Interactive Effect of Temperature, Atmosphere and Storage Time on the Probability of Colony Formation on Blood Agar by Four Listeria Species.

The probability (P) of one Listeria cell and the cells needed to initiate colony formation on sheep blood agar plates as affected by atmospheric conditions (AC), storage temperature (T), time (t), and Listeria species was evaluated. The factorial design experiments included Listeria monocytogenes (2 strains), Listeria seeligeri , Listeria ivanovii , and Listeria innocua , as test organisms, storage of the plates at 4, 8, 20, and 30°C under air (A), modified atmosphere (MA) of 5% O, + 10% CO + 85% N, 100% CO (CO), vacuum (V), and candle jar (CJ) for 7, 14, 21, 42, and 56 d. Statistical analysis indicated the significant effect of AC (p<0.0004), T (p<0.0001), t (p<0.0001) but not of species (p>0.71). None of the interactions with atmospheric conditions were significant (AC × species, AC × t, AC × T, all with p>0.46). Pairwise comparison of the P's for each of the AC's indicated that 100% CO was significantly more inhibitory to growth initiation than any other AC (p<0.004). No difference among the other AC's was shown. The effect of CO on delaying growth of Listeria was enhanced with decreasing storage T. Thus, under CO, ≤6 cells of L. monocytogenes formed a colony within 7 d at 20°C and 42 d at 4°C. L. ivanovii was the most sensitive to CO and required 4.7 × 10 cells to form a colony after 42 d at 4°C. CO (100%) extended the lag phase at ≤8°C and decreased the rate of growth of the test organisms at 4°C but not at higher temperatures.