarantee aseptic handling of samples are taken.