This research was conducted to understand the interactions between normal background microorganisms (Pseudomonas and Lactobacillus) and Escherichia coli on solid food such as fresh ground beef. By using aseptically-obtained fresh ground beef as a model, different levels of background bacteria along with different levels of E. coli were inoculated and applied in three experiments at different storage temperatures.
In Experiment I, three levels (zero, 3 logs and 6 logs) of Pseudomonas were combined with three levels (zero, 2 logs and 4 logs) of E. coli and stored at 7 °C for 7 days. One log increase of VRBA (E. coli) counts was observed for treatments with 2 log E. coli inoculation but no changes were found for treatments with 4 log E. coli inoculation during the 7 days storage at 7 °C. No effects of Pseudomonas inoculation levels on E. coli counts were observed during the 7 days storage at 7 °C.
The same levels (zero, 2 logs and 4 logs) of Pseudomonas were combined with same levels (zero, 2 logs and 4 logs) of E. coli in Experiment II and stored at 4 °C for 14 days or 25 °C for 30 hours for a better understanding of bacteria interaction during storage which occurred in Experiment I when the product was stored at 7 °C. The results indicated Pseudomonas was still the dominating strain in both storage temperatures (4 and 25 °C) due to their faster growth rate and short lag time. E. coli counts decreased until Day 10 and then increased at Day 14 when stored at 4 °C, but E. coli counts increased until Hour 25 and then decreased at Hour 30 when stored at 25 °C. Higher Pseudomonas inoculation had higher E. coli counts at 25 °C storage (due to metabiosis; Gram et al., 2002) but no effects of Pseudomonas on E. coli counts was observed when stored at 4 °C.
Different from Experiment II, an additional background bacterium, Lactobacillus was added to Experiment III. A total of three bacteria strains were used and more interactions were observed only with the dominating strains at both 4 °C for 14 days and 25 °C for 30 hours. Pseudomonas dominated at both storage temperatures. Lactobacillus counts did increase at 25 °C storage but not at 4 °C storage. Lower E. coli counts were observed with higher Pseudomonas inoculation treatments and E. coli counts decreased with storage time when stored at 4 °C. E. coli counts increased with storage time when stored at 25 °C. Both Lactobacillus and E. coli showed higher counts while Pseudomonas was co-existing when stored at 25 °C.
More reductions of E. coli by higher Pseudomonas inoculation were observed in Experiment III but not in Experiment II or I when stored at 4 °C. This implied that the addition of Lactobacillus had a synergistic effect on reducing E. coli at 4 °C storage. This also suggested the probability that more strains of background bacteria would result in more interactions between species and probably might have a better inhibition effects on the growth of fecal source bacteria with refrigerated storage.