# A thermo-chemical disinfection process

1 (b) 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,

(iv) Calculate the D values and comment briefly

Word limit : 350
* the D value is the term used to describe the rate of inactivation under defined
conditions and may be calculated as
Note Revised Equation

T2 – T1
D = ______________________
Log10 N1 – Log10 N2

Where 10 N 0 log N is the initial population at time T1
and log 10 N1 is the population at time T2

Table

Staphylococcus aureus

Staphylococcus aureus

Colony forming units (cfu) / ml

Time (min)
0
1
2
3
4
5

5oC
4.0 x10 7
5.0 x 106
6.0 x 105
7.0 x 104
8.0 x 103
9.0 x 102

20oC
4.0 x107
3.5 x 106
3.0 x 105
2.5 x104
2.0 x 103

1.5 x 102

35oC
4.0 x107
4.0 x 106
4.2 x 106
9.0 x103
7.2 x 102
5.8 x 101

Streptococcus faecalis

Colony forming units (cfu) / ml

Time (min)
0
1
2
3
4
5

5oC
3.2 x10 7
4.2 x 106
5.2 x 105
6.2 x 104
7.2 x 103
8.2 x 102

20oC
3.2 x107
2.3 x 106
1.8 x 105
1.3 x104
8.2 x 102

3.0 x 101

35oC
3.2 x107
1.2 x 106
2.4 x 106
7.8 x103
6.0 x 102
4.6 x 101

Escherichia coli.

Colony forming units (cfu) / ml

Time (min)
0
1
2
3
4
5

5oC
4.6 x10 7
5.2 x 106
4.8 x 105
4.8 x 104
4.6 x 103
4.4 x 102

20oC
4.6 x107
4.6 x 106
4.4 x 105
3.2 x 104
2.8 x 103
1.2 x 102

35oC
4.6 x107
4.2 x 106
3.8 x 105
2.8 x 104
1.6 x 103
9.6 x 101

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