Microbial survival curves - the reality of flat "shoulders" and absolute thermal death times

Occasionally, experimental survival curves of micro-organisms exposed to a lethal agent have a flat region and traditionally it has been interpreted as evidence of the existence of a "shoulder". However, if the survival curve is considered the cumulative distribution of lethal events, which reflects a spectrum of resistances, or sensitivities, then when the distribution's mean, or mode, is large relative to its spread, a region resembling a "shoulder" will be observed irrespective of whether the distribution is symmetric or skewed. Computer simulated survival curves generated with the Fermi and Weibull distributions as models demonstrate that the shape of the survival curve alone is, therefore, insufficient to confirm any specific inactivation mechanisms at the cellular and molecular level, although it can refute the existence of some. Microbial mortality has also been assumed to be a process following an exponential decay and hence that a certain degree of survival is inevitable. It is not inconceivable, however, that there can be an absolute thermal death time if the survivors are being progressively weakened by a prolonged exposure to the lethal agent. This testable possibility is demonstrated with simulated survival curves generated with two mathematical models. (C) 2000 Elsevier Science Ltd. All rights reserved.