This experiment was done in order to investigate certain factors affecting the killing of the bacterium Escherichia coli. The two factors investigated were: a) the length of time of exposure to Domestos and b) the concentration of this agent on killing E.coli. The methods used in the experiment were serial dilution and pouring Petri dishes. The results indicated that both the above mentioned factors influence the behaviour of this disinfectant agent and affect the growth of the micro-organism tested.

This experiment's procedures and results are analysed and its design is assessed for accuracy and precision.IntroductionEscherichia coli is considered to be a model organism for studying a variety of life's essential processes because of its rapid growth rate and simple nutritional requirements. The average human intestines contain about 1 kg of bacteria. Approximately 0.1% of these are normally E. coli.

Their presence within the intestines is necessary for normal development and health - E. coli, together with other types of enterobacteria, produce vitamins which are subsequently absorbed by the body, e.g. vitamin K and B-complex vitamins. Although the normal habitat of E.coli is the intestines, they are able to survive quite well outside the body in faecally-contaminated environments like water or mud.

Under those circumstances, they can be useful indicators of sewage contamination ("faecal coliforms").Domestos is a powerful disinfectant designed to kill germs and eliminate dirt. It does not contain ammonia. Like available chlorine it contains Sodium Hypochlorite. Domestos is considered effective against viruses, but weak in heavily soiled situations.

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.MethodsFor 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.