Influence of raw meat natural background flora on growth of Escherichia coli O157: H7 in ground beef

Abstract

Escherichia coli O157:H7 is a foodborne pathogen of increasing importance. It has been involved in several threatening outbreaks, most of them associated with meat products. In this study, the influence of some bacteria from the natural background flora of raw meat over E.coli O157:H7 in ground beef stored under refrigeration and at room temperature was evaluated. Different levels of E.coli O157:H7 (10 1 -10 2 , 10 3 -10 4 and 10 6 -10 7 CFU/g), inoculated in ground beef samples, were challenged with strains of non-pathogenic E.coli, Pseudomonas putida or Leuconostoc sp. Growth of the pathogen was monitored using standard cultural methods and an ELISA-type rapid method. Nonpathogenic E.coli, Pseudomonas putida and Leuconostoc sp. did not affect growth of E.coli O157:H7 in ground beef, both under refrigeration and at room temperature. Based on these findings, the low occurrence of E.coli O157:H7 in raw meat may not be attributed to antagonistic effects of bacteria from the natural background flora.

Survival of Shiga toxin-producing Escherichia coli O157:H7 in Minas frescal cheese

“…However, this step increases the bacterial population and does not portray the real number of bacteria in the original product. Saad & Franco (1999) assessed the influence of the natural microbiota of raw meat on the growth of E. coli D157:H7 in ground beef, which was maintained at 8.5 °C for 96 h. When intermediate levels of 10 3 to 10 4 CFU.g -1 of E. coli D157:H7 were incubated with 10 6 to 10 7 CFU.g -1 of non-pathogenic E. coli, it was only possible to count E. coli D157:H7 up to 24 h of storage at 8.5 °C, and this count was equal to the initial contamination level, which most likely was observed due to the difficulty of enumerating the pathogen in the midst of high counts of non-pathogenic E. coli. When the contamination level was smaller, i.e., from 10 1 to 10 2 CFU.g -1 , the difficulties increased, and it was not possible count the pathogenic E. coli.…”

Role of Glucose in Enhancing the Temperature-Dependent Growth Inhibition ofEscherichia coliO157:H7 ATCC 43895 by aPseudomonassp

“…It needs to be stressed, however, that a complete growth inhibition of the pathogen at 10°C, and to a lesser extent at 15°C, occurred in mixed broth cultures with 1% glucose. Thus, the concentrations of glucose naturally present in fresh meat (e.g., 0.1 to 0.5%) (18) or as a supplement (e.g., 0.2%) in minced meat in the study of the effects of microbial association on spoilage (13) the background flora may be expected in culture broth (9) or foods (17,20,21) stored in air without any previous glucose supplementation. Consistent with this expectation, we have recently shown that the inhibitory effects of the Pseudomonas strain used in this study against E. coli O157:H7 ATCC 43895RIFϩ are reduced in mixed cultures grown in normal TSBYE with 0.25% glucose at incubation temperatures of 10 to 25°C and are finally decreased to levels (unpublished data) similar to those of the cocultures without glucose ( Fig.…”

“…Similarly a high level of background flora inhibited the growth of E. coli O157:H7 in ground beef by a factor of 1 to 3 logs at 10 to 12°C; however, this inhibition was maximized anaerobically rather than aerobically due to the natural selection of Lactobacillus instead of Pseudomonas under limited-oxygen conditions (17,24). Additional coinoculation studies with Pseudomonas strains, though, have shown only weak or no inhibitory effects of these bacteria on E. coli O157:H7 in skim milk (20) and ground beef (21), especially at abusive storage temperatures.…”

Occurrence of Salmonella spp and Escherichia coli O157:H7 in raw meat marketed in São Paulo city, Brazil, and evaluation of its cold tolerance in ground beef

“…Those researchers concluded that the death of E. coli O157:H7 in ground beef after freezing and thawing ranged from 0.62 to 2.52 log10 CFU/g. On the other hand, Saad and Franco 27 , observed that the counting of E. coli O157:H7 inoculated in ground beef kept under refrigeration was relatively constant throughout the 96 hours the experiment lasted, and Doyle and Schoeni 10 showed that E. coli O157:H7 in ground beef can survive up to nine months at -20ºC. The presence of high levels of total aerobes (>7 log 10 CFU/g) in ground beef stored at 2ºC could result in unfavorable changes in the environment and it could consequently affect the survival of E.coli O157:H7 1 .…”