Procaryotes = no nucleus in cell - bacteria. Eucaryotes = have nucleus - fungi, protozoa, plants and animals. Microorganisms are living entities that are too small to be seen with the naked eye. Compromise bacteria, viruses, protozoa, and fungi (yeasts and molds) Some bacteria are able to develop into spores under unfavorable conditions thus being more able to increase survival in heating, drying and irradiation. Molds are multi or unicellular found in decaying organic matter. Yeasts are unicellular and can grow over a wide range of conditions. Protozoa are single-celled eucaryotes, classified by morphology, locomotion and life cycle. Gram-positive (thick cell wall and no outer membrane) Gram-negative (thin cell wall and an outer membrane) They can then germinate into vegetative cells when conditions improve. Phototrophic organisms require energy in the form of light. May become more exacting in requirement during refrigeration. Bacteria need quite a bit of water, minimum aw of 0. 90.