What is "sporulation"?

A friend asked me today "What is sporulation?"

There are essentially two genera of bacteria - Clostridium and Bacillus - that can produce very resistant stages called "spores" in response to unfavourable environmental conditions.  In contrast to the vegetative, or growing, cells, the spores are resistant to many environmental conditions, including heating and drying.  While the vegetative cells may be killed by heating to about 75C, the spores of C. botulinum can withstand boiling for more than four hours.

History gives us an example of how resistant spores can be:

In 1946, the British undertook some biological warfare experiments on Gruinard Island in the Inner Hebrides.  Small bombs containing anthrax spores (from Bacillus anthracis) were detonated near to groups of tethered sheep.  Within days, the animals were dying.  The government realised that the island would be hazardous to all mammals and quarantined the island indefinitely, occasionally testing the soil for viable anthrax spores.  On every testing occasion, the spores were shown to be viable.  This continued until a public campaign in 1981 forced the decontamination of the island.  This was achieved in 1986 by spraying 280 tonnes of formaldehyde solution onto the island and removing the most heavily contaminated topsoil.  The island was demonstrated to be safe and returned to its original owners.

It's important to note that soil normally contains some bacterial spores, including those of C. botulinum.  That means that raw vegetables and fruit carrying soil will often also be carrying spores.  Even milk drawn from healthy cows may initially be contaminated with soil organisms from their hides.  The normal pasteurisation process will kill the vegetative cells, but not the spores.