Bacteria are found everywhere. Set up a model of Pasteur's experiment which showed there are microbes in the air, but use test tubes, straight and bent glass tubing and a clear solution of Marmite instead of swan-necked flasks and broth. Grow bacteria from students' fingertips on sterile agar plates; but be sure to follow safety guidelines.
Processes such as decay, the recycling of elements and nutrients in nature, sewage treatment, and fermentation are all ways in which bacteria are useful to us. In biotechnology, the manufacture of insulin is a key benefit. These positive values outweigh the negative image of bacteria as trouble-makers. Bacteria and fungi can cause diseases, but are also the source of antibiotics used to fight these same diseases. This information, and the "ambivalent" role of Clostridium botulinum (a harmful food poisoner but useful Botox producer), can trigger interesting research and discussion.