WHAT IS A micro-organism ?

Micro (Invisible with naked eye) + organism (Being alive)


Bacteria: unicellular organism measuring from 0.5 to 7 microns; Main origin= mammals and environment; proliferate through cell division

Yeast: unicellular organism measuring 6 to 25 microns;  Main origin =environment; proliferates by budding

Mold: Organsm forming multi-cellular mycelium; Main origin = soil; proliferates by producing billions of conidia (2-50 microns) = spores.

The microscopic algae:  Organisms capable of photosynthesis and growth  in liquid media in the presence of light. They have extremely varied shapes and sizes. They are the basis of many food chains.


A mold is an eukaryotic organism, multi-cellular belonging to the kingdom of  Fungi, can colonize numerous environments and is highly contaminant. They have a large and varied enzyme potential (cellulases, proteases …). In sum, molds can grow with wery limited substrate and on a huge variety of media: plasterboard, wood, wallpaper, food, dust, plants … They also have the ability of release spores and contaminate the air.


Molds development factors are :

  • Water activity (aw): the most important because it is the water available for growth of microorganisms. Several factors can affect the aw of a place or product: the temperature (condensation), weather, leaks and water damage and the water content of a support (material, plant, food … )

  • Temperature: in general, growth from 4 ° to 30 ° C. Optimum between 18-28 ° C.
  • Oxygen: molds are able to grow in a very wide range of oxygen (from ambient air to the product having only trace of oxygen)

  • pH: yeasts / molds  Optimum pH 5-6 but some can grow in pH ranges from 2 to 12.


The mold growth can cause the deterioration of the structure of a building, health risks to its occupants, problems for the products quality…



Contaminated material 3

A food product that has changed color due to microbial growth

Contaminated material 1

Mould growth on materials will lead to changes in their physical properties but will also contribute to the degradation of air quality.



Mycelium 4

During germination, spores form germinative tubes which will divide to create mycelium.




Contaminated vegetable 2

Molds are responsible for many plant diseases. Here, the leaves of potato plants infected by fungi of the genus Alternaria.

Conidiophore 4

Scopulariopsis sp. is a mold which is considered a contaminant but which in certain circumstances is desired as quality proof (for example on specific cheese).




Conidiophore 3

A typical conidiophore of an Aspergillus.




Mycelium 3

The mycelium is composed of thousands of filaments visible under microscope.




Conidiophores 2

Another example of Penicillium, typical mold of our latitudes.



Conidiophore 1



A microscopic picture of Penicillium sp. Conidiophores are structures where born spores.