Modeling the Physiological State of the Inoculum and CO2 Atmosphere on the Lag Phase and Growth Rate of Listeria monocytogenes

In previous studies, the growth of L. monocytogenes has been modeled under different CO2 headspace concentrations; however, the inoculum cells were always in the stationary phase. In this study, the growth of L. monocytogenes under different CO2 concentrations as affected by the physiological state of the cells was investigated. Exponential-growth-phase, stationary-phase, dried, and starved cells were prepared and inoculated at 5°C into brain heart infusion broths that had been preequilibrated under atmospheres of 0, 20, 40, 60, or 80% CO2 (the balance was N2). Lag-phase duration times (LDTs) and exponential growth rates were determined by enumerating cells at appropriate time intervals and by fitting the data to a three-phase linear function that has a lag period before the initiation of exponential growth. Longer LDTs were observed as the CO2 concentration increased, with no growth observed at 80% CO2. For example, the LDTs for exponential-phase, stationary-phase, starved, and dried cells were 2.21, 8.27, 9.17, and 9.67 days, respectively, under the 40% CO2 atmosphere. In general, exponential-growth-phase cells had the shortest LDT followed by starved cells and stationary-phase cells. Dried cells had the longest LDT. Exponential growth rates decreased as the CO2 concentrations increased. Once exponential growth was attained, no retained differences among the various initial physiological states of the cells for any of the atmospheres were observed in the exponential growth rates. The exponential growth rates under 0, 20, 40, 60, and 80% CO2 averaged 0.39, 0.37, 0.23, 0.23, and 0.0 log CFU/day, respectively. Dimensionless factors were calculated that describe the inhibitory action of CO2 on the LDTs and exponential growth rates for the various physiological states.