Feature

Shedding Light on Bacteria

Getting bacteria to glow has its uses in food production.

Manfred Plagmann

Bacteria are all around us. Most of them are harmless, some even living inside us. We can live quite comfortably with those which are harmless to us; all the others must be kept at bay.

Food is one of the places where no one wants bacteria, harmless or not. But those places are actually preferred by them. Particularly food that tends to decay rather fast such as milk products or meat are areas where bacteria contamination might occur. The meat industry, for example, has therefore developed meat handling and cleanliness procedures to minimise the possibility of a bacterial contamination.

The number of bacteria on the meat is monitored by taking samples, storing them in a warm place and counting the bacteria which have developed after about three days. This method is very thorough but also very slow. At Industrial Research Ltd, I have been looking at developing a method which will give instantaneous results avoiding the long waiting period of three days.

Organic substances tend to become fluorescent when excited by ultraviolet light. Bacteria and food are made of organic materials and therefore emit fluorescent light when placed in the ultraviolet light of a laser. The fluorescence of the bacterium Escherichia coli and different types of meat such as lamb, pork, and chicken, is measured to find differences in their fluorescence spectra to distinguish between the bacteria and the meat.

Once the differences are known, a camera can measure the fluorescence of a whole piece of meat and the bacteria will show in the obtained fluorescence image.

Why use E. coli for the measurements when it lives in our body and could therefore be regarded as harmless? Why not use other, pathogenic bacteria?

There are a couple of reasons. E. coli is used in lots of laboratories every day and is therefore easily obtainable. It also shows fluorescence spectra similar to other, more dangerous species. Handling E. coli in our lab is easy as the petrie dishes they come in can be opened for measurement purposes without risking the contamination of the lab equipment. Another reason is that not all strains of E. coli are harmless -- some strains have become famous for being very toxic.

The E. coli strain O157:H7 has been in the media in conjunction with food poisoning, especially in minced meat. The origin of this relatively new strain is unknown but it is thought to have originated in the late seventies, perhaps in Argentina. It is believed that a shigella bacterium (a very toxic bacterium) gave an E. coli bacterium the DNA necessary to make a shigella-like toxin.

How can an E. coli bacterium get hold of the DNA of a shigella bacterium? A bacterium multiplies by division and the result are two identical bacteria. As this system of multiplication means evolution is not possible, nature invented conjugation. Conjugation is bacteria's answer to sexual reproduction, a method of genetic shuffling. It was probably this way that the E. coli strain O157:H7 inherited not only the DNA to produce the shigella-like toxin but also the resistance to a particular antibiotic.

With these two new properties this particular E. coli strain has become a dangerous bacterium. A real-time detection system will therefore be of benefit as it is not a statistical method, but tests each individual piece of food for bacterial contamination.

Manfred Plagmann is a researcher with Industrial Research Ltd.