Notes for demonstrators

Overview

This is the first real chance the students have had to do some proper experimental design – there is no schedule for them to follow and no right or wrong way to go about things (a minority of students absolutely hate this; the majority embrace it!)

The main learning objectives for these sessions are for students to be able to devise suitable methods, controls and replicates, to plan their work, to appraise and adjust their approaches as necessary, and to analyse their data and present it with appropriate statistical analyses.

General tips

Students are used to being given precise instructions and some will want you to tell them exactly what they should do. Try not to indulge them! Emphasise that science is about trial and error – there’s no right or wrong, and they need to make their own decisions. You can provide suggestions or prompts, but this is about getting them to think for themselves.

There is no precise timetable for what should happen when, but we will encourage the students to follow roughly this schedule.

Day 1

Planning! They are only too keen to jump in and start chucking cheese on the floor, but if they don’t think everything through beforehand (e.g., how are they going to sample and quantify the contamination, what levels of contamination are they expecting, what basic controls do they need, etc.) they are going to have problems down the line. I am therefore going to suggest that no-one does any experimental work for the first hour at least. It would be good if we can check with each group what they are planning and offer advice if they are heading in a dubious direction.

Towards the end of the session they will set up some simple tests to see if their approach is going to work. Encourage them to try several methods of sampling as they have no idea what the bacterial load is going to be, so methods that will give quantifiable results if picking up either 10 bacteria or 10 million should be explored at this stage to maximise their chance of success tomorrow. Remind them about basic controls too, otherwise their results might be meaningless tomorrow.

To note, the floor is surprisingly clean; scraping a loop across the surface of the food is not a good sampling method. It’s fine for them to try both these things today, but please persuade any groups only thinking of these routes to try other options too, to avoid too much disappointment tomorrow.

Day 2

This is when some of them discover that nothing has worked! Often they have (almost) nothing on their plates if they planned for much higher levels of contamination (sometimes they have too much). This is where they need to adjust their methods and will probably need some help with this.

Everyone will then set up more experiments before the end of the session, this time to investigate several variables (food types, surface types, contamination type/levels, transfer time etc) to see what’s going to give the best results.

Day 3

Hopefully everyone is now having success, but if there are still groups that do not have a workable approach, they might be getting a bit frustrated! It’s a good idea to remind them that this is normal in science and that it’s not unusual for researchers to spend weeks, even months, coming up with a suitable method. At this stage, we should give them some strong direction on what to do to improve their results, as today they need to start setting up their final experiments which will give their final data on Day 4.

Reassure them that their lab report will not be marked down if they do not get brilliant results. That is not how it’s marked.

This is their last chance to do any experimental work and so everyone now needs to think about biological/technical replicates (they can’t do statistical analysis on a sample number of one) and make sure that they are including all appropriate controls. Stress to them that controls and replicates are far more important at this stage than the number of variables, if the number of plates etc they are needing to do is getting out of control.

In this session they will receive a printout to help get them started with planning their summative lab report. If they have questions, remind them that the information is very likely covered in one of the following: the assessment criteria, the student instructions, the document on what to include in each section (all of which can be found on [VLE]). I think it would be best to point them to these documents (or to me (JMC) if something isn’t covered).

They might be a bit busy to do anything with this today, but encourage them to do it tomorrow, if they haven’t.

Day 4

They will analyse their results ready for their lab report. They’ve learned about descriptive stats (means, confidence intervals, possibly standard deviation / standard error) but might need reminders (you can tell them that they covered these things in the Autumn data skills sessions). They haven’t yet covered t-tests and p-values, so are not required to do this (unless they want to and are confident in it).

Group work – planning handout

Data for non-attendees

Method

I sterilised grapes by wiping them with ethanol. Then I dropped them onto a smooth surface which was contaminated with 0.2 ml S. marcescens. I left the grapes for different lengths of time (4 grapes per time point) and then transferred them to 5 ml LB broth and vortexed them for 5 seconds to transfer any bacteria into the broth. I then took out 2 x 100 μl (technical repeats) of the broth and plated them onto an LB agar plate, incubated them overnight, and counted the red colonies, which would have been the S. marcescens from the contaminated surface. I averaged the data from the technical replicates before proceeding.

As a negative control I used grapes which had not been dropped onto the surface prior to being put into the 5 ml LB broth (i.e. 0 second time point).

As a positive control I swabbed the contaminated surface with a sterile swab and transferred this to 5 ml LB broth, before proceeding as for the test samples above.

Results

References

CBBC (2017) Is it safe to eat food you’ve dropped on the floor? [Available here].

Dawson, P., Han, I., Cox, M., Black, C. and Simmons, L. (2007) ‘Residence time and food contact time effects on transfer of Salmonella Typhimurium from tile, wood and carpet: testing the five-second rule’, J Appl Microbiol, 102(4), pp. 945-53. [Available here].

Food Standards Agency (2022) ‘FSA research suggests new higher estimates for the role of food in UK illness’. [Available here].

Miranda, R. C. and Schaffner, D. W. (2016) ‘Longer Contact Times Increase Cross-Contamination of Enterobacter aerogenes from Surfaces to Food’, Applied and Environmental Microbiology, 82(21), pp. 6490-6496. [Available here].