The criteria used for the classification of microorganisms include morphology, reproductive mechanisms, pigment presence, means of motility, physiology and structural features.
2.2. Microbial activity
Microorganisms in the process of self replication, produce numerous complex macromolecules from about 100 different monomer units. In the biochemical pathways to achieve this a bacterial cell uses well over 1000 different enzymes and a eukaryotic cell may employ twice as many.
The biochemical metabolism can be divided into two broad classes: the anabolic pathways (anabolism) synthesize the complex molecules and their intermediate precursors, and the catabolic pathways (catabolism) supply the energy needed for the anabolic processes. These two divergent activities are closely linked.
Microorganisms that carry out their metabolism using oxygen are referred to as aerobic microorganisms. Some microorganisms can substitute nitrate, others sulfate or ferric ion, for oxygen and thus grow in the absence of oxygen. These microorganisms are referred to as anaerobic.
Microorganisms can be classified according to the lowest temperatures at which significant growth occurs. See Table 3
Growth of yeast is optimal in the region of 20-30C for mesophiles species.
In general a shift in the incubation temperature from the optimum to a lower temperature results in a temperature-dependent reduction of metabolic activity. An increase in the incubation temperature can cause a reduction in both the biomass concentration and cell viability due to a temperature- and exposure-dependent decrease in enzyme activity.