Antibacterial Cream Challenge Glossary

Antiseptics are applied to the skin and mucous membranes to help prevent infection.

Disinfectants are chemicals that destroy germs on non-living objects such as lab benches.

Antibiotics are drugs that treat infection after it occurs in living organisms, often internally.

Bacteria were first discovered in 1683 by Anthony Van Leeuwenhoek. He gave them the name 'animalcules'. Although he did not understand them or know exactly what they were, his discovery has been called the foundation of microbiology. However, the significance of bacteria was not fully realized for another two hundred years.

One micron is one one-millionth of a meter, or about 1/25,400 of an inch. In comparison, a human hair is 100-300 microns in diameter, the eye of a needle is about 1,250 microns, and the period at the end of this sentence is about 600 microns.

Proteins have been called the building blocks of living organisms. Our skin, muscles and hormones all consist of protein.

DNA carries the genetic code which is inherited by an organism's progeny. The code determines which proteins are made by the organism.

Petri Dish - Richard Petri invented the famous Petri dish in 1887, which is essentially a flat bottomed glass saucer with deep rims. Influential microbiologist Robert Koch soon saw the benefits of Petri dishes as they enabled him to study individual colonies of bacteria.

Agar is made by boiling certain types of red seaweed or kelp. Only a limited number of seaweed families contain true agar. We have a 19th century Dutch lady by the name of Fannie Eilshemius to thank for its use in science labs today. She used it to prepare fruit jellies, and realising that it might be useful to her scientist husband, Fannie passed on the tip. Agar has one very weird but useful property - it gels at 34 to 42°C (93-108°F) but won't melt again until it reaches 85°C - 90°C (185-194°F).

Gelatin can be made by boiling up old hides and bones. The disadvantage of gelatin is that it can be broken down by some bacteria and melts at around 25°C (77°F).

Nutrient broth is made by boiling chopped meat, beef stock cubes or Marmite and water.

Equipment - This includes Petri dishes, pipettes, bottles, swabs, agar.

Sterilizing will kill all bacteria. We need to ensure we are only growing the bacteria we want. Without sterilizing we would not only grow the bacteria from our chosen human but also those from insects, animal excrement and the soil. More importantly, we need to destroy any contaminating fungi, which would make a hairy mess on our plates!

Autoclaves work on the same principle as a domestic pressure cooker. Water boils at a higher temperature under pressure so when water is heated in a sealed container the very high temperatures and pressures that result can kill germs very effectively.

Baking in an oven - Laboratory ovens typically heat to between 200°C and 400°C (about 400-750°F). Ovens are fine for glass and metal tools but aren't much good for plastic equipment or the media (the heat would caramelise any sugars in it and the proteins would be destroyed).

Controls - Experiments to which the answers are known are used as a reference. So a negative control would be included — in this case, a Petri dish with agar, bacteria and nutrient but nothing else. If bacteria grows in the untreated Petri dish but not in the dishes with added antiseptic, we would know that all the antiseptics worked. If there were no bacteria on any of the plates, we would know that something was wrong.

Replicates - Often called reps, this means that three or four Petri dishes are used for each experiment. If an 'unusual' result appears in one Petri dish, we know, by comparing it to the others, that an 'operator error' has been made. If reps hadn't been used, we may have assumed that this was a genuine result.

Calculate the mean - This is the basis of many microbiological experiments and medical trials but, of course, complex formulae and powerful computers are used in the real world.