Kultureller Nachweis von Salmonellen in Rohmilch. II: Eigene Untersuchungen

Several cultural methods for isolation of salmonella from foods were tested with regard to their suitability for detection of the pathogen in raw milk. The following schemes were applied: preenrichment in buffered peptone water (BPW), preenrichment in the sample itself with or without inhibitory dyestuffs added, and direct enrichment in MULLER KAUFFMANN tetrathionate broth (MK), selenite cystine broth (SC) as well as RAPPAPORT VASSILIADIS medium (RV). The samples were inoculated with different salmonella serovars, and, in some experiments, also contaminated with other bacteria eventually influencing the detection of the pathogen. The highest recovery (100 %) of salmonellae was observed using BPW as preenrichment medium (18-24 h) followed by selective enrichment in MK/RV simultaneously. Combining BPW with the other selective media lower recoveries were obtained. The same was true for the combination of BPW with SC/RV which is presently recommended in several standards. Shortening the period of preenrichment from 18-24 h to 8 h adversely affected the recovery of salmonellae. Highest recoveries with a single broth (80 %) could be obtained with the RV medium. A combined enrichment in MK/RV yielded a 96.7 % recovery. An incubation period of 8 h was superior to 18-24 h using milk with malachite green added as preenrichment medium and MK, SC, MK/SC or MK/RV for enrichment. Direct enrichment in MK/SC as described in IDF 93A: 1985 and ISO 6785 was inferior to direct enrichment in SC/RV (91.9 % vs. 97.8 %). Using the last-mentioned method better results could be obtained than with preenrichment in BPW followed by selective enrichment in SC/RV (92.9 %) and MK/SC (94.2 %), respectively. RV gave the highest recoveries (92.6 %) when direct enrichment in only one single selective medium was carried out. On the other hand, the use of multiple selective broths is highly recommended. Salmonella serovars differ in their sensitivity to the selective ingredients and may not grow if only one medium is applied. In the experiments described here, for instance, S. Kedougou could not be isolated with the MK medium. Plating out additionally after 48 h improved the recovery. Plating out after 24 h, however, cannot be omitted. Especially enrichment in SC yielded positive results after 24 h but negative ones after 48 h. Larger sample volumes may be examined by incubation of the sample itself with (e. g. malachite green, 60 mg/l) or without addition of inhibitory substances. The recoveries, however, were inferior to those obtained with BPW as preenrichment medium. Best results were observed using a preenrichment period of 8 h followed by selective enrichment in MK/RV (88,9 and 90 % recovery, respectively).