Bacteria Spotting

Millions of people in the developing world die from drinking water contaminated by bacteria. Current methods for detecting pathogens require laboratory equipment and specialised training which can be quite taxing on remote communities to operate. A cheap and simple test is sorely needed.

A collaboration between the University of Massachusetts, University of Puerto Rico and the Georgia Institute of Technology has led to the development of a colour test strip to detect bacteria published in the Journal of the American Chemical Society. The strip changes from yellow to red in 10 minutes if bacteria are present.

On a piece of filter paper, researchers applied a mixture of gold nanoparticles, the enzyme, β-galactosidase, and a yellow dye, chlorophenol-red-β-D-galactopyranoside in a particular way. The surface of the nanoparticles are positively charged to it attracts the negatively charged enzyme making it inactive. A freely moving enzyme reacts can react with the yellow dye changing it into a red colour so unless the enzyme is freed, the test strip remains yellow.

How would an enzyme particle become freed? This is where the bacteria comes into play. Microbes are negatively charged and bind to the nanoparticles much more strongly than the enzyme particle so the enzyme particles are effectively bumped out of place and freed. The enzyme is then free to react with the yellow dye to turn it red, indicating the presence of a microbe.

At the moment the colour strip can detect 10,000 bacteria per mL of water but this is not yet sensitive enough to be of use in the field because the most virulent bacteria can cause disease in as low concentrations between 10 and 100 bacteria/mL. At the moment, the nanoparticle, enzyme and bacteria are not very specific but if selectivity can be demonstrated, it would most likely bring a revolution to testing water quality.

Related articles

• A Simple Test Spots Bacteria (pubs.acs.org)

Pineapple Ruins Jelly

Pineapple, Source: Wikipedia

I have a tendency to experiment in the kitchen and most of the time it goes well. One particular experiment involved adding fresh pineapple to jelly. It didn’t set.

It was a disaster. I was making a tropical fruit inspired trifle. I was so incredibly confident of my predicted success that I had talked up this dessert and even incorporated components from some of the people attending. Instead of having set jelly to layer in amongst layers of cake and fruit, I had slush. I had three hours to make a replacement dessert. The pineapple slush was exiled to the back of the fridge while I frantically I made a pavlova covered in cream and tropical fruit.

After everyone had left and with the dishwasher whirring away, I brought out the bowl of pineapple slush. After 10 hours, it still had not properly set. I declared war on pineapple that night in the kitchen. I had to find out why the jelly hadn’t set. I was so annoyed because my stomach was looking forward to a tropical fruit trifle that never eventuated. Pavlova just wasn’t the same.

The reason why my pineapple jelly did not set and was not ever going to set despite the length of time spent in the fridge was because fresh pineapple contains an enzyme called bromelain. It is a plant extract that is usually used as a meat tenderiser. Jelly is made from gelatine which is made from the collagen found inside the skin and bones of animals. It is a common gelling agent in not just jelly but also in pharmaceuticals, marshmallows, photography and even found in some low-fat yoghurt.

Bromelain and other enzymes in the pineapple interfere with the preparation gelatine-based desserts because it breaks down the gelatine making it impossible for a wobbly jelly to form. This enzyme can be deactivated by heating the pineapple to a temperature above 65^oC. A quick boil of fresh pineapple in either its own juices or water for a few minutes should suffice. Another alternative is to use canned pineapple where the heating processes before being canned has been high enough to deactivate the bromelain.

Other fruits to be wary of adding to jelly include fresh papaya and kiwi fruit. They also contain enzymes that interfere with the setting of jelly.