Effects of Preenrichment Media and Their Incubation Conditions on Isolating Salmonellae from Fish Meal

Various combinations of preenrichment media, their incubation times and temperatures and atmospheres were examined for their efficacy in recovering salmonellae naturally occurring in fish meal. Variations included three preenrichment media (lactose broth, lactose broth supplemented with 10% defibrinated horse blood, and lactose broth with 10% egg yolk), three (25, 37 and 43 C) incubation temperatures, two (24 and 48 h) incubation times, and two (aerobic and anaerobic) incubation atmospheres. Three hundred and twenty samples (50 g each) of various consignments of fish meal known to be contaminated with salmonellae were examined. The lowest number (282) of isolations of salmonellae was obtained using lactose broth for preenrichment. Lactose broth with addition of blood gave 321 isolations, and lactose broth with egg yolk 357 isolations. In general, advantage was observed for preenrichment for 48 h (504) over preenrichment for 24 h (456). This was particularly evident at 25 C with 201 and 144 isolations, respectively. However. at 43 C the results were reverse (135 isolations after 48 h. and 156 after 24 h). At 37 C not so significant differences were found (168 - 48 h, and 156 - 24 h). Salmonellae were isolated 345 times with preenrichment at 25 C. 324 times at 37 C and 291 times at 43 C. Anaerobic preenrichment gave more (510) positives as compared with aerobic preenrichment (450 positives).