May: Antibacterial Action

We wanted to compare different substances for their antibacterial properties.  Students chose 4 similar test substances from a range including: toothpastes, mouthwashes, acne treatments, household cleaners, and foods including different honeys, onions, garlic, and chillies.

Preparation

We each made pour plates of nutritional agar, seeded with a safe bacterium. Everybody successfully made their own pour plate. This can be quite tricky - you have to keep the agar at around 37-40C so that it pours freely but does not kill the bacteria.  Pouring at higher temperatures also causes excessive condensation in the petri dish later.  We put a small amount of the microbial broth culture in the petri dish, swirled it around, then poured the agar, swirled again gently, put the lid on and left it to cool.  As soon as the agar set, we inverted the plates until we were ready to use them.

Small discs of paper were sterilised in an autoclave (aka pressure cooker!) and then soaked in a test substance. These discs were blotted lightly to stop excess liquid running off them, then using forceps we placed them on the surface of the agar plates.

The plates were left for bacteria to develop. Students took their own plates home to observe, and then to measure the diameter of the 'zone of inhibition' - the circle around each disc where bacteria could not be seen in some cases.

Once some growth had occurred, the agar went cloudy and some plates had surface colonies. We observed whether bacterial growth could be seen near each paper disc. If growth couldn't be seen near a disc, but could be seen elsewhere, then it was likely that the substance on the disc had inhibited the bacteria from growing there. If this had happened, we measured the diameter of the zone of inhibition.

This activity was based on the Nuffield Practical Science activity, 'Investigating Anti-Microbial Action'

The bacterial culture that we used was Micrococcus Luteus, which was obtained from Blades Biological.  This culture is considered safe for educational use, and is apparently considered particularly suitable for comparing mouthwashes .  Angie made several new cultures from the original one, and stored more for future use.

Alison recalling the price of the Manuka honey.

We had some surprises in our results.  Some products were very effective against our bacterial culture, but this doesn't mean that we can generalise and say that these products would be effective against all bacteria.  Ideally, we would like antibacterial products to be effective against harmful bacteria and not against anything else.  To know how useful our test bacteria are, we would need to know how similar it is to the 'target bacteria' which are likely to be causing harm in that situation.   There are potential risks involved in using antibacterial products, eg encouraging resistant organisms to develop, or triggering allergies.  Or they might just be a waste of money.  The USA's Food and Drug Administration recently advised against antibacterial handwashes, for instance.

Now, on to the photos and the results!

Some samples had been prepared earlier and kept in an incubator so we could accelerate the results.  However, for the group, we cultivated our bacteria at room temperature to avoid accidentally cultivating something else which was harmful to humans.

Petri dishes were labelled on the underside so that we would know which test substance was which later.

Each person chose their own test substances.  Those being compared in this picture include chillies, garlic, onion and chilli powder.

The test discs were carefully handled with forceps to avoid contamination.

Very Serious Scientists deliberating.

Angie managed to sneak in for a Science Selfie!

Some slide-viewing for those who had time to spare - practising microscope skills.

Results

We had some striking results:

Mouthwashes - Corsodyl was very effective.  Neal's Yard 'natural mouthwash' had no detectible effect.  Curasept was effective, but not as much as Corsodyl.

Another trial of mouthwashes, compared to 6% Hydrogen Peroxide solution.  In this one, Corsodyl was very effective, Hydrogen Peroxide was a little ffective, and Curasept and Neal's Yard were not effective.  The photo does not show the results very clearly, but there are some large colonies very close to the Neal's Yard and Curasept discs, although the overall density of bacteria in those zones is low.  I think this may have been due to poor distribution of the bacteria in the agar initially.

Comparison of things you might put on a cut or a spot: Germolene, Savlon, Tea-tree oil, and Peroxiben (spot cream).  It was hard to see what had happened here - bacteria stayed away from the whole lot!  We think that tea-tree oil might have run off the surface of the disc and into other areas.  

Face washes

When comparing face washes for spotty teenagers, Superdrug's cheap own-brand version worked better at inhibiting m.Luteus than more expensive branded products.

Chillies and chilli powder

Comparison of fresh chopped chilli, dried chilli, chilli powder (which is a mix of spices) and alcohol for a control.

The perils of cryptic labels.  Anyone remember what this was?

Household cleaning sprays

Cleaning sprays - Sainsburys' cheap spray, Domestos cleaning spray, Method 'natural' spray, and Ecover spray.

Onion, garlic and chilli

Onion, garlic anc chilli - none had any detectible effect on our bacterial culture.

Handwashes

Anti-bacterial handwashes: Method 'natural' antibacterial wash was very effective, as was Simple handwash.  Palmolive wash was less effective, and tea-tree oil appeared not to be effective.

Honeys

Honeys - expensive Manuka honey, runny honey, Sainsbury's Basics honey, and Golden Syrup for a control.  We left this to develop for a few more days.  None of the honeys appeared to have any effect against the microbe in our experiment.

Honeys and propolis

Manuka honey, cheap honey, Propolis, and alcohol hand gel for a control.  The propolis stopped anything growing on the disc itself, but did not appear to have any further inhibiting effect.  The honeys didn't appear to have any effect.

Angie removed the lid for a photo, which is not ideal as it can spread mould spores.  This was done quickly and carefully!

We kept some of the experiments running for longer.  After the initial bacterial bloom, in some cases moulds took over quite rapidly.  Interestingly, the honeys seemed to promote the growth of mould, though the propolis remained untouched.