Student article – What has more bacteria, a door knob or a toilet seat?

What has more bacteria, a door knob or a toilet seat?

Evan Hobson and Rhys Gibbs,

Plasmawr School , Cardiff, Wales, UK

Reviewed by Prof. Caroline Sewry, PhD  FRCPath, Professor of Muscle Pathology,  Dubowitz Neuromuscular Centre, Institute of Child Health and Great Ormond Street Hospital


The reason we did this experiment was to see whether there is truth the rumour that there are more germs on a doorknob than on a toilet seat. Our two hypotheses were 1) there being more bacteria on the door handle and 2) there being more bacteria on the toilet seat.


We collected bacteria from a public house. We used a swab on a doorknob to the lounge, and another swab from a toilet seat. We put these swabs into petrie dishes we made out of plastic takeaway dishes (sanitised in the dishwasher). We then poured in agar (a gel that bacteria thrives in and grows into spores) which we made out of beef stock sugar and boiling water. The boiling water was there in order to kill the bacteria already in the beef stock and sugar. The dishes were covered with cling film. We left the mix for 3 days and then looked at the number of spores which had grown in the agar.

Fig. Spores on toilet seats

Conclusion and discussion

The doorknob accumulated a quantity of bacteria superior to the toilet seat. The major culprit behind this outcome is some individuals refrain from sanitizing their hands correctly. Studies show that the majority of the population wash their hands incorrectly after attending the rest room. 

(Advice from G Pritchard, Science Teacher; Research team also included E Davies, O Morgan and Z Mills) 


This study addresses a very important issue with regard to correct washing of hands after going to the toilet. The authors have highlighted that many people do not do this adequately and, of course, this also applies to other areas of life. The implications of not washing hands correctly could have been discussed. For example, if there are bacteria on our hands what are the consequences of this. Is it always bad? Do the authors consider there are important areas where hand cleanliness is important, perhaps in the kitchen could be discussed. Where have the bacteria found on the door knobs come from?

The door knob to the lounge was studied. It is not clear from the methods if the door knob on the outside or inside was studied. Is there a difference? Are there other areas that are equally or more or less contaminated. Perhaps the door handle of a cubicle, tap handles, or the flush handle or button. Was the door knob of the lounge the most appropriate area to swab (from ‘lounge’ I assume the authors studied the main entrance door)?

Most experiments in science require a control as a comparison for testing an hypothesis. What control could have used in this study? Perhaps cleaning some knobs with disinfectant and seeing if swabs from these grew bacteria in comparison with the uncleaned ones. Is disinfectant the best control or is there something else better to kill bacteria?

In the methods what swab was used? Could anything in this have affected or reduced the number of bacteria that grew? The authors took appropriate measures to ensure  that the plastic trays and agar did not have bacteria  that would have influenced their results

Well done to the designers of this project who have raised hypotheses, designed a study to answer their questions and drawn conclusions from their results, Scientific studies often raise further questions to answer, these could be raised at the end of the report.

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