The effectiveness of a disinfectant depends on the product’s contact with the micro-organisms and the absorption onto the microbe’s surface or penetration into the cell. The type of disinfectant, its concentration, the exposure time, the temperature, the presence of organic material and the type of micro-organism all have a significant role in the disinfectant’s effectiveness.
Methods
For this experiment Domestos was used as disinfectant, Escherichia coli was the micro-organism and sterile ringer was used to make the serial dilution of the disinfectant. Four bottles were prepared, three by serial dilution and one containing pure Domestos, in order to have different concentrations. 5ml of Ringer were taken and added to 3 empty bottles to prepare the serial dilution of Domestos. The 1st bottle remained initially empty. Afterwards, 10ml of Domestos were added to the first bottle, thus there was no dilution (Dilution factor = 1). In the 2nd bottle containing 5ml of ringer, 5 ml from the 1st bottle were added giving a 50% (v/v) Domestos solution. Then from the 2nd bottle 5 ml were taken and added to the 3rd bottle, which already contained 5 ml of ringer, giving a v/v concentration of 25%. Finally 5 ml were taken from the 3rd bottle and added to the 4th, so the v/v concentration in the 4th bottle became 12.5%.
After the dilutions were prepared, four petri dishes were labeled with the different concentrations and the time of exposure. Following the preparation of the four plates, 1ml of Escherichia coli was added to each of 4 bottles containing the different concentrations of Domestos and Ringer and was exposed for 5 minutes. When the exposure time was elapsed, cooled molten nutrient agar was used to prepare the plates where each solution was put, so four plates with different concentrations and 5 min exposure time were made.
The same procedure followed but when 1 ml of Escherichia coli was added to the dilutions it was exposed for 10 minutes. Petri dishes were labeled as appropriate with the new exposure time. The same process followed again twice with different exposure times which were 15 and 20 minutes.
Sixteen plates with different times of exposure and four different concentrations for each time of exposure were ready to incubate. After one week, the effect of different concentrations of Domestos on killing Escherichia coli and the length of time of exposure of this agent on killing the micro-organism could be observed.
Results
The laboratory experiment showed that both the disinfectant’s concentration and the length of the exposure time clearly affect the killing of the bacteria. The effectiveness of Domestos was improving as the concentration or the exposure time was increasing.
Figure 1
Table 1
Figure 1 presents the effectiveness of the disinfectant at four different concentrations (100%, 50%, 25% and 12.5%) in relation to the time exposure. The longer the exposure time the more efficient Domestos becomes at all tested concentrations. Table 1 describes the microbial growth associated with each scale used in this experiment, where impermeable indicates high growth, therefore low effectiveness of the disinfectant, and permeable indicates low growth, therefore high effectiveness of Domestos.
Discussion
Disinfectants are not effective for all micro-organisms. Their effect varies depending on their composition and the micro-organism’s structure. Non-diluted (clear) Domestos was used in order to observe the reaction of the pure agent and to what extend it is effective. Also the maximum effectiveness was measured and used as a point of comparison to assess how much effectiveness is affected by dilution.
Both factors (time exposure, concentration) investigated were proven to affect the effectiveness of Domestos. At all concentrations, there was a relatively sharp increase to the effectiveness as the exposure time was increasing. It became clear that the more the micro-organism was exposed to the disinfectant, of any concentration, the more effective the agent was becoming. It was also interestingly observed that after 20 minutes of exposure the last two (25%, 12.5%) concentrations presented the same effectiveness.
Assessing the concentration factor, by diluting the pure agent 50%, its effectiveness at the 5 minute mark was decreased by 25% (from scale 4 to scale 3). By further diluting the agent to 25%, it was very interestingly observed that its effectiveness remained unaltered for the same time exposure (5 minutes). It was only after 10 minutes when a difference has been noted. An explanation for the same results for these two concentrations (50%, 25%) after 5 minutes could be that the difference was so slight that could not be observed without calculating the exact number of colonies. By further diluting the agent to 12.5% the effectiveness was decreased by 33.3% (from scale 3 to scale 2) compared to the previous two concentrations at the 5 minute mark.
Assessing the experiment’s procedure and results, it is observed that its design could be improved if the bacterium volume used was smaller. The results from this experiment were not very clear, because of the volume of E.coli which was added to the dilutions. Since in 0.1ml were 10000000 cells, then in 1ml, which was added in each dilution, the number of cells was extremely high. Thus it was impossible to count the cells from the plate as it was very cloudy. This lead to the utilisation of a scale to measure the effect of Dometos. Should the volume of E.coli used was lower, the colonies might have been seen and calculated and this would lead to more specific results.
A successful disinfection depends on the selection of the correct chemical agent associated with an appropriate disinfecting procedure. A detailed understanding of the characteristics of each chemical agent, including the limitations and appropriate applications, is important. Also, it is essential that the chemical components used in commercial products meet established quality requirements.
Reference
-
Russell, A. D., (1982)Principles and Practise of Disinfection, Preservation and Sterilization. Blackwell.
-
Tortora, G. J., Funke, B. R. & Case, C. L. (1995) Microbiology. An introduction (5th ed). Benjamin/Cummings.
