Posted: April 2nd, 2015

Scientific Principles Underlying Decontamination (

Paper details:
– A thermo-chemical disinfection process is tested using three cultures of vegetative bacteria:- Staphylococcus aureus, Streptococcus faecalis and Escherichia coli in suspension against the disinfectant ‘X ’ at a concentration of 50 mg/l. The results are given in the Table below. ‘Zero’ exposure time refers to the suspension before exposure to temperature/chemical conditions. An inactivator, established as effectively quenching the disinfectant without (either alone or with the reaction products from interaction with the disinfectant) affecting the growth and recovery of any of the organisms tested, was used.

(I) Plot inactivation curves for each set of results.
(ii) What explanation can you offer for the shapes of the inactivation curves?
(iii) Select appropriate data to determine the D values* for each organism,
giving reasons for your choice.
(iv) Calculate the D values and comment briefly

I would like to know if somebody will be able to rewrite my assessment following the feedback:

“The answer begins with an error – the inactivation curve is not done by “ plotting temperature against time”, the cited references do not state that and this is not how you presented the data. It is a plot of population v time for a specified temperature.

The incorrect formula has been used for the D value – the references listed include a number of sources that would have given the correct version. Consideration of the D value definition would have shown that the formula had been inverted.

You have not used a log /linear plot for the inactivation curve. In consequence the population scale was 5,000,000 cfu steps, thus most data points could not be differentiated. Spurious significance appears to have been on the shape of the curve at 1min and 2 min – although this is described as 1 degree of temperature and 2 degree temperature. Regrettably, the text is utterly incomprehensible.

In response to 1b (iii), you have correctly stated that “using log to base 10 the data becomes linear” and this has been applied to the calculations. The calculations were all arithmetically correct and marks have not been deducted for using the wrong formula in these calculations.

The units for D values are incorrect – by giving log10 values, this converts the population to a number rather than remaining as cfu/ml. The D value is then stated in minutes, with the specific temperature given as a subscript eg T35.”

Comments from Support Team:     Discipline: Scientific Principles underlying Decontamination Hospital sterile services

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