The effect of aerobic mesophilic microfloral concentration on the isolation of Listeria monocytogenes LM82 was studied in 31 (18 cheeses and 7 noncheese) retail ,foods having standard plate counts of 10' to l @ colony forming units (CFU)Ig. Foods were spiked with L. monocytogenes and enriched at 30°C for 24 h in a selective enrichment broth used in a U.S. Food and Drug Administration method. Inoculum levels for isolation on modijied McBride agar ranged from 0.1 to > 5 X lo' with a geometric mean value of 5 inoculated CFUIg or 1.4CFUIg. Pure Enterococcus (Streptococcus) faecalis (0 to 6 x 10' inoculated CFUImL) in the absence of food matrix had no effect on the enrichment of L.monocytogenes . Ease of isolation of LM82 was independent of the food microfrora concentration both generally and in the speciJc food type of 9 Brie cheeses, Competition, when it occurs, therefore, may be due to speciJic bacterial competitors rather than bacterial numbers.
Commercially shredded cabbage distributed at the retail level is usually packaged under vacuum or in a modified atmosphere of nitrogen and carbon dioxide to suppress proliferation of aerobic spoilage microorganisms. The ability of Shigella sonnei to survive and grow in shredded cabbage packaged under these conditions was determined by the 3-tube most probable number procedure. After artificial inoculation with S. sonnei at one of three different levels, the shredded cabbage was packaged aerobically, under vacuum, or under modified atmosphere (30% nitrogen and 70% carbon dioxide) and stored at room temperature (24 ± 2°C) or under refrigeration (0–6°C). For most levels of inoculation, S. sonnei tended to increase or remain relatively high for 1–3 d in cabbage packaged under all three conditions and stored at room temperature. However, after 3 d of storage at this temperature, S. sonnei approached indeterminately low levels. The steady decrease of pH values of shredded cabbage over the storage period may have contributed to the decrease of S. sonnei. Under refrigeration, however, both the S. sonnei levels and the pH values remained relatively constant. The results indicate that S. sonnei can survive and even proliferate in shredded cabbage packaged and stored under a vacuum or modified atmosphere as well as aerobic conditions, thereby posing a potential hazard to the consumer.
Five high-moisture foods were used to evaluate both the effect of a 6 h, rather than the standard 24 h, selective enrichment incubation period, and the efficiency of Rappaport-Vassiliadis (RV) medium relative to the use of selenite cystine (SC) and tetrathionate (TT) broths for the recovery of Salmonella. Cheese and lettuce were artificially inoculated with a pool of two serotypes, whereas the other foods were naturally contaminated. Significantly higher numbers of Salmonella-positive test portions were obtained at 24 h with the following food and media combinations: cheese (TT and RV media), lettuce (SC, TT, and RV media), raw chicken (RV medium), and pork sausage (SC, TT, and RV media). There were no significant differences between the two incubation periods in recovery of Salmonella from turkey. Overall, more Salmonella-positive test portions were obtained from samples of lettuce, chicken, and pork sausage selectively enriched in RV medium than in SC or TT broths. The results of this study indicate that not all high-moisture foods can be selectively enriched for 6 h without a significant loss in recovery of Salmonella. RV medium was superior to SC and TT broths for recovery of Salmonella from some meats and was at least as productive in its recovery from the other high-moisture foods tested.
A preenrlchment procedure and a direct selective enrichment procedure were compared for recovery of Salmonella artificially inoculated into liquid whole egg, egg yolk, and egg albumen. For liquid whole egg and egg yolk, the 2 procedures were comparable. With egg albumen, however, preenrlchment In lactose broth gave significantly higher recoveries than did direct selective enrichment in either selenite cystine or tetrathionate broths. The lactose preenrlchment procedure was used to determine the survival of S. enterltldla in egg yolk and egg albumen over a period of 7 days. As shown by most probable number determinations, counts of S. enterltldla Inoculated Into egg albumen decreased by 3 log units, whereas those in egg yolk did not change significantly. It is recommended, therefore, that only the egg yolk be examined for this pathogen. In a comparison of 5 different preenrlchment media (lactose broth, brain heart Infusion broth, trypticase soy broth, buffered peptone water, and nutrient broth), lactose broth was somewhat less productive than the other 4 media for the recovery of Salmonella from egg yolks. Trypticase soy broth gave the highest recovery.
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