Microbiologie Industrielle

Several types of micro-organisms are used in industrial microbiology: bacteria, yeasts and moulds, Archeae, algae and viruses. These germs can be used whole (the whole cell) or simply with enzymes extracted from them.

    II.1.1 Bacteria: 

Bacteria are unicellular organisms which possess the relatively simple elements essential to cellular life, including genetic material (DNA), represented by a single circular chromosome and possibly plasmids, without a nuclear envelope (nucleoid). Bacterial cells come in several forms: bacilli, cocci or spirochetes, with a very wide range of sizes (0.2 to 2 µm in diameter and 2 to 8 µm long). They have a rigid wall composed mainly of peptidoglycans. There are two main classes of bacteria characterised by differences in their cell wall: Gram-positive bacteria and Gram-negative bacteria.

The bacteria most commonly used in industry include : 

• Lactic acid bacteria: 

The lactic acid bacteria group includes several genera characterised by their ability to ferment carbohydrates by producing lactic acid.

Lactic acid bacteria are Gram-positive bacteria, immobile, asporulated, with negative catalase and oxidase , negativenitrate reductase , anaerobic or aerotolerant, in the form of shells or bacilli.

Lactic acid bacteria are generally mesophilic, growing mainly at pH 4.0-4.5, and some are still active at pH 9.6 or pH 3.2. They have highly variable salt tolerances and low proteolytic and lipolytic activity. 

All lactic acid bacteria have a strictly saccharolytic fermentative metabolism which, using carbohydrates, can produce either lactic acid exclusively (strict homolactic bacteria), or lactic acid and acetic acid (facultative heterolactic bacteria), or lactic acid, acetic acid or ethanol and CO2 (strict heterolactic bacteria).

• Role of lactic acid bacteria :

Lactic acid bacteria are mainly involved in :

- cheese production: coagulation: acidification of the milk by producing lactic acids; ripening: the molecules produced by lactic bacteria during ripening contribute to the flavour and aromas of the cheese. Bacterial enzymes also play a part in the proteolysis that takes place during the cheese maturing stage.

- Two strains of bacteria are used to make yoghurt: Streptococcus thermophilus and Lactobacillus delbrueckii bulgaricus.

- Lactic bacteria such as Lactobacillus and Bifidobacterium are also used as probiotics for their nutritional and therapeutic qualities.

• Corynebacteria : 

The Corynebacterium glutamicum species is used for the industrial production of amino acids, and is also capable of producing organic acids, including succinate, under microaerobic or anaerobic conditions. 

• Actinomycetes: 

These are Gram-positive filamentous bacteria with a slower growth rate. Most of them are aerobic, but some species are anaerobic.

Actinomycetes in general, and Streptomyces in particular, are of medical and industrial importance because they synthesise antibiotics with a wide range of structures and in abundant quantities. More than fifty different antibiotics have been detected in the Streptomyces genus, including streptomycin, neomycin, chloramphenicol and tetracyclines.

 

     II.1.2. Yeasts : 

These are unicellular fungi that reproduce by budding or fission. All yeasts have a plasma membrane protected from external aggression by a cell wall. As eukaryotes, they have a clearly delimited nucleus and different chromosomes. They also have mitochondria, cellular organelles capable of providing energy from oxygen. Yeasts reproduce sexually or asexually. They are capable of fermenting animal or vegetable matter.

The most famous yeast is Saccharomyces cerevisae, but it is not the only one. There are also many species of unicellular fungi, including the Candida genus, which is known to cause mycosis in humans, Leucosporidium, Rhodotolura, Torulopsis and Trichosporon.

 

   II.1.3. Moulds : 

These are microscopic fungi that grow in multicellular or unicellular filamentous forms. They are strict aerobes and derive their energy from the respiration and fermentation of soluble organic materials found in their environment. Most species thrive in broad pH zones of between 3 and 8, and at optimum temperatures of between 20 and 25°C.

 Moulds are best known for their food spoilage properties, but they are also of great industrial interest in the preparation of food (cheeses by Penicillium roquefortii), and in the synthesis of substances such as enzymes, organic acids and antibiotics.

 

   II.1.4. Archaea" Archaeobacteria 

Archaea (from the Greek archaios=ancient), a particular type of prokaryotic cell, are distinguished from eubacteria by certain chemical characteristics, including a cell wall without peptidoglycan. Archaeobacteria are organisms that often develop under very difficult environmental conditions (some species develop at temperatures ranging from 200°C to 350°C, while others can proliferate in the presence of 30% NaCl at pH=0).

Archaeobacteria have a wide range of morphologies: spherical, helical or rod-shaped. Some are Gram-positive and others Gram-negative. They multiply by scissiparity, fragmentation or budding.

Archaeobacteria have been divided into three main groups: 

- Methanogenic archae: These are strict anaerobes that use carbon dioxide or other simple organic compounds, and hydrogen, to produce methane.

- Halophilic archae: can tolerate high salt concentrations.

- Thermophilic archaeobacteria: which survive in extreme temperatures.

Methanosarcina acetivorans

 

   II.1.5. Microalgae :

From the cyanobacteria family, these are unicellular or multicellular microorganisms, generally photosynthetic eukaryotes or prokaryotes, of great interest to the pharmaceutical, cosmetics and more particularly the food industry (algae are rich in iodine, polysaccharides, etc.). One example is the micro-alga spirulina, which contains around 70% protein.

Chlorella algae                                         Spirulina algea

 

   II.1.6. Viruses :

The virus (acaryotic microorganism) is a biological entity incapable of reproducing independently, requiring a host cell, whose constituents it uses to multiply, hence the name obligate cellular parasite. It is a microorganism consisting essentially of a nucleic acid (DNA or RNA) surrounded by a protein shell. 

ü Viruses (attenuated viruses, inactivated viruses) are used in the pharmaceutical industry to produce vaccines.

ü Bacteriophages, viruses that specifically infect bacteria, are used to eliminate pathogenic bacteria and treat certain bacterial infections.

    II.2 Properties of microorganisms of industrial interest :

A microorganism of industrial interest must be capable of growing and secreting the desired substance in large-scale cultures, and of growing in an inexpensive and available culture medium (agri-food waste), without any particular requirements (growth factors). It must also meet a number of criteria: 

- Rapid growth: short generation time in an inexpensive and available culture medium. 

- It must be non-toxic (no production of toxins) and must not be pathogenic for humans, animals or plants.

- Produce the substance of interest in a short period of time

- Grow and secrete this substance in a large volume culture.

- Produce spores or vegetative cells, facilitating inoculation in large fermenters.

- Can be genetically manipulated (genetic engineering) to increase yields. 

    II.2.1. Means of obtaining microorganisms of industrial interest:

There are two ways of obtaining microorganisms of industrial interest:

1.            Isolate them from natural fermentations, water, air, soil, tissues or more or less decomposed organic or even inorganic materials.

2.            Obtain them already isolated in the culture state from strain collections: ATCC (American Type Culture Collection) in the United States, the Institut Pasteur's biological resource centre, or the NCTC (National Collection of Type Culture).

     II.2.2. Selection and improvement of microorganisms of industrial interest :

The improvement strategies aim to increase the final product concentration and reduce the production of undesirable cometabolites. Considerations such as how the producing strains are used are also taken into account. Two broad categories of approaches are used for breeding: random mutagenesis and metabolic engineering.

• Random mutagenesis requires no prior knowledge of either the genome or the physiology of the strains treated. Mutagenesis induces genetic changes within the genome without it being possible to know where these changes will occur. Scientists subject a producing strain to mutagenic agents (radiation, UV) and select the most prolific strains in the progeny.
• Metabolic engineering can be defined as the improvement of a cell's potential through the manipulation of well-targeted enzymatic functions, using recombinant DNA technology; it is based on precise knowledge of the cellular functions to be modified and the genes associated with them.

For example, the original strains of Penicillium chrysogenum produced 1.2 mg of penicillin per litre. After genetic manipulation and selection, they produce more than 50,000 mg/l.

Human insulin is produced by genetic engineering: the insulin-producing gene is inserted into a host cell, an Escherichia coli bacterium, which, modified in this way, multiplies in a fermenter, automatically producing human insulin.