By far the most important and troublesome are the bacteria. A number of rod shaped bacteria produce spores, a non-vegetative dormant stage in its life cycle. They can survive a wide range of unfavorable conditions. Spores can be compared to plant seeds. They will germinmate and grow when conditions are suitable.
Bacterial spores are extremely resistant to heat and cold.
Some spores can survive in boiling water for several hours. The same bacterium in the vegetative state will not survive heating to boiling. Soils or waters from which raw foods are obtained are the sources of the microorganisms and their spores. Leafy vegetables such as spinach and other greens which grow close to the soil harbor high numbers of bacteria and spores. Asparagus and mushrooms which grow through the soil are always contaminated with high numbers of spores.