Effect of Temperature and Media on Adenosine Triphosphate Cell Content in Enterobacter aerogenes

During cold storage (2°C) of Enterobacter aerogenes for 15 min, 2, 4, 6 and 8 d in glucose mineral salts medium (GMS) and skim milk (SM), substantial differences were noticed in ATP/cell levels. At the appropriate interval, ATP was determined with the firefly bioluminescent method on a portion of the culture, while another portion was activated at 30°C for 40 min and ATP determined at 10-min intervals. Initial ATP content after 15 min of storage in SM was 0.55 femtogram (fg)/cell as compared to 0.84 fg/cell in GMS. Upon cold storage for longer periods (2, 4, 6 and 8 d), the ATP/cell concentrations were relatively stable (0.82, 0.73,0.62 and 0.72 fg) in GMS, while they declined to levels below 0.2 fg in SM. However, activation of only 30 min increased cell ATP concentration in SM to 1.06, 0.92, 0.77, 0.66 and 0.72 fg, and narrowed the difference between ATP/cell levels in the two media. As a realistic ATP concentration/cell value is necessary to calculate bacterial numbers in milk by the bioluminescent technique, a 30-min activation period at 30°C is recommended to replenish the A TP lost during refrigerated storage.