[Comparative tests of various liquid media for the preenrichment of salmonellae from milk powder (author's transl)]

Abstract

In applying S. mendoza the following four liquid media were tested as to whether they can be used as enrichment broths for salmonellae in milk powder: phosphate buffered water, correspondingly buffered peptone broth, tetrathionate and selenite lactose broth. The yield of salmonellae in these tests is largely independent of the starting pH value of the prepared milk suspension which was varied between 5.8 and 8.2 (Fig. 1). In water the pH dropped within 24 hours (Fig. 2), in peptone and tetrathionate broth within 48 hours down to figures between 4.1 and 4.4 (Fig. 3 and 4), in selenite broth final figures of pH 5.7 were not attained before the lapse of 6 days (Fig. 5). The absolutely highest germ figures were observed in selenite broth. Quite in general the bacteria figure maximum was not attained until the 3rd--5th day, whereupon the germ figures dropped again. In peptone broth the introduced salmonellae were recovered in 73% of all cases after the lapse of 24 hours. Also in tetrathionate the success quota was about 73%, but part of it (8%) did not increase until a lapse of more than 72 hours incubation time, although the pH had already become strongly acid some days previously. In water the introduced salmonellae were traced only in 67%, in selenite broth even only in 61%. Also the time until the first detection increased a little in these media (Table 1). The period during which salmonellae were traceable was equally highest in peptone broth being 57% relative to the complete testing period (tetrathionate: 55%, water: 51%, selenite: 39% (Table 1). This permits the conclusion that peptone broth is the best preenrichment medium in which salmonellae may become traceable after a lapse of approx. 24 hours. Not considerably more unfavourable is the tetrathionate broth which so far had been considered as an exclusive selectivity medium. In this context it is, however, necessary to face the possibility that predamaged salmonellae do not begin to increase until the end of some days incubation and even at a strongly acid pH.