Welcome to the Microbiology Information

Microbiology: An Introduction Media Update No Synopsis Available

Prepared culture media dispensed in plates, tubes, or bottles, which are not used immediately should be protected against light and desiccation, so that the composition and the performance is not changing during storage. The stability of prepared culture media is limited. For most of the prepared agar plates and bottled media the optimal storage temperature is 4-12 °C in the dark, and wrapped in a bag to avoid contamination and dehydration. Prepared culture media containing blood, egg yolk, egg yolk tellurite emulsion and antibiotics should be stored in the refrigerator at 2-8 °C.

If plating agars are to be stored for a longer period of time, they must be prevented from drying out by sealing each Petridish with adhesive tape along the joint between the lid and base or by packing several dishes into airtight plastic bags. Before packaging the plates are not dried. They should be cooled down, because hot to warm agar plates produces condense water. Excessive condense water may ultimately result in contamination of the plates. The storage of agar plates before drying limits the adverse effects of drying of the agar surface during storage. A loss of more than 15 % of the water content (during storage and incubation) can adversely affect the growth of microorganisms, particularly Gram-negative. Liquid media in test tubes or flasks should also be sealed airtight. Loss of water can result in precipitation and crystallization of certain substances in the culture media.

The extent of water loss depends on the composition, the amount of medium in the plates, the type of incubator i.e. fan-assisted or otherwise, the humidity of the atmosphere in the incubator, the position and number of the plates in the incubator and the incubation temperature.

In the case of culture media which contain unstable additives, it is often better to store the prepared medium without required additives and to add these later when the final medium is needed.

Some culture media contain ingredients which are extremely light sensitive, e.g. rose bengal in yeast agars like Rose Bengal Chloramphenicol Agar. Upon exposure to light sensitive an inhibitory substance is formed. Rose bengal containing culture media must be kept in the dark both during storage and incubation.

The expiration date of prepared culture media depends on the quality of the basic ingredients, the formulation, the quality of the preparation procedures, the sterilization, the packaging and the storage conditions. Each laboratory should assess the expiration date of each prepared culture medium for its typical preparation, packaging and storage conditions.

ISO 11133 part 1 (2000-06-01) recommends that media to which final components are added immediately before use shall be kept in a refrigerator for not more than 3 months, at room temperature for not more than 1 month and in sealed bags for a maximum period of one week.

For its continual maintenance and its multiplication, microorganisms must draw from its environment the substances required for the synthesis of its cell material and the generation of energy. The substances microorganisms require are termed nutrients. The nutrient requirement of microorganisms varies with type of microorganism and can be very complex. Escherichia coli is very simple in its nutritional requirement, whereas Lactobacilus spp. are very demanding (fastidious). A culture medium must supply not only the nutrients a specific microorganism requires, but these must also be present in the appropriate concentration. A too high concentration of a nutrient e.g. amino acid, may inhibit the growth.

Composition of microorganisms-macromolecules
The solid matter of microorganisms contains in addition to hydrogen and oxygen (derivable from water) carbon, nitrogen, phosphorus and sulfurs. These six element account for 95 % of the cellular dry weight.

Microorganisms consists of water and macromolecules. Apart from lipids, the macromolecules are build from monomers. Monomers are the precursors of the macromolecules. Examples of macromolecules are:

Protein
Polysaccharides
Lipid
Lipopolysaccharide
DNA
RNA

Proteins are the most abundant class of macromolecules and consists of a polymers of the monomers amino acids. After protein ribonucleic acid (RNA) is the most abundant macromolecule. Ribonucleic acid is a polymer of nucleotides and occurs in ribosomes, messenger and transfer RNA’s, the key players in the protein synthesis. Lipids rank in abundance as third. Fatty acids are the main constituent of lipids. The simplest form of a lipid is a tri-glyceride and the more complex forms are phospholipid and glycolipid. Lipids are crucial for the membrane structure and serve also storage depots of excess of carbon.

The smaller fractions of cell constitute polysaccharide, lipopolysacharide and DNA. Polysaccharides are polymers of sugars and are primarily present in cell walls. They also serve as carbon and energy source (e.g. glycogen) Lipopolysachharides, such as glycolipid and glycoprotein, play an important role in cell membrane and cell surface receptor molecules. DNA is the other polymer of nucleotides and its contribution to the bacterial cell weight is small. Its function as the repistory of genetic information is, however, crucial to microorganisms.

Nutrient requirements
Microorganisms differ in the specific form under which carbon, nitrogen, sulfur and oxygen must be provided as nutrients. Nutritional studies have shown that microorganism that do not perform photosynthesis or bacteria that obtain energy from the oxidation of inorganic compounds obtain carbon simply from organic nutrients. These include i.e. amino acids, fatty acids, organic acids, sugars, nitrogen bases, aromatic compounds. The C source has a dual function and serves both as source of carbon and source of energy. Carbon is the major element in all classes of macromolecules. Some organism require a single organic compound whereas other can not grow with only one compound. Microorganism are extremely divers in the kind and the number of organic compounds they require as C source. After carbon nitrogen is the most important element and is found i.e. in proteins as amino acids and in nucleic acids.

The nitrogen (N) source for most organisms are inorganic compounds, that is ammonia, and nitrate or, organic compounds, that is, amino acids, nitrogen bases of nucleotides and many N-containing organic compounds. Nitrogen fixing microorganisms require nitrogen gas.

In addition to C and N source a microorganisms requires macronutrients such as phosphor, sulfur, potassium, magnesium, calcium, sodium and iron. Phosphor is required for the synthesis of nucleic acids and phospholipids. Sulfur is required in the amino acids cysteine and methionine and in vitamins such as thiamine, biotin, lipoic acid, and co-enzyme A. Most cell sulfur originates from inorganic source such as sulfate or sulfide. Potassium is required for the protein synthesis and plays an important role in the homoeostasis. Magnesium functions to stabilise ribosomes, cell and nucleic acids. It is also required for the activity of many enzymes. Calcium helps to stabilize the cell wall and plays a key role in the heat stability of endospores. Sodium plays a role in the homeostasis. Iron plays a major role in the cellular respiration and is a key component of cytochromes and iron-sulfur proteins involved in the electron transport.

Microelements or trace elements are elements such as cobalt, nickel, chromium, copper, manganese, selenium tungsten, vanadium and zinc. Many of the trace elements play a structural role in enzymes.

Any compound that a microorganism can not synthesise from simpler carbon sources must be provided as a nutrient. Such organic compounds are termed growth factors. These include vitamins, amino acids, purines and pyrimidines. Growth factors fulfill specific needs in biosynthesis and they are required in only small amounts. Vitamins function as co-enzymes. Lactic acid bacteria are renowned for their complex vitamin requirement.

Microbiology: An Introduction Media Update No Synopsis Available

Now here are some basic microbiology information, the culture media and its intended use. You see a microbiology laboratory cannot function without its core ingredient, media. Everything in microbiology is about growing microscopic organims and therefore knowledge of its growth media is critical. That is the composition of a culture medium formulation determines its purpose.

Preservation medium
A preservation culture medium preserves and maintain the viability of microorganisms over an extended period. During long-term storage the preservation medium protects microorganisms against the adverse influences (e.g. Dorset egg medium).

Resuscitation medium
A resuscitation medium is a non selective nutrient rich medium enabling stressed and damaged microorganisms to repair and to recover their capacity for normal growth (e.g.Tryptic soya agar with 0.3 % yeast extract or Tryptic soy broth).

Enrichment medium
A liquid culture medium provides nutrients for multiplication of microorganisms (e.g. Buffered peptone water or Nutrient broth).

Fermentation medium
A liquid culture medium formulated to achieve the nutrients for an optimal yield of a specific microorganisms (e.g. Yeast) or metabolism product (e.g. toxin).

Selective enrichment medium
A selective enrichment medium is formulated to support the multiplication of target microorganism or a group of microorganisms whilst partially or totally inhibiting the growth of accompanying interfering organisms (e.g. Muller-Kauffmann Tetrathionate broth with novobiocin or L-PALCAM broth).

Isolation medium
A solid culture medium which supports the growth of microorganisms (e.g. Plate Count Agar).

Selective isolation medium
A selective isolation medium which supports the growth of specific target microorganisms, whilst inhibiting other interfering microorganisms (e.g. PALCAM agar or MacConkey agar).

Differential medium
A culture medium which permits the testing of one or more physiological/biochemical characteristics of a microorganisms for their identification (e.g. Fluorocult LMX broth or Simmons Citrate Agar).

Identification medium
A culture medium designed to produce a specific identification reaction which does not require any further confirmatory test (e.g.Triple Sugar (TSI ) Agar).

General-purpose media
Some culture media may be assigned to several categories. Blood Agar, for example can be used as a resuscitation medium, as isolation medium or as a differential medium for the detection of haemolysis.

Culture Media for Food Microbiology No Synopsis Available