11BI Microbe Experiment

Ubiquity and Colony Morphology (Abundance/Diversity)

The Handwash Test and Students’ Choices

The IB Biology microbial experiment (third term 2012) had several facets, but was primarily run to help make the students aware of the ubiquity of microorganisms (they are indeed everywhere !) and the idea that each microbial species displays a particular colony morphology, which is heritable. The handwash test and the other chosen test by the students were especially to help in terms of design of experiments: each student was responsible for their own controls and doing a test for which sufficient relevant data could be collected, processed and analyzed.

Every possible factor that could affect the measurement of the dependent variable (in this case number and type of colonies that grew on the plate), unfortunately, was not considered by the students who did at least turn in their lab write ups. In particular, the amount of soap/time for effective handwashing was a factor that was entirely neglected. Furthermore, few students included relevant controls for their chosen conditions. Because the students’ results for this handwash test were contrary to the usual hypothesis (more bacteria could be found on average after ‘handwashing’ by most students, but not to any significant level, as discussed further below), the experiment’s evaluation ideally would have included further testable hypotheses and or controls to improve interpretations.

For instance, if bacteria are killed by the skin’s dryness and slight acidity, similar inoculations at different times after hand washing might be useful controls. Or if handwashing time changes the amount of bacterial colonies detectable, varying this might be a useful independent variable for another round of experiments, for instance. Furthermore, if the faucet or the towel dispensor is a reservoir for bacteria, other control experiments could test this directly. For successful internal assessement, you need to learn to consider all these aspects, and an additional exercise for you this summer should be to go through and determine the aspect grades you would expect for the lab write-ups you turned in for this experiment !

Essential to the summary and ‘take-home message’ for you from this experiment, however, is that you do not go into your summer thinking that hand-washing results in more bacteria on your hands !!!'

On the contrary, proper hand-washing, at least 20s worth, with plenty of soap, can significantly reduce levels of bacteria found on your hands, and you should always wash your hands before touching your eyes or mucous membranes.

If you compare some averaged students results with those from your teacher (who frequently washes her hands), it is clear that the number of colonies detected shows just a slight trend of increase for the student data after handwashing (Graph1), with the differences between the average values just as likely to be due to chance as anything else, while the data from your teacher (Graph2) clearly shows a significant decrease in bacterial levels after hand washing of about three fold. In contrast, the data for colony diversity, even for your teacher, did not significantly change, with the same colony types found both before and after hand-washing. (To note : this also lends some support to the idea that the faucet could have been the source of bacteria observed… but direct tests of this could be readily performed. )



Other comparisons, based on the students’ choice of inoculation conditions, for instance swabs from table vs chair or shoe vs foot, could have been much more meaningful with more thought to controls. Remember, for your final internal assessment experiments, design is also an important part!

Finally, the inheritance of macroscopic morphological traits of the colonies, admittedly, was not a huge part of this experiment in the end, primarily because few students bothered to do any further streaks from their initial plates. However, your teacher clearly demonstrated that the pigmentation of the bacterial colonies, white, yellow or dark yellow, were inherited by the subsequent generation (Streak picture).



Additionally, there was one more take home message from this experiment, as a bit of water analysis was performed with the special bacterial plates, ECA Easygel (example from Micrology Labs site, below), that can distinguish bacterial species based on their metabolism of substrates to form colorful products, with dark blue denoting E. coli (the main inhabitant of the human gut), light blue for other ‘coliforms’ and pink or green reaction products for other bacteria, which could potentially cause infections or food poisoning, respectively.

These last data showed that not only the tap water but the WC in the villa are fairly ‘clean’ in terms of these contaminants, but that all these microbes were detectable in river samples, which could also be streaked to isolate single colonies that display the same characteristics at the parental colonies.

