The effect of a competitive microflora, pH and temperature on the growth kinetics of Escherichia coli O157 : H7

Growth curves were generated for Escherichia coli O157:H7 in brain-heart in fusion broth incubated at 37 or 15 degrees C in the presence of individual and combinations of competing microflora. Broths were inoculated with E. co li O157:H7 (log(10)3.00 cfu ml(-1)) and competitors (log(10)4.00 cfu ml(-1) ) and the initial pH of the broth was either neutral (7.0) or adjusted to 5 .8 and then sequentially reduced to 4.8 over 10 h to simulate fermentation conditions Growth curves were also generated for the competitors in these c ultures, including Pseudomonas fragi, Hafnia alvei, Pediococcus acidilactic i (pepperoni starter culture) and Brochothrix thermosphacta. Gompertz equat ions were fitted to the data and growth kinetics including lag phase durati on, exponential growth rates and maximum population densities (MPD) calcula ted, in pure culture, the growth parameters for E. coli O157:H7 in neutral pH broths were significantly different from those recorded in simulated fer mentation broths (P < 0.05). The presence of competitors in the broth also had a significant effect on the growth kinetics of the pathogen. H. alvei s ignificantly inhibited the growth (lag phase, growth rate and MPD) of E. co li O157:H7 at 37 degrees C, neutral pH and outgrew the pathogen under these conditions. in neutral pH cultures, two other competitors, B. thermosphact a and P, acidilactici also inhibited the lag phase of the pathogen but had no effect on the of her growth parameters. in simulated fermentation broths , the growth rate of E. coli O157:H7 was consistently slower and the MPD lo wer in the presence of a competitive microflora than when grown individuall y, At 15 degrees C, only one competitor, P. fragi significantly inhibited t he lag phase of the pathogen. The implications of these findings for food s afety are discussed. (C) 1999 Academic Press.