Occasionally, experimental survival curves of micro-organisms exposed to a lethal agent have a ¯at region and traditionally it has
been interpreted as evidence of the existence of a ``shoulder''. However, if the survival curve is considered the cumulative distribu-
tion of lethal events, which re¯ects 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, insucient to con®rm any speci®c 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. # 2000 Elsevier
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