Killing by irradiation

Microorganisms as a group are much more resistant to the effects of ionising radiation than are higher organisms, and some more resistant than others. A plot of log surviving numbers against time of irradiation is shown in Figure 13.11. The decimal reduction value (D 10) is analogous to the D value used for heat sterilisation (see above).

Figure 13.8 The kinetics of cell death. (a) During heat sterilisation, the number of living cells decreases by the same proportion per unit time, giving an exponential curve. (b) When plotted on a logarithmic scale, the decrease in numbers is seen as a straight line, whose slope is a reflection of the rate of killing. The time period between A and A' is the decimal reduction time (D): the time taken to reduce the population to one-tenth of its size. The total period until the point SA is the sterility assurance value, when there is only a one in a million probability of any cells having survived. From Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services

Figure 13.8 The kinetics of cell death. (a) During heat sterilisation, the number of living cells decreases by the same proportion per unit time, giving an exponential curve. (b) When plotted on a logarithmic scale, the decrease in numbers is seen as a straight line, whose slope is a reflection of the rate of killing. The time period between A and A' is the decimal reduction time (D): the time taken to reduce the population to one-tenth of its size. The total period until the point SA is the sterility assurance value, when there is only a one in a million probability of any cells having survived. From Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services

Figure 13.9 Sterilisation time is dependent on starting population. Populations A, B and C all have the same decimal reduction rate, but different starting populations, therefore after a given time they have different numbers of survivors. Population D has the same starting numbers as C, but because a smaller proportion are killed per minute, (i.e. the slope is less steep), a greater number survive after time X

Figure 13.9 Sterilisation time is dependent on starting population. Populations A, B and C all have the same decimal reduction rate, but different starting populations, therefore after a given time they have different numbers of survivors. Population D has the same starting numbers as C, but because a smaller proportion are killed per minute, (i.e. the slope is less steep), a greater number survive after time X

Surviving percentage of bacteria

100

10

-\----------- —

1

\

45o

0.1

65o

D-value

Figure 13.10 D value is reduced at higher temperatures. The survival rate of a mesophilic bacterium at three different temperatures. The D value is the time taken to reduce the population to one tenth of its starting value: note how it falls from 50min (45°C) to 5min (65°C)

Surviving 100 percentage of bacteria

10 1

Radiation dosage

Figure 13.11 Killing by irradiation. The decimal reduction value (D10) is the dose of radiation necessary to reduce the population of microorganisms to one-tenth of its value. The value for D10 differs greatly between organisms

Figure 13.10 D value is reduced at higher temperatures. The survival rate of a mesophilic bacterium at three different temperatures. The D value is the time taken to reduce the population to one tenth of its starting value: note how it falls from 50min (45°C) to 5min (65°C)

Was this article helpful?

0 0
Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

Get My Free Ebook


Post a comment