Tailed survival curves

Tailed Survival Curves Tailed survival curves cannot be extrapolated. They are characterized by a slope that diminishes with increasing exposure to the sterilization treatment. They are often described as concave. ... 

There is one basic question that ought to be addressed about tailed survival curves are they artefacts or are they genuine They have certainly been noted in many experiments. Beside the undoubted occurrence of examples of genetic heterogeneity and heterogeneity of treatment, there are three theories [21 ] that merit some consideration. 

Survival curves for irradiated pure cultures of microbial populations conform to the idealized types described in Chapter 2. In radiation microbiology the D-value is invariably called the D]q value. It is doubtful whether a truly tailed survival curve has ever been seen tn a property conducted irradiation study of pure cultures. There are, however, major differences seen from microorganism to microorganism and from condition to condition. 

Beside bioburden, the other determinant of sterility assurance is the survival curve and its shape and its slope. It is not correct to assume that all survival curves are of the simple linear type when data is plotted on semilogarithmic graph paper. Three general types of survival curve have been reported, the exponential curve, the shouldered" curve, and the "tailed" curve (Fig. 4). 

Cerf, O. (1977). Tailing of survival curves of bacterial spores. Journal of Applied Bacteriology 42 1-19. 

In order to discuss quantitatively the DoD droplet formation process, the positions of several representative points in the ejected liquid can be plotted as a function of time to produce the curves of the DoD droplet formation, as depicted in Figure 10. The axial distances of points 1-5 from the nozzle exit are denoted as Xi( ) to Xs( ), respectively, with measured from the first appearance of liquid from the nozzle. Initially, point 1 is the leading edge of the liquid ejected from the nozzle that later becomes the tip of the primary drop. Point 2 is the first pinch-off point of the liquid from the nozzle tip, and also the tail of the free liquid thread its first appearance corresponds to the initial breakup time bi. Points 3 and 4 are the lower and upper points produced by the second pinch-off, and the curves associated with these points initiated at the second breakup time b2) these curves form a closed loop if (in the case of a single satellite drop) the satellite recombines with the main drop (as is the case in Figure 10(a)) or may continue separately if the satellite survives as a discrete body of liquid. Later, point 3 becomes the tail of the primary drop, and point 4 becomes the head of the secondary free liquid thread or satellite. Between points 2 and 4, further pinch-off points may occur. Point 5 is the tip of liquid protruding from the nozzle orifice due to multiple reflections of the pressure wave inside the ink chamber. 

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