Open by asking students, "Do you know why dirt smells like dirt?" "Why does yeast smells like yeast?" "Why do rotten eggs smell like rotten eggs?" Have students write their ideas in their notebooks while the teacher writes them on the board (or whatever presentation surface is used in the classroom). Most likely they will guess correctly that each smell comes from the microorganisms they just observed. Then give a brief explanation, having students take notes, of the chemicals that cause each smell.
Dirt smells like dirt because of chemicals called "geosmins" which means "earth smell" (Greek: geo-earth, osmi-smell). These chemical compounds of carbon, oxygen and hydrogen, are produced when bacteria, called streptomyces coelicolor, ingests dead plant biomass. There are many different kinds of streptomyces coelicolor that are very beneficial to humans. Most antibiotic medicines are made from certain varieties of streptomyces coelicolor.
Temperatures between 105
o
F and 115
o
F are ideal for yeast growth and reproduction. Sugar, a chemical compound made from carbon, oxygen and hydrogen, is food for the yeast. As the yeast uses the sugar for energy it releases CO
2
and ethyl alcohol. The smell and taste of fresh baked bread comes from the combination of those two substances.
Hydrogen sulfide (H
2
S) is produced by anaerobic bacteria, usually in moist environments where oxygen is absent such as the mud at the beach which is revealed at low tide. The sulfur is what makes it smell like rotten eggs. The bacteria feed on the biomass of dead organisms to obtain energy for their own growth and reproduction and release the H
2
S into the air. Small amounts of H
2
S can be very smelly, however, the smell of more concentrated amounts cannot be detected by human smell and is very toxic and flammable (Cane 2010), (H. Kimberly 2010), (Wordinfo 2010), (Glass 2010), (Madigan and Martinko 390-394).