The Nature of Microorganisms

Microscopic organisms are presently divided into three major groups: (1) Eubacteria (bacteria), which lack a discernible nucleus and mitochondria; (2) Archaebacteria (bacteria), which also lack a discernible nucleus and mitochondria; and (3) Eukaryotes (yeasts, molds, algae, and protozoa), which possess both a clearly discernible nucleus and mitochondria, plus filamentous structures known as endothelial reticulum. Mitochondria are self-replicating organelles and contain their own deoxyribonucleic acid (DNA), referred to as mitochondrial DNA. In Eukaryotes, the cytochrome and tricarboxylic acid (TCA) enzymes required for aerobic synthesis of ATP are located in the mitochondrial membrane, while with prokaryotes and Archaebacteria the cytochromes are in the cytoplasmic membrane and the TCA enzymes are in the cytoplasm.

All microorganisms are allocated to a specific group with respect to growth temperature. Obligate psychrophiles are defined as those organisms capable of growth at or near 0°C but not at 20°C. Such organisms usually have a maximum growth temperature of 15-17C. Psychrotrophic organisms are capable of growth at or near 0°C but exhibit optimum growth at approximately 25°C and are frequently unable to grow at 30°C. Mesophiles exhibit growth from 20-45C with an optimum growth temperature usually in the range of 30-35 C. Thermophiles exhibit growth in the range of 45-65 C. Hyperthermophiles are organisms from oceanic thermal vents and hot springs that are restricted to growth temperatures from 70-120C. Hyperthermophiles have not yet been isolated from foods.

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