Microbial safety of meat and meat products is of great public and industrial significance. However, some would claim that the numbers of disease outbreaks caused by meat products have not decreased, even through a lot of research and various approaches had been evaluated to ensure the increased cleanliness of meat and meat products. The question then arises is why are more problems surfacing in meat and meat products which are presumably cleaner than product more contaminated. Don Murphy (1996), in his article of "how clean is too clean?" reported that "Jay (1996a) suggested that the current approach to food safety should be achieving the goal of safe meat, not just in reducing the numbers of microorganisms". Jay (1996a) asked a thought provoking question "Is our meat too clean?" which is based on the hypothesis that lack of microbial interference and competition might be encouraging pathogen growth and causing food poisoning. His hypothesis suggested that antagonistic effect of background microflora against the growth of pathogens, and that 105 - 106 background bacteria/g of meat were inherently safer than 103 background bacteria/g of meat.
Beneficial attachments of non-pathogenic microflora in fighting foodborne diseases of live poultry, called competitive exclusion, has been applied widely (Jay, 1996; Scanlan, 1997; Palmu and Camelin, 1997; Nuotio et al., 1992; and Hume et al., 1996 and 1997). The presence of other competitive microflora have been known as an effective method in preserving food, and lactic acid bacteria have been suggested as suitable "barriers" against pathogenic organisms (Gombas, 1989). The most common microbial interferences used in meat and meat products are the use of LAB (lactic acid bacteria) which are bacteria that have received a great attention in food. Abdel-Bar and Harris (1984) had reported that Lactobacillus bulgaricus with levels of 104 to 106 CFU/ml had inhibitory effects on pathogens in ground beef. However, the problem with the use of LAB is the production of a lactic acid flavor in meat.
Gram (1993) examined the antibacterial effects of 209 Pseudomonas strains isolated from spoiled iced fish and found 67 strains can inhibit some growth of certain target pathogens which suggested the microbial interaction may influence the selection of a microflora for some chilled food products. Ragruhbeer et al. (1994) had inoculated two strains of E. coli O157:H7, one non-pathogenic E. coli and one strain of non-fecal Enterobacter aerogenes in commercial food and the results indicated that the high inoculation levels of non-pathogenic E. coli (> 106 CFU/g) showed inhibition of E. coli O157:H7 at room temperature after 96 hours; however, E. coli O157:H7 still survived in salad dressing after 17 days in the high inoculation treatment after refrigerated storage.
The single most dominant microflora for refrigerated aerobic stored
meat is Pseudomonas (Gill, 1983) with an average level of 104
CFU/g (Foegeding et al., 1983). In the beef study conducted by
Foegeding et al. (1983), Pseudomonas had maintained 104
CFU/g from day 0 to day 6 in beef samples. Enterobacteriaceae had
also been reported as the spoilage flora of fresh meat that undergo
aerobic refrigerated storage (Eribo and Jay, 1985; Gamage et al.,
1997). Giannuzzi et al. (1998) checked microbial flora at
temperature of 0, 4, 7, 9, and 10oC and the results of
Enterobacteriaceae was 102 CFU/g to 103-5 CFU/g, LAB was 103
CFU/g to 104-6 CFU/g and Pseudomonas was 102 to 103 CFU/g to 104
to 107 CFU/g from day 0 to day 7 by using gas permeable packaging. In
the study of Palumbo et al. (1997), they used ground beef with
background bacteria of 103 CFU/g and 106 CFU/g and E. coli with
103 CFU/g.
Murphy (1996) also quoted the hypothesis from Jay (1996)
who reported "When processors implement stringent sanitation
controls, they reduce the level of pathogenic bacteria. But in the
process, the protective microorganisms are also eliminated - leaving the
remaining or re-contaminated pathogens with a clear field to
reproduce". Therefore, the hypothesis of this experiment is that
higher numbers of normal background bacteria (Pseudomonas,
Acinetobacter, Lactobacillus) will retard the growth of
E. coli (fecal source) more than that of lower numbers of
background bacteria on aseptically obtained raw ground beef under
aerobic refrigerated storage.
Different background microorganisms (Pseudomonas, Acinetobacter and Lactobacillus) commonly found in raw ground beef will be inoculated with two different levels of non-pathogenic E. coli in non-denatured, aseptically obtained fresh lean ground beef in different studies. The use of different levels of microorganisms was to understand the microbial interaction between normal microflora and E coli to see if different levels of background flora will have various effects on the growth of E. coli. The levels of Pseudomonas utilized were 103 and 106 CFU/g, and the levels of E. coli were 102 and 104 CFU/g. The levels of Acinetobacter and Lactobacillus which will be inoculated in continuation of this research will be at two different levels of 102 and 104 CFU/g. In this study, only Pseudomonas and E. coli were evaluated. Others (Acinetobacter, Lactobacillus) will be evaluated in subsequent research. This study is important because it will provide information helpful in evaluating the possibility of pathogen reduction (from fecal sources) in fresh raw ground beef.