Cells of Listeria monocytogenes or Salmonella enterica serovar typhimurium taken from six characteristic stages of growth were submitted to an acidic stress (pH=3.3). As expected, the bacterial resistance increased from the end of the exponential phase to the late stationary phase. Moreover, the shape of the survival curves gradually evolved as the physiological states of the cells changed. A new primary model, based on a two mixed Weibull distributions of cell resistances, was proposed to describe survival curves and the change of pattern with the modifications of resistance of two assumed subpopulations. This model resulted from the simplification of a first proposed model. Theses models were compared to the Whiting's model. The parameters of the proposed model were stable and showed a consistent evolution according to the initial physiological state of the bacterial population. Compared to the Whiting's model, it allowed best fit and more accurate estimation of the parameters. Lastly, the parameters of the simplified model had a biological significance which facilitated their interpretation.