Inhibition of enterotoxicogenic strains Bacillus cereus GT1 and Staphylococcus aureus S1 isolated from double white cream cheese

In this study three different food preservatives (sodium nitrate, sodium benzoate and sodium sorbate) were used to evaluate their effect on two enterotoxicogenic strains (Bacillus cereus GT1 and Staphylococcus aureus S1). A significant decrease in the viability and production of virulence factors was observed. Yet, obvious tolerance to increasing concentrations (from 1 to 6 g/l) of the three preservatives was recorded reflecting possible resistance mechanisms within the tested strains. The two strains were subjected to increasing doses of gamma radiation (1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 kGy). Obvious correlation was observed between the initial counts of bacterial spores contaminating the products and the required dose for their complete elimination. Whereas the eliminating dose of B. cereus GT1 strain was 10 kGy only 4 kGy was sufficient to suppress the growth and virulence of S. aureus S1, reflecting its sensitivity to low doses of gamma irradiation. Also, inhibition of the two strains by probiotic strain Bacillus pumilus G4 was studied both in situ (in cheese) and in vitro (in culture media). The viable cell population of B. cereus GT1 increased from 10(6) to 2.9 x 10(8) CFU/g within 60 h in controls but decreased from 2.1 x 10(6) to 2.3 x 10(3) CFU/g after treatment with the probiotic strain. No viable count was observed after 60 h of incubation. Whereas, the viable cell population of S. aureus S1 increased from 10(6) to 2.5 x 10(8) CFU/g within 60 h in controls but decreased from 1.5 x 10(6) to 4.3 x 10(2) CFU/g after treatment with the probiotic strain. No viable count was observed after 54 h of incubation.