Micro-organisms – mono or pluricellular complex systems, with prokaryotic or eukaryotic cells, endowed with their own metabolism and genetic continuity, very diversified in shape, size and metabolic activity, and which share some common defining features:

• very small dimensions that make these organisms visible as individuals isolated only at microscope – the dimensions are appreciated by the current metric system in µm, and substructures in nm or Å;
• cellular organization – microorganisms have a cellular organization, either prokaryotic type or eukaryotic type;
• they are biological systems – bacteria sum up the minimal features that characterize life, bringing together the attributes of: cellular organization, own metabolism, genetic continuity, which ensures the species survival;
• large ratio between surface and volume – this particularity favours the survival strategy of micro-organisms in nature, because this ratio influences the ability to interact with the environment and nutrient absorption, causing the high intensity of metabolic reactions (biosynthesis and biodegradation) and in the same time the high rate of multiplication in order to compensate the losses due to the abiotic factors variation, as well as antagonistic relations with other species. As a rule, the surface/volume ratio is much higher in the case of bacteria compared to other organisms.

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