Growth of Escherichia coli and Staphylococcus aureus on the surface of TrypticaseSoy Agar (TSA) packaged with various CO 2 partial pressures (0, 20, 40, 60, 80, 100%, balance N 2 ) was compared to the control (N 2 100%) on TSA in which the pH was adjusted to equal that in CO 2 atmospheres at 15ЊC and 30ЊC. At 15ЊC, the biostatic effect was noted with all CO 2 partial pressures for both species. At 30ЊC, the biostatic effect of CO 2 was almost completely nullified for E. coli, but that for S. aureus was still effective. S. aureus was more sensitive to the inhibitory effects of CO 2 than E. coli at both the temperatures.mission rate of the film was 8.7 cc/m 2 /24h/ atm. Details as to the packaging methods and gas analyses have been described previously (Kimura et al., 1996).The surface pH of the agar was measured with a pH meter (Horiba M-8, Kyoto, Japan) equipped with a flat combination electrode (T-8) immediately (with a few seconds) after opening. The bags were opened 3, 30, 60, 90, and 120 min after packaging. The pH was averaged from the values of three replicate plates contained in different packages.We examined the growth inhibitory effect of CO 2 alone, not in combination with decreasing pH. For this purpose, the pH of N 2 -packaged media was adjusted to the same level as that which occurred on dissolution of headspace CO 2 . Since high CO 2 decreased the pH to Ͻ6.0 when unbuffered TSA medium was used, the media to be packaged with CO 2 were also Tris buffered (0.02M) to minimize the pH decrease. After the pH had been adjusted to 8.0 initially, the fall in pH after MA treatments was less pronounced. The surface pH on TSA stored for 120 min after MAP treatments was 6.7 for 20% CO 2 , 6.3 for 40% CO 2 , 6.2 for 60% CO 2 , 6.0 for 80% CO 2 , 5.9 for 100% CO 2 in 15°C experiments and pH 7.0 for 20% CO 2 , 6.8 for 40% CO 2 , 6.6 for 60% CO 2 , pH 6.4 for 80% CO 2 , and 6.2 for 100% CO 2 in 30° C experiments. The pH of N 2 -packaged media (also buffered with 0.02M Tris) for comparing bacterial growth was adjusted to the corresponding values using 0.1N HCl. The MA treatment with N 2 did not change pH on the agar surface.
