Posted: April 2nd, 2015

Scientific Principles Underlying Decontamination

ASSESSMENT NUMBER: Rewritten

The inactivation curve is done by plotting population against time at a specified temperature (Gazso & Ponta, 2005). But since the population is usually in very large numbers, it is always advisable to use the log of microorganisms. The ensuing issue is therefore that the D-value is read from the line of best fit. Afterwards the D – value of the thermal processes can be determined by considering the change in temperature over change in the log and is therefore an estimate of one log cycle at a specified temperature (Gera, 2011; Ogata, & Shibata, 2008).

For all the graphs, X axis are the plot for time (sec) whereas y axis represents microbial population (cfu/ml).

1. Inactivation Curves for each Set of Results

 

 

 

 

 

 

 

 

 

2. Explanation for the shapes of the inactivation curves

For all the curves, the population decreases as the time of decontamination. This is a clear indication that the bacteria population is reduced on further decontamination. However, the situation changes after a minute of heating as time increase but population for the Streptococcus faecalis and E.Coli almost remains the same. The population of Staphylococcus aureus changes at 2 minutes of decontamination.

This indicates that the bacteria became inactive and succumbed to the high temperatures long time.

3. Appropriate data to determine the D values* for each organism and reasons for the choice

The data selected for the three vegetative bacteria are as follows: 20^oC for Staphylococcus aureus, 20^oC for Streptococcus faecalis and 35^oC for Escherichia coli. The reason is that from the representation of the curves, these will give best linear lines for ease of D-value calculation (Frascella, Gilbert, Fernandez, & Hendler, 2000).

 

 

The Graph showing D-value of Staphylococcus aureus at 20^oC

 

The Graph showing D-value of Streptococcus faecalis at 20^oC

 

 

The Graph showing D-value of Escherichia coli at 35^oC

4. Determination of D values* for each organism and comments

The overall formula for D_value is

T2 – T1
D = ______________________
Log10 N1 – Log10 N2

For Staphylococcus aureus the D values* are:

D values* at 5^oC

D_t5=5-1/log (4*10^7)-log(9*10^2)

=1.07573

D values* at 20^oC

D_t20=5-1/log (4*10^7)-log (1.5*10^2)

=1.082

D values* at 35^oC

D_t35=5-1/log (4*10^7)-log(5.8*10^1)

=0.8563

 

The D values* for Streptococcus feacalis are:

D values * at 5^oC

D_t5=5-1/log(3.2*10^7)-log(8.2*10^2)

=1.0889

D values* at 20^oC

D_t20=5-1/log (3.2*10^7)-log(3.0*10^1)

=0.8294

D values* at 35^oC

D_t35=5-1/log (3.2*10^7)-log(4.6*10^1)

=0.8557

 

The D values* for E. coli are:

D values* at 5^oC

D_t5=5-1/log (4.6*10^7)-log (4.4*10^2)

=0.9961

D values * at 20^oC

D_t20=5-1/log(4.6*10^7)-log(1.2*10^1)

=0.8954

D values * at 35^oC

D_t35=log (4.6*10^7)-log(9.6*10^1)

=0.88020

 

It can be concluded that for Staphylococcus aureus, the highest inactivation rate was at 35^oC, whereas the lowest was at 5^oC. This shows it takes longer to reduce microbial load by 90% at low temperature this applies to the three organisms.

The highest inactivation rate for E.coli was 0.88020 cfu/ml/minute at 35^oC, and the lowest rate was at 5^oC (Block, 2000; Taniguchi, Stanley, H., & Ludwig, 2002; Mullan, 2007; Young & Reichert, 1997; Juneja, Huang, & Marks, 2001; Zhang, Zheng, & Zhi, 2007).

Conclusion

In conclusion, as the temperature increased the rate of inactivation increased, and for Staphylococcus aureus and E.coli, 35^oC was the optimum temperature for inactivation by thermal-chemical disinfection.