Imagine that you are an early microbiologist attempting to reproduce Louis Pasteur’s famous experiments which helped disprove spontaneous generation. You first prepare a hay infusion (a type of nutrient broth), then boil it for 20 minutes in a swan-necked flask of the same design that Pasteur used. Upon cooling and incubation at room temperature, you notice that your broth has become cloudy (turbid) only a few days later. What did your broth contain that Pasteur’s likely did not?

Microorganisms usually adapt to changes in their environment. One example of an extreme survival strategy employed by certain low G+C Gram-positive bacteria, generally of the Firmicute family, is the formation of endospores. They have a very peculiar structure that helps with this survival feature: an outer proteinaceous coat surrounding the spore provides much of the chemical and enzymatic resistance. Beneath the coat there is a very thick layer of specialized peptidoglycan called the cortex, which aids in resistance to high temperature. They are resistant to desiccation, lysozyme, ultraviolet and gamma radiation, chemical disinfectants, starvation AND temperature. So, in this experiment, it is probable that the boiled broth contained endospores, which survived this heating and then reactivated its metabolism.