Bacterial nutritional media

Bacterial nutrition

The microbes draw all the necessary nutrients for the synthesis of their cell metabolite and for the generation of energy from the environment alone. These substances are termed as nutrients.

A substance or solution, full of nutrients is needed for the growth of , so the microbes can grow in it, therefore called as culture media.Or in other words the food material on which microbes are grown in the lab condition is known as culture medium . The growth of the microbes is called culture. Media may be vary in form and composition depending upon the species to be cultivated. Basically culture medium can be liquid or solid.

1. Liquid culture medium or broth

A sterile liquid medium in which no solidifying agent has been added, is known as broth

2. Solid culture medium

Sterile medium which has been solidified by adding Agar Agar or any other solidifying agent. Example gelatin is known as solid medium. Agar Agar is the most common solidifying agent which is obtained from a red algae . It liquefies on heating to 96 degrees Celsius and hardens into a Jelly on cooling to 40 to 45 degrees Celsius.

On the basis of composition there are three types of culture media

1. Natural culture media

The exact chemical composition of the natural medium is not known . The natural culture medium include milk ,urine ,diluted blood ,vegetables, juice ,meat extract and peptones etc

2. Semisynthetic culture medium

The medium whose chemical composition is partly known. Example PDA (potato dextrose Agar) oat meal agar, any medium which contains Agar becomes semi synthetic medium.

3. Synthetic medium

Synthetic medium or chemically defined culture medium is composed of special substances of known composition . Example EMB AGAR (eosine methylene blue Agar), mannitol agar. Further a synthetic medium can be divided into three categories

• General purpose medium it is used for a wide variety of microorganisms . Example nutrients Agar medium

• Selective medium these are used for selective isolation of microbes. These media encourage the growth of certain type of bacteria and restrict or inhibit other type of bacteria . Example mannitol salt Agar this is used for selective isolation of pathogenic Staphylococci (because most of the bacteria are inhibited by high concentration of salt added in the medium).

Mannitol Agar in which nitrogen sources omitted . This is used for isolation of nitrogen fixer.

Rose Bengal agar it is used for selective isolation of fungi because it inhibits all bacteria

• Differential media

These are used for differential isolation of microbes. These media allow the growth of most of the microbe but help us to identify a particular type of microbe

Example : EMB (eosin methylene blue Agar), salt mannitol Agar, in which phenyl red has been added and MRS agar medium for isolation of Lactobacillus. In this medium a pH sensitive dye BCP (bromocresol purple) is added . Bacteria ferment lactose present in the medium and pH of the medium decreases in the area surrounding these bacterial colonies . The colour of the medium at these places changes from purple to yellow

• Blood Agar medium

It is used for isolation of hemolytic streptococci. These bacteria hemolyzed blood and a clear zone is produced around these colonies.

Important ingredients of culture media

Various ingredients, some of which are plant origin and some of animal origin , are used in various culture media . Some chemical substances are also added, depending upon the microbe to be grown in the medium. Some important ingredients are as follows

• Routine hydrolysates

These are the substances used as source of protein . Some are meat, gelatin ,carotene and soybeans based.

• Meat extract and infusions

these substances are rich in minerals, organic micro nutrients, protein derivatives ,carbohydrates and vitamins

• Yeast extract

These are prepared by autolysis and plasmolysis of cells of saccharomyces and is available in powder or paste form. It is a mixture of amino acids, peptides, water soluble vitamins, carbohydrates and minerals

• Malt extract

It contains carbohydrates 90 to 92% and Nitrogenous bacteria 4 to 5% . It also contains vitamins ,fatty acids, organic acid, phosphorus,sulfur, vitamins such as biotene , folic acid. In addition to these it contains maltose ,hexose ,sucrose and proteins etc.

• Agar

A solidifying agent, forms gel at temperature below 40 degrees Celsius is also required. It is a polysaccharide which is extracted from several red algae.

• Gelatin

it was used for earlier for solidification

• Carbohydrates

Various carbohydrates are used in media as energy source and source of carbon for hetetotrophic microbes

• Chilates

Chelating agents are organic compounds which form complex substances with heavy metals and keep them in solution. In culture media, EDTA (ethylene diamine Tetra acetic acid ) is the best known chelating agent and is widely used.

Preparation of culture medium

Preparation of culture medium involves the following steps

• Ingredients are weighed and dissolved one by one in distilled water

• PH of the dissolved medium is checked and adjusted if not correct

• The medium is dispensed into suitable container example test tubes or flask

• the mouth of container is closed with cotton plugs

• cotton plugs are covered with aluminium foil to protect it from moisture

• Medium is sterilised by moist heat in an autoclave at temperature 121 degree Celsius and pressure 15 lbs/mm for 20 minutes

• Addition to the media after steriliation ,sometimes some heat labile substances example antibiotics , blood serum etc. are to be added to the medium. These solutions are sterilised separately by filtration (Millipore filter ) and added to the media after autoclaving.

Some examples of cultural media which is used in the routine work in the microbiological labs are

• Nutrient Agar medium it is a general purpose medium which supports the growth of most of the microbes

• Malt extract medium it is used for culture of yeast ,streptomyces. Rose Bengal Agar for molds.

• Ashby’s medium it is used for the growth of free living bacteria like Azotobacter .

• MacConkey agar it is used for growing gram negative bacteria

• Terrific broth it is used for growing Escherichia coli.It is used with glycerol

• Xylose lysine desoxyscholate (XLD) it is selective for gram negative bacteria buffered charcoal yeast extract agar,is used for selective gram negative bacteria like Legionella pneumophila.