Project Summary

Isolation and Identification of Bacteria Found in Raw Ground Beef that Inhibit the Growth of Eschericia coli O157:H7

Principle Investigator(s):
T.A. Bridgeman and E.A. Zottola
 University of Minnesota
Completion Date:
February 1998


Layman’s Summary 

The natural microflora of ground beef appears to exhibit a bacteriostatic effect on the growth of E. coli O157:H7. This project took raw ground beef and screened it for naturally occurring organisms that possess the ability to inhibit the growth of E. coli O157:H7. After isolating approximately 500 Gram-negative organisms, individual organisms were tested for their effectiveness in inhibiting E. coli O157:H7. Several Gram-negative organisms (13 total) had inhibitory properties with 11 identified as Pseudomonas aeruginosa and two isolates identified as Serratia liquefaciens. The majority (90%) of the isolated lactic acid bacteria (LAB) showed inhibitory effects against E. coli O157:H7. By controlling the pH of the growth agar, it was determined that the inhibitory effect from LAB was due to acid production and not some other bacteriocin or antimicrobial substance.   

When E. coli O157:H7 was introduced into retail ground beef samples, growth did not occur when stored at 4° C. It merely survived, held constant by a natural environment of spoilage bacteria at refrigerated temperatures. However, when E. coli O157:H7 was transferred to sterile ground beef that had no competing organisms, an increase in numbers did occur. The results from this study indicate that the natural flora of ground beef may be responsible for the low incidence of rate and low numbers of E. coli O157:H7 in raw ground beef. Therefore, it is possible that high numbers of these organisms can control the survival of E. coli O157:H7 in foods. However, these competitive bacterial systems could keep E. coli O157:H7 at low levels and thus make them difficult to detect and enumerate.