Effect of growing bacteria isolated from food on staphylococcal growth and thermonuclease activity

The effect of growing Bacillus subtilis, Streptococcus faecalis var. liquefaciens, Escherichia coli and Lactobacillus plantarum on staphylococcal growth and thermonuclease (TNase) activity was investigated in liquid media and in foods. Growth of S. aureus at 37°C for 24 h under aerobic conditions was not inhibited by the four test strains. However, staphylococcal TNase activity decreased by 70 and 80% in the presence of B. subtilis and S. faecalis var. liquefaciens respectively. Staphylococcal growth and TNase activity were strongly inhibited by L. plantarum under anaerobic conditions at pH 5.5 but not at pH 7.0. Furthermore, optimal TNase production by S. aureus occurred in cooked meat medium containing 0.5 to 5.0% NaCl. TNase production significantly decreased at higher concentrations of NaCl. In the presence of B. subtilis and S. faecalis var. liquefaciens. TNase activity decreased at NaCl levels of 0.5 to 5.0% but not at NaCl concentrations>5.0%. TNase activity was also inhibited by growing B. subtilis and S. faecalis var. Liquefaciens at pH 5.0 to 7.0. The rate of inhibition increased with increasing pH. TNase activity was not inhibited after 48 h incubation at 20° in the presence of B. subtilis and S. faecalis var. liquefaciens but significant inactivation could be demonstrated at 25° to 37°C. The results obtained with artificially contaminated, sterile food samples were similar to those obtained with brain-heart infusion broth, but the degree of decrease in TNase activity in food was much lower than that in brain-heart infusion broth.